Patent Publication Number: US-2015083138-A1

Title: Laryngeal mask airway device

Description:
CROSS-REFERENCE SECTION 
     This application is a continuation of currently pending U.S. patent application Ser. No. 13/399,680, filed Feb. 17, 2012, which is a continuation of Ser. No. 11/939,149, filed Nov. 13, 2007, which is a continuation of Ser. No. 11/350,470, filed Feb. 10, 2006, now U.S. Pat. No. 7,305,985, which is a continuation of U.S. patent application Ser. No. 10/225,678, filed Aug. 22, 2002, which is a continuation of U.S. patent application Ser. No. 09/289,319, filed Apr. 9, 1999, now U.S. Pat. No. 6,439,232, which claims the benefit of United Kingdom patent application 9817537.5, filed Aug. 13, 1998, all of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to laryngeal mask airway devices (LMA-devices). Such devices are useful in facilitating lung ventilation in unconscious patients by forming a low pressure seal around the patient&#39;s laryngeal inlet, avoiding the known harmful effects of the endotracheal tube, which forms a seal within the windpipe (trachea). 
     LMA-devices of the types disclosed in UK Patent Nos. 2111394 and 2205499 have become accepted items of equipment for rapidly and reliably establishing an unobstructed airway in a patient in emergency situations and in the administration of anaesthetic gases, and have found use in most countries of the world. A disadvantage associated with the use of such a mask is encountered in a patient who is at risk from vomiting or regurgitating stomach contents while unconscious since although the device forms a seal around the laryngeal inlet sufficient to permit artificial ventilation of the lungs, the seal is sometimes insufficient to prevent lung contamination during retching, vomiting or regurgitation. 
     A partial solution to this problem is disclosed in U.S. Pat. No. 4,995,388 in which reliance is made upon a combination of an improved peripheral continuity of seal pressure against the laryngeal inlet and the provision of a drainage tube to conduct gastric contents away from the laryngeal inlet. However, one embodiment of such a system is itself disadvantaged by the fact that the removal of such gastric discharges can be achieved only after the seal between the LMA device and the laryngeal inlet/oesophagus has been breached. Another embodiment provides for removal of gastric drainage without breaching the seal between the LMA device and laryngeal inlet/oesophagus, but this proved awkward to insert and caused throat irritation. 
     A more successful solution to this problem has been provided by the gastro-laryngeal mask airway device disclosed in U.S. Pat. No. 5,241,956 and European Patent 651664. In that device, a drainage tube passes through the posterior aspect of the mask and through the distal end of the inflatable cuff of the mask to open in alignment with the patient&#39;s oesophagus. However, the drainage tube must be sufficiently rigid at its distal end to withstand the pressure within the inflated cuff and it has been found that this may make proper insertion of the deflated device into the patient&#39;s throat more difficult than either necessary or desirable. 
     In a modified gastro-laryngeal mask airway device disclosed in International Patent Application WO 97/12680, provision is made for the distal half of the mask to be of softly compliant construction, and to ensure against collapse of the drainage tube when the cuff is inflated. Also, the mask has a flexible leading edge for facilitating correct insertion into the throat of the patient. 
     European Patent Application 796631 and U.S. Pat. No. 5,632,271 disclose an LMA device which further facilitates insertion into the throat of the patient, an LMA device includes a drainage tube, which opens into the distal tip of the mask, passes along the posterior aspect of the flexible airway tube and emerges from the mouth of the patient just below the upper incisor teeth. For practical purposes this device works well but has the following limitations. 
     A disadvantage of this back-to-back tube orientation is that it confers a degree of instability to the mask when the device is in place, permitting the possibility of loss of seal between the mask and laryngeal inlet. Another disadvantage of the back-to-back tube configuration is that it confers to the tubular elements of the device an undesirable degree of stiffness so that movements of the head and neck of the patient occasioned, for example, by surgical manipulation or positioning, may result in undue harmful pressure being exerted on the surrounding tissues of the upper airway passages. 
     Another disadvantage is that the inserting index finger tends to slip off the airway and drainage tube due to lack of purchase. A still further disadvantage is that the inserting index finger may be damaged by the teeth of the patient because of the greater combined diameter of the back-to-back tubes. 
     SUMMARY OF THE INVENTION 
     The present invention has as its overall objective to provide an LMA device of the types described above, i.e., incorporating means for draining gastric discharge from the region of the oesophageal inlet of the patient, which substantially avoids the disadvantages described above in relation to various of the known types of LMA-devices. 
     In accordance with the invention, this objective is achieved by first modifying the bowl of the mask such that its interior curvature has a significantly deeper shape than previous constructions. This is accomplished by either making the posterior wall or backplate of the mask to generally the same peripheral dimensions to permit its attachment to the posterior aspect of the inflatable cuff formation (in contrast to attachment to the inner rim or equator of the cuff formation), or by changing the cross section shape of the cuff so that its seam is placed at offset from the major or equatorial plane. Hence, the backplate is located substantially behind, i.e., posteriorly of the cuff and not, as previously, within the annulus of the cuff. The backplate edge, or rim, is attached roughly tangentially with respect to the roughly ring-shaped cross-section of the inflatable toroidal shape of the cuff annulus. It will be evident that with this construction, the depth of the bowl of the mask, i.e., the distance between the anterior aspect of the cuff when inflated and the anterior aspect of the backplate, will be greater than in previous constructions by approximately half the posterior-anterior dimension of the inflated cuff. Since most adult-size LMA devices have cuff inflation diameters in the range of 12 to 16 millimeters, it is clear that the additional bowl depth will be of the order of 6 to 8 millimeters. This additional bowl depth permits the gastric drain tube to be on the anterior surface of the backplate instead of running posteriorly as in previous designs, increasing the stability of the mask when installed in the throat of the patient and reducing the tendency of the installed device to migrate outwardly. 
     This anterior placement of the drain tube also eliminates the requirement to guard the aperture of the airway tube against obstruction by the anatomical structure known as the epiglottis. To prevent such obstruction, former cuffs were provided with paired parallel bars running across the airway aperture. These bars proved effective in preventing epiglottis obstruction but offered unwanted resistance to airflow and tended to obstruct passage of suction or inspection tubing. Anterior positioning of the drain tube allows it to act as an epiglottic prop, holding back the epiglottic rim from the floor of the mask and the airway port. The paired bars described above were not able to prevent obstruction occurring as a result of the epiglottic rim lying in contact with the bowl or floor of the mask. The anterior location of the drain tube in the present invention overcomes the problems of epiglottic misplacement more effectively than the previous design. 
     The second modification to the backplate is to replace the single tube joint port adapted to accept the flexible airway tube with a double-barrelled port in which said ports are arranged side-by-side, that is to say laterally, permitting easy assembly of said side-by-side airway and drainage tubes. This provides better correspondence with the cross section space within the throat, the major axis of which runs laterally, and reduces stiffness and consequent pressure on the throat from movements of the head and neck of the patient. Also, the side-by-side adjacency reduces the pressure exerted on the drainage tube by the incisor teeth of the patient, and facilitates manufacturing since the portions of the tubes in the throat of the patient describe similar radii. 
     The double-barrelled tube joint additionally provides a desirable locating point for the tip of the index finger used to insert the device, thus reducing possible slipping of the finger on the tube-joint. Also, the reduced transverse diameter in the vertical direction between the teeth of the patient resulting from the side-by-side adjacency of the drainage and airway tubes reduces possible injury to the finger from contact with the teeth. 
     A third modification to the backplate is the incorporation of a well or depression covering an area of approximately 3 square centimeters and having a 2 to 5 millimeters depth situated in the anterior surface of the backplate under the drain tube where it connects with the distal end of the drain port of the backplate. The well has the dual functions of permitting gas circulation and allowing secretions from the trachea to be drained away. 
     The LMA device of the invention is readily distinguished from the devices proposed hitherto in which the backplate of the mask has been located within the annulus of the inflatable cuff, and in which the gastric drainage tube has been routed across the posterior surface of the backplate. 
     According to the invention, therefore, there is provided a laryngeal mask airway device equipped for drainage of gastric discharge, the device comprising an inflatable main-cuff and a backplate having a laryngeal-side and a pharyngeal-side. The backplate also has an external tube joint adjacent to the proximal end of the main-cuff. The backplate is hermetically bonded to the periphery of the main-cuff establishing separation between a laryngeal-chamber region and a pharyngeal region. A distally open evacuation tube includes a distal portion which longitudinally traverses the interior of the distal region of the main-cuff in sealed relation therewith for operative engagement and communication with the inlet of the oesophagus. The evacuation tube traverses the laryngeal-chamber region generally adjacent to the laryngeal-side of the backplate and passages through a proximally located tube joint to the pharyngeal region. An airway tube also extends into the tube joint for communication with an airway port to provide a flowpath between the airway tube and laryngeal-chamber region. 
     These and other objects, features, and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a simplified overall view to show an LMA-device of the invention, installed in a patient whose relevant anatomical features are shown by phantom outlines; 
         FIG. 2  is a perspective view showing the LMA-device of  FIG. 1  installed in a patient, the patient being shown in the quarter neck direction from the front right-side omitting most neck structures and showing a sagittal section of the larynx, the epiglottis being shown displaced anteriorly relative to the main-cuff to show the internal-drain tube (normally, the epiglottis extends into the main-cuff), the right lateral portion and proximal region, including the hemispherical posterior bulge, of the main-cuff being shown; 
         FIG. 3  is a plan view of the anterior side of the LMA-device of  FIG. 1 , the main-cuff being inflated and illustrated in enlarged scale relative to  FIG. 1 , the airway and external-drain tubes being cut-off, the well hidden behind the internal-drain tube also being shown; 
         FIG. 4  is an enlarged perspective view of a detail of  FIG. 3  with the airway and evacuation tubes removed, showing the anterior surface of the tube joint and the posterior bulge of the main-cuff; 
         FIG. 5  is an enlarged plan view of a detail of  FIG. 3  with the airway and evacuation tubes removed, showing the anterior surface of the tube joint and the posterior bulge of the main-cuff; 
         FIG. 6  is an enlarged end view of a detail of  FIG. 3  with the airway and evacuation tubes removed, showing the proximal end surface of the tube joint and the posterior bulge of the main-cuff; 
         FIG. 7  is a plan view of the posterior side of the device of  FIG. 1 , in the same inflated condition as and to the scale of  FIG. 3 , the portions of the airway tube and the external- and internal-drain tubes hidden in the tube joint being shown, the well hidden behind the backplate also being shown; 
         FIG. 8  is a lateral view in partial section, in the plane indicated by the line  8 - 8  of  FIG. 7  which is parallel to the sagittal plane and which coincides with the central longitudinal axis of the evacuation tube, except in the distal region of the main-cuff where the evacuation tube is transversely offset from the sagittal plane, showing the longitudinal traverse of the internal-drain tube along the backplate; 
         FIG. 9  is a view corresponding to  FIG. 8  with portions broken away to show the anterior-posterior dimension of the internal-drain tube relative to a plane containing the anterior surface of the main-cuff; 
         FIG. 10  is a sectional plan view in the plane indicated by the line  10 - 10  of  FIG. 9  showing the location of the anterior-posterior dimension of  FIG. 9  relative to the proximal region of the main-cuff; 
         FIG. 11  is a distal view in cross section, in the plane indicated by the line  11 - 11  of  FIG. 7  showing the engagement between the internal-drain tube and backplate, and the adjacency between the seam in the main-cuff and backplate; 
         FIG. 12  is a distal view in cross-section of a second embodiment of the backplate and back-cuff in a plane corresponding to the plane indicated by line  11 - 11  of  FIG. 7 , showing a reduced wall thickness of the backplate in the sagittal plane, and the back-cuff tethered to the backplate; 
         FIG. 13  is a distal view in cross section, in the plane indicated by the line  13 - 13  of  FIG. 7  showing a portion of the LMA-device between lines  11 - 11  and  13 - 13 , the clearance between the internal-drain tube and base of the well being illustrated; 
         FIG. 14  is an enlarged fragmentary view of a detail of  FIG. 8  showing the connection between the external-drain tube and distal region of the main-cuff, the angles between selected parts and respective reference planes also being shown; 
         FIG. 15  is an enlarged fragmentary view of a detail of  FIG. 8  showing the connection between the internal and external-drain tubes; 
         FIG. 16  is an anterior perspective view of the backplate removed from the LMA-device of  FIGS. 3 and 7 ; 
         FIG. 17  is a perspective view, in the aspect indicated by line  17  of  FIG. 16 , showing the recessed heel portion and well, and also showing the double-barrelled passage for the connections of the airway and external-drain tubes; 
         FIG. 18  is an anterior view of a second embodiment of the backplate of  FIG. 16 ; 
         FIG. 19  is a perspective view of the second embodiment of the backplate illustrated in  FIG. 18 , in the aspect indicated by line  20 , showing the recessed heel portion, and the double-barrelled passage for the connections for the airway and external-drain tubes; 
         FIG. 20  is a perspective view of the anterior surface of the LMA-device of  FIGS. 3 and 7  in a deflated condition; 
         FIG. 21  is a lateral view of the main-cuff in the direction indicated by line  22  of  FIG. 20  showing the preferred deflection characteristic of the main-cuff; 
         FIG. 22  is a perspective view in the aspect of  FIG. 20  showing the LMA-device of  FIGS. 3 and 7  in an inflated condition; 
         FIG. 23  is a plan view of the anterior side of a third embodiment of the LMA-device of  FIGS. 3 and 7  showing one-way valves incorporated in the anterior wall of the main-cuff; and 
         FIG. 24  is a lateral view of the main-cuff of the embodiment illustrated in  FIG. 23  in the direction indicated by line  23 - 23  showing one of the one-way valves and its associated housing. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     As used herein, the anatomical terms “anterior” and “posterior”, with respect to the human body, refer to locations nearer to the front of and to the back of the body, respectively, relative to other locations. The term “anterior-posterior (A-P)” refers to a direction, orientation or the like pointing either anteriorly or posteriorly. The anatomical terms “proximal” and “distal”, with respect to applying an instrument to the human body, refer to locations nearer to the operator and to the inside of the body, respectively. Alternatively, “distal”, as opposed to “proximal”, means further away from a given point; in this case, “distal” is used to refer to positions on the LMA-device  20  or in the body relative to the extreme outer or connector end of the LMA-device. “Proximal” is the opposite of “distal”. The term “lateral” refers to a location to the right or left sides of the body, relative to other locations. Alternatively, “lateral” means to one or other side of the mid-line, with respect to the major axis of the body, or to a device lying in the body&#39;s major axis. The term “bilateral” refers to locations both to the left and right of the body, relative to the sagittal plane. The term “sagittal” or “sagittally” refers to a vertical longitudinal plane through the center or midline of the body that divides a bilaterally symmetrical body into right and left halves. The sagittal plane is the plane passing antero-posteriorly through the middle of the body in its major axis. The term “medial” means nearer to the mid-line. 
     A laryngeal-mask airway device (LMA-device) of the present invention, is designated generally by the reference numeral  20  in  FIGS. 1 and 2 . The LMA-device  20 , in a deflated condition, is inserted into the throat  32  the upper surface of which is bounded by hard and soft palates  192 ,  195 . The LMA-device  20  is lodged in the pharynx  197  of the throat  32  at the base of the hypo-pharynx  212  where the throat divides into the trachea  36  (i.e., windpipe) and oesophagus  57 . A lower portion of the LMA-device  20  reaches to the base of the hypo-pharynx  212 . After the LMA-device  20  is so lodged in the pharynx  197  such that the lower portion of the LMA-device reaches the base of the hypo-pharynx  212 , the LMA-device is inflated. Disposed in the junction between the throat  32  and trachea  36  is the flexible epiglottis  35  (i.e., a lid-shaped structure) which forms the upper border of the larynx  37 , entry through which is provided by the laryngeal inlet  67 . To facilitate understanding of the relations between the LMA-device  20  and anatomy of the throat  32  and related structures, a glossary of the anatomical structures related to the LMA-device is provided herein below. 
     Referring to  FIGS. 1 and 2 , the laryngeal-mask airway device (LMA-device)  20  is shown comprising an airway tube  22 , installed through the mouth  25  of a patient. The LMA-device  20  further comprises a backplate  27  having an airway port  30  through which the airway tube  22  can establish a free externally accessible ventilation passage, via the patient&#39;s mouth  25  and throat  32 , and past the epiglottis  35  to the larynx  37 . The backplate  27  is preferably of an elastomer such as silicone rubber and relatively stiff, for example, of 80 Shore durometer. 
     As further shown in  FIGS. 3 and 7 , the backplate  27  is surrounded by a main-cuff  40  comprising an inflatable ring which, when inflated, has the shape of a torus generated by an asymmetrical oval or ellipse having a wider proximal region  42  and narrower distal region  45 . The main-cuff  40  is circumferentially united to the backplate  27  in essentially a single plane, except for the portion of the main-cuff extending into a recess  47  in a heel  50  of the backplate  27 . The portion of the main-cuff  40  extending into the recess  47  may or may not be united to the backplate  27 , as described further hereinbelow. 
     The main-cuff  40  may also be of silicone rubber, although preferably relatively soft and flexible compared to the backplate  27 . The material of the main-cuff  40  is preferably of 20 to 30 Shore durometer. Except for a plastic connector (not shown) attached to the proximal end of the airway tube  22  and a check valve  52 , all parts of the LMA-device  20  disclosed herein are preferably made of silicone, possibly with different additives. 
     An externally accessible tube  55  and inflation port  56  on the main-cuff  40  are the means of supplying air to the main-cuff and of extracting air from (and therefore collapsing) the main-cuff for purposes of insertion in or removal from the patient. The check-valve  52  is disposed in the tube  55  for holding a given inflation or holding a given deflation of the main-cuff  40 . 
     In the installed position of  FIGS. 1 and 2 , the projecting but blunted distal region  45  of the main-cuff  40  is shaped to conform with the base of the hypo-pharynx  212  where it has established limited entry into the upper sphincteral region of the oesophagus  57 . The pharyngeal-side  60  of the backplate  27  is covered by a thin flexible panel  62 , as shown in  FIGS. 7 ,  11  and  13 , which is peripherally bonded to a margin  63  on the posterior surface of the main-cuff  40 , to define an inflatable back-cuff  65  comprising a cushion which assures referencing to the posterior wall of the pharynx and thus is able to load the inflated main-cuff forward for enhanced effectiveness of sealing engagement to the inlet  67  of the larynx  37 . The inflated main-cuff  40 , thus-engaged to the laryngeal inlet  67 , orients a distal-end  72  of the airway tube  22  at an acute angle to a mid-line major plane  75  of the main-cuff  40  and in substantial alignment with the axis of the laryngeal inlet  67 , for direct airway communication only with the larynx  37 . 
     The major plane  75  is a plane containing the major axis  77  of main-cuff  40  extending between proximal and distal regions  42 ,  45 . The major plane  75  is disposed between, and parallel to, the anterior and posterior surfaces of the main-cuff  40 . Additionally, the major plane  75  is equidistant from the anterior and posterior surfaces of the main-cuff  40 , except for posterior bulge  100 . 
     The LMA-device  20  is of the GLM (gastro-laryngeal mask) variety in which an evacuation tube, designated generally by  80 , as shown in  FIGS. 1 ,  2 ,  3  and  7 , serves for extraction and external removal of gastric-discharge products from the oesophagus  57 . Additionally, the evacuation tube  80  provides a pathway into the oesophagus  57  for insertion, for example, of a gastric feeding tube, suction catheter, temperature probe or other monitoring device, probes carrying stimulating electrodes such as pacing wires, sengstaken balloons, or other catheters bearing inflatable cuffs, fiber optic endoscopes or medication. The evacuation tube  80  follows the general course of the airway tube  22 , with sealed entry through the backplate  27  alongside the airway tube, on the laryngeal-side  81  of the backplate, and with sealed passage through the interior of the main-cuff  40  and open through the distal region  45  of the main-cuff. Inflation-air supply to the back-cuff  65  may be via the same tube  55  as for the main-cuff  40 , or separate inflating means (not shown) may be provided for the back-cuff  65 . The disclosures of U.S. Pat. Nos. 5,241,956, and 5,632,271, and 5,878,745 disclosing various laryngeal mask devices, are hereby incorporated by reference herein. 
     More specifically, the toroidal-shaped main-cuff  40  is formed by first moulding it in an intermediate stage having opposing edges, each of which has an elliptical shape. The opposing edges of the main-cuff  40 , when in generally edge-to-edge relation, are welded together to form a seam  85 , as shown in  FIGS. 5 ,  11  and  13 . The seam  85  defines an oval contained in a plane which is parallel to the major plane  75 , corresponding to the internal surface of the main-cuff  40 . When the backplate  27  is attached to the main-cuff  40 , the seam  85  abuts the periphery of the oval portion  87  in anterior relation to the backplate, as best shown in  FIGS. 11 and 13 . The seam  85  may be inserted in a corresponding groove in the oval portion  87 . Alternatively, the backplate  27  and main-cuff  40  may be extruded as a single, unitary piece. 
     As used herein, the term “welding” describes the bonding together of two components having the same or similar chemical compositions, either by adhesive having the same or similar chemical composition as the components, or by high pressure or temperature fusion, or a combination of any of them. 
     A separate tube (not shown), preferably with multiple perforations along its length, may be contained within the main-cuff  40  between the opening of the tube  55  into the main-cuff such that each perforation communicates with a port between the interiors of the main-cuff and back-cuff  65 . Such a separate tube preserves a flowpath between the tube  55  and back-cuff  65  if the main-cuff  40  is completely collapsed from deflation, thereby providing for further deflation of the back-cuff  65  via the tube  55 . Alternatively, a channel (not shown) may be formed on the inner surface of the main-cuff  40  between the opening of the tube  55  into the main-cuff and at least one of the one or more ports between the interiors of the main-cuff and back-cuff  65 . Such a channel preserves a flowpath between the tube  55  and back-cuff  65  if the main-cuff  40  is completely collapsed from deflation. 
     The backplate  27  has a one-piece, integral spoon-shape which, with the oval portion  87 , also has an external tube joint  92 . The tube joint  92  is oriented proximally relative to the oval portion  87 . Opposite sides of the oval portion  87  are defined by a convex pharyngeal-side  60  and concave laryngeal-side  81 . The periphery of the oval portion  87  is hermetically bonded to the periphery of the main-cuff  40  to establish separation between the laryngeal-chamber region  110  and pharyngeal region  112 . 
     The periphery of the oval portion  87  of the backplate  27  abuts, in proximal relation to, the seam  85  of the main-cuff  40  in its inflated condition, as shown in  FIGS. 10 and 12 . This more posterior location of the backplate  27 , as compared to locating the periphery of the oval portion  87  in the major plane  75 , provides additional space for the internal-drain tube  115 . The oval portion  87  may be located at various positions in the anterior-posterior direction relative to the main-cuff  40  because of the generally constant cross-section of the laryngeal-chamber region  110  in planes parallel to major plane  75 , as shown in  FIGS. 10 and 12 . 
     Formed in the laryngeal-side  81  is a well  95  defined by a depression adjacent to the tube joint  92 . The well  95  faces the evacuation tube  80  such that the well is offset relative to the sagittal plane  97  of the main-cuff  40 . The well  95  thereby provides a radial clearance between the evacuation tube  80  and laryngeal-side  81 . 
     The portions of the laryngeal-side  81  which are proximal and distal of the well  95  are inclined relative to the base of the well such that the laryngeal-side ramps anteriorly as it approaches the well in the distal and proximal directions, as shown in  FIG. 8 . 
     The periphery of the oval portion  87  adjacent to the tube joint  92  is included in the heel  50 . A portion of the heel  50  contiguous with its anterior edge is removed to define a crescent-shaped recess  47 . The proximal region  42  of the main-cuff  40  has an approximately hemispherical posterior bulge  100  arising from its posterior surface, as shown in  FIG. 8 . The posterior bulge  100  extends posteriority symmetrically relative to the sagittal plane  97  to fit into the mid-line groove  102  forming part of the anterior surface of the double-barrelled tube joint  92  of the backplate  27 . The mid-line groove  102  is shown in  FIG. 16 . The posterior bulge  100  also extends into the crescent-shaped recess  47  to compensate for the reduced support provided by the backplate  27  resulting from the recess  47 . 
     Less than the entire width of the main-cuff  40  extends posteriorly from the proximal region  42  because the recess  47  of the backplate  27  allows space for the main-cuff  40  to extend posteriorly in the approximately hemispherical posterior bulge  100 . The posterior bulge  100  is partially supported bilaterally by the backplate  27  thus preventing ballooning-out of this portion of the main-cuff  40 . Such ballooning-out of the main-cuff  40  would result in the flow of internal gases from other interior regions of the main-cuff resulting from redistribution of the pressure in the main-cuff, thereby resulting in an uneven seal between the main-cuff and the tissues surrounding the laryngeal inlet  67 . Such an uneven seal might result in loss of seal, particularly at the pointed distal end of the main-cuff  40 . 
     The recess  47  and mid-line groove  102  together form a partial socket which provides mechanical support posteriority, bilaterally and distally for the posterior bulge  100 . 
     The posterior bulge  100  may be separable from the recess  47  to define a normally closed and therefore self-sealing port for insertion of an elongate member such as a probe, endotracheal tube, endoscope or the like from the pharyngeal-region  112  into the laryngeal-chamber region  110 . This enables such-an elongate member to be inserted into the laryngeal-chamber region  110  without occupying the interior of the airway tube  22  which may obstruct air flow through the airway tube. Additionally, throughout insertion of such an elongate member through the port and the laryngeal-chamber region  110 , the elongate member is anterior of the internal-drain tube  115 . 
     In comparison, if such an elongate member is inserted through the airway port  30  into the laryngeal-chamber region  110 , upon entry into the laryngeal-chamber region, the distal end of the elongate member lies substantially parallel to the internal-drain tube  115 . Accordingly, shortly after entry into the laryngeal-chamber region  110 , upon continued insertion into the laryngeal-chamber region, the insertion direction of such an elongate member must normally be sharply changed to enable entry into or viewing of the larynx  37  or bronchial tree. Additionally, insertion of such an elongate member through the airway port  30  into the laryngeal-chamber region  110  results in the elongate member being laterally offset from the sagittal plane  97  since the airway port is so offset from the sagittal plane. Such an elongate member must therefore be suitably steered if it is to be aligned in the sagittal plane  97 . Aligning such an elongate member in the sagittal plane  97  may facilitate its further insertion through the larynx  37  into the trachea. 
     The elongate tube joint  92  is formed on the pharyngeal-side  60  and extends posteriorly and proximally relative to the oval portion  87 . The tube joint  92  includes a longitudinal passageway  105  extending from its proximal end  107  distally to the concave laryngeal-side  81 . The passageway  105  has a double-barrelled cross section for supporting the airway tube  22  and evacuation tube  80 , described more fully herein below. The longitudinal central axis of the passageway  105  is contained in the sagittal plane  97  and inclined posteriority at an angle of approximately 30 degrees relative to the major plane  75 , as viewed in the sagittal plane  97 . 
     A strap  200  is moulded to the external anterior surface of the proximal tube joint  92  in arching relation over the mid-line groove  102 . The moulding of the strap  200  onto the anterior surface of the proximal portion of the tube joint  92  defines an introducer tool slot  201 . The distal edge of the strap  200  has an internal curved edge  203  against which abuts the posterior bulge  100  (which is an extension of the main cuff  40 ), as shown in  FIGS. 4 ,  6  and  16 . The introducer tool slot  201  and curved edge  203  avoid becoming dirt trap because when the main-cuff  40  is deflated, the posterior bulge  100  (i.e., the main-cuff extension) pulls away from the strap  200 , thus avoiding the formation of a blind pocket which could be a dirt trap. 
       FIGS. 18 and 19  show a second embodiment of the backplate  27   b . Parts in  FIGS. 18 and 19  having corresponding parts in  FIGS. 16 and 17  have the same reference numeral with the addition of suffix b. The backplate  27   b  is similar to the backplate  27  illustrated in  FIGS. 16 and 17  except that the backplate  27   b  does not a strap similar to strap  100 . 
     The evacuation tube  80  comprises an internal-drain tube  115  extending between the tube joint  92  and the distal region  45  of the main-cuff  40  on the laryngeal-side  81  of the backplate  27 . The internal-drain tube  115  longitudinally traverses the interior of the distal region  45  of the main-cuff  40  in sealed relation therewith for operative engagement and communication with the inlet of the oesophagus  57 . The internal-drain tube  115  is anterior relative to the seam  85  of the main-cuff  40  such that the seam is disposed between the internal-drain tube and the distal end of the oval portion  87 . 
     The internal-drain tube  115  therefore pierces the distal region  45  at the proximal crotch-region  117  and the longitudinally opposing distal crotch-region  120 , both of which are portions of the distal region  45 . The edges of the main-cuff  40  in the crotch-regions  117 ,  120  surrounding the internal-drain tube  115  are hermetically sealed to the tube such that the enclosure of the main-cuff  40  is defined in part by the external cylindrical surface of the internal-drain tube. 
     The internal-drain tube  115  terminates in an oblique distal orifice  123  opening out on the anterior distal aspect of the distal region  45  of the main-cuff  40 . The oblique distal orifice  123  results in partial flattening of the distal region  45  such that the flattening is in a transverse plane inclined relative to the major plane  75  by an angle a of preferably approximately 45 to 50 degrees when main-cuff  40  is inflated, as shown in  FIG. 14 . When the main-cuff  40  is deflated, angle a is preferably approximately 40 to 45 degrees. In adult sizes of the LMA-device  20 , the surface area of the distal region  45  removed to accommodate the orifice  123  is approximately 1 square centimeter which is therefore no longer available to contribute to expansion of the main-cuff  40  when the main-cuff is inflated for sealing around the laryngeal inlet  67 . Accordingly, to prevent inspired gas leakage across the distal region  45  resulting from insufficient local expansion of the main-cuff  40 , additional circumferential area of the anterior surface of the distal region may be required for sealing. This may be provided by inversion of the anterior-facing lip  127  of the distal region  45  surrounding the orifice  123  resulting from longitudinal withdrawal of the intra-cuff portion  130  of internal-drain tube  115  approximately 3.5 millimeters relative to the plane containing the distal end of the distal region  45  of the main-cuff  40 . This inversion produces a corresponding lateral bulging of the distal region  45  around the orifice  123 . The anterior position of the distal orifice  123  ensures less compressive force resulting from the fluid pressure inside the main-cuff  40  on the intra-cuff portion  130  in the anterior-posterior direction, thus compensating for anterior-posterior compression from anatomical structures in the throat  32  so that the internal-drain tube  115  is subject to approximately equal compressive forces laterally and anterior-posteriorly, hence avoiding collapse. 
     The part of the intra-cuff portion  130  containing the distal orifice  123  has a longitudinal central axis inclined relative to the plane containing the distal orifice by an angle .gamma. of preferably 60 degrees, and inclined relative to the major plane  75  by an angle Δ of preferably 20 degrees. The longitudinal central axis of the intra-cuff portion  130  is contained in the sagittal plane  97 . 
     The distal orifice  123  has diametrically opposed posterior and anterior apexes  135 ,  137 . The distal orifice  123  is contained in a transverse elliptical plane preferably inclined by an angle β, which is preferably 40 degrees, relative to the major plane  75 , as shown in  FIG. 14 . The inclination of the distal orifice  123  is such that the posterior apex  135  is offset distally relative to the anterior apex  137  along the longitudinal axis of the portion of the internal-drain tube  115  containing the distal orifice  123 . 
     Integral with the external anterior surface of the intra-cuff portion  130  adjacent to the distal orifice  123  is a semicircular transverse shoulder  142 , as shown in  FIG. 9 . The anterior and the adjacent lateral portions of the distal edge of the distal region  45  of the main-cuff  40  are bonded to the proximal surface of the shoulder  142 . The posterior and remaining lateral portions of the distal edge of the distal region  45  are bonded to the unshouldered external surface adjacent to the distal orifice  123 . 
     The lateral termination of each end of the shoulder  142  facilitates collapse of the distal orifice  123  in the major plane  75  when the main-cuff  40  is deflated since the un-reinforced posterior portion of the intra-cuff portion  130  is able to collapse more readily when the pressure inside the main-cuff  40  is reduced (i.e., negative pressure is applied to the main-cuff). Also, by limiting the circumferential dimension of the shoulder  142 , its peripheral length which must be deflected is reduced. In contrast, if the shoulder  142  extended posteriorly a sufficient amount such that it traversed the major plane  75 , the portions of the shoulder that traversed the major plane would require closure to close distal orifice upon deflation of the main-cuff  40 . Such closure of such a shoulder would require significantly more force than required to flatten the shoulder  142 , shown in  FIG. 14 . Such increased force may require stronger material for the main-cuff  40  and application of higher deflation vacuums to the main-cuff. 
     The distal orifice  123  is withdrawn proximally relative to the distal region  45  of the main-cuff  40  resulting in the portion of the distal region  45  adjacent to the distal orifice  123  being invaginated when the main-cuff  40  is inflated, as shown in  FIG. 14 . The bonding of the distal end of the distal region  45  to the distal surface of the shoulder  142  results in the transversely-arcuate inverted anterior-facing lip  127  of the invaginated surface having the greatest radial bulge. The transversely-arcuate lateral portions  145 ,  147  of the invaginated surface have the next largest radial bulge with the transversely-arcuate posterior portion  150  having the least radial bulge. The opposed lateral portions  145 ,  147  are symmetrical about the sagittal plane  97  of the main-cuff  40 . 
     The portion of the internal-drain tube  115  longitudinally traversing the interior of the distal region  45  of the main-cuff  40  defines intra-cuff portion  130 . The outer surface of the intra-cuff portion  130  has at least one circumferential strengthening rib  152  proximal of the shoulder  142  to resist radial collapse of the intra-cuff portion  130  by internally directed radial forces resulting from the fluid pressure within the main-cuff  40 . The rib  152  is contained in a transverse elliptical plane preferably inclined at an angle θ, preferably of 60 degrees and equal to angle γ, relative to the longitudinal axis of the intra-cuff portion  130 , as shown in  FIG. 14 . The inclination of the rib  152  enables its posterior pivoting about its posterior apex during deflation of the distal region  45  to facilitate flattening of the main-cuff  40 . 
     The portion of the internal-drain tube  115  proximal of the intra-cuff portion  130  is laterally offset from the sagittal plane  97 , as shown in  FIGS. 3 and 7 . The portion of the internal-drain tube  115  where it emerges from the proximal crotch-region  117  and extends to the well  95  is received in a groove  157  formed in the oval portion  87 , as shown in  FIG. 7 . The groove  157  is defined laterally by fillets  160  which laterally abut the internal-drain tube  115 . As much as 50% of the posterior portion of the cross-sectional area of the internal-drain tube  115  may be contained in the distal portion of the groove  157 , except where its circumference is free posteriority, i.e., where it runs over the well  95 . In one size of the main-cuff  40 , the longitudinal dimension of the groove  157  is 2.5 centimeters. The internal-drain tube  115  is welded to the groove  157 . 
     The fillets  160  resist anterior deflection of the oval portion  87  since the fillets provide increased surface area for the weld between the internal-drain tube  115  and oval portion. This additional resistance compensates for the reduced resistance resulting from a reduction in the anterior-posterior thickness of the part of the oval portion  87  defining the base of the groove  157 . Such reduced anterior-posterior thickness is desirable to increase the anterior-posterior dimension a between the anterior surface of the main-cuff  40 , and the portion of the internal-drain tube  115  between the proximal crotch region  117  and well  95 , shown in  FIG. 8 , especially at the location of dimension b, shown in  FIGS. 9 and 10 , which should have a depth of at least 10 millimeters in adult sizes, described further herein below. 
       FIG. 12  illustrates a second embodiment of the LMA-device  20   a  in which the flexible panel  62   a  is tethered to the backplate  27   a . The parts in  FIG. 12  having corresponding parts in  FIGS. 1 to 11  have the same reference numeral with the addition of suffix a. Tethering of the panel  62   a  to the backplate  27   a  provides additional resistance to anterior inversion of the oval portion  87   a . This enables further reduction in the anterior-posterior thickness of the part of the oval portion  87   a  defining the base of the groove  157   a . As discussed above, such reduced anterior-posterior thickness is desirable to increase the anterior-posterior dimension, corresponding to the dimension a in  FIG. 8 . 
     A longitudinal portion of the internal-drain tube  115  extends over well  95 , as shown in  FIG. 8 . The anterior-inclination of the portions of the laryngeal-side  81  proximal and distal of the well  95 , described herein above, anteriorly props the portion of the internal-drain tube  115  extending over the well to increase the anterior-posterior clearance between the internal-drain tube and base of the well. The internal-drain tube  115  arches over the well  95  defining a slight posterior curve and simultaneously curving laterally to its insertion in the tube joint  92 . 
     The evacuation tube  80  includes an external-drain tube  165  having a distal end  167  connected in end-to-end relation to the proximal end  170  of the internal drain-tube  115 . The joint between the internal and external-drain tubes  115 ,  165  is located where the tube joint  92  opens into laryngeal-chamber region  110 , as shown in  FIGS. 8 and 15 . 
     The inner diameters of the internal-drain tube  115  and external-drain tube  165  are the same. The outer diameter of the internal-drain tube  115  is less than the outer diameter of the external-drain tube  165 . The distal end  167  of the external-drain tube  165  has an internal countersunk portion  172  defined by a bevelled internal axial wall, as shown in  FIG. 15 . The outer diameter of the countersunk portion  172  is greater than the outer diameter of the internal-drain tube  115 . The proximal end  170  of the internal-drain tube  115  abuts the countersunk portion  172  resulting in coaxial self-alignment of the central longitudinal axes of the distal and proximal ends  167 ,  170 . 
     As shown in  FIGS. 8 and 15 , the external-drain tube  165  is supported in the cylindrical drain barrel  175  of the double-barrelled passageway  105  which is longitudinally offset from the well  95  at an angle of approximately 9 degrees. The internal-drain tube  115  is thereby disposed anteriorly of the well  95  and is also offset at 9 degrees from the major axis of drain barrel  175  to increase the lateral clearance. 
     The evacuation tube  80  is preferably moulded to the backplate  27 . Alternatively, for making a prototype, assembly of the evacuation tube  80  to the backplate  27  may be by first welding the distal portion of the internal-drain tube  115  into the distal region  45  of the main-cuff  40 . Before connecting the proximal end of the internal-drain tube  115  to tube joint  92 , the main-cuff  40  is welded to the backplate  27 . The external-drain tube  165  is then welded into the drain barrel  162  of the tube joint  92 , for example, by an adhesive  173 . Hardening of these welds effectively clamps and fixes the distance between the distal end of the proximal crotch-region  117  of the main-cuff  40  and the distal end  167  of the external-drain tube  165 . The internal-drain tube  115  is cut, as needed, such that it is slightly longer than this distance. The proximal end  170  of the internal-drain tube  115  is then inserted into the countersunk portion  172  of the external-drain tube  165  with the countersunk portion resulting in coaxial self-alignment of the longitudinal central axes of the distal and proximal ends  167 ,  170 . The internal-drain tube  115  is then welded to the tube joint  92 , for example, by an adhesive  174 . 
     The slightly longer length of the internal-drain tube  115  relative to the distance between the proximal crotch-region  117  and distal end  167  results in a slight longitudinal compression of the internal-drain tube causing lateral curvature of it away from the adjacent side-wall  177  of the backplate  27 . Lateral curvature of the internal-drain tube  115  away from the adjacent side-wall  177  increases the lateral clearance between them, reducing the likelihood of dirt collecting between them. 
     As shown in  FIGS. 3 and 7 , the airway tube  22  is supported in the cylindrical airway barrel  180  of the double-barrelled passageway  105  in communication with the airway port  30  defined by the opening of the airway barrel  180  into the laryngeal-side  81 . Such communication provides a flowpath between the airway tube  22  and laryngeal-chamber region  110 . The airway tube  22  is connected to the tube joint  92  by welding using an adhesive or, alternatively, connected by high-pressure or temperature fusion. 
     The airway tube  22  and external-drain tube  165  are welded together in side-by-side tangential relation, as shown in  FIG. 2 . The welding is accomplished by depositing adhesive in one or both of the crevices defined by the outer surfaces of the tubes  22 ,  165  adjoining the line of tangential contact between them. The adhesive preferably extends longitudinally from the tube-joint  92  proximally for approximately 4¼ inches. Alternatively, the tubes  22 ,  165  may be connected together by high pressure or temperature fusion. Also, the tubes  22 ,  165  may be manufactured by simultaneous extrusion. Additionally, the tubes  22 ,  165  may remain separate for certain clinical applications, e.g., operations on the tongue  202  in the mid-line or other mid-line structures in the pharynx  197 . 
     The airway tube  22  and external-drain tube  165  are inserted through a bite-plate  176  comprising a sleeve which is telescopically fitted around the tubes  165 ,  176 , as shown in  FIG. 2 . The bite-plate  176  is positioned longitudinally on the tubes  22 ,  165  such that, when the LMA-device  20  is completely inserted into the throat  32  and pharynx  197 , the bite-plate is positioned between the upper and lower teeth, described further herein below. 
     In embodiments in which the airway tube  22  is bonded to the external-drain tube  165 , the tubes  22 ,  165  are bent away from one another, laterally at the proximal extent of the adhesive to facilitate routing of the airway tube to a ventilating apparatus (not shown) and the external-drain tube  165  to a suction-apparatus (not shown), if required. The separation of the airway tube  22  and external-drain tube  165  is achieved by placing a sleeve  182  on the airway tube to cover the proximal 3 centimeters of the airway tube. The sleeve  182  is proximally oriented relative to the bite-plate  176 . Connected to the distal end of the sleeve  182  is a triangular wedge  185  oriented toward the external-drain tube  165  to force the softer external-drain tube to incline away from the airway tube  22  by an angle C, preferably approximately 15 degrees. The sleeve  182  and wedge  185  are a single moulding and are welded to the airway tube  22 . Additionally, the wedge  185  is welded to the external-drain tube  165 . The sleeve  182  also stiffens the proximal end of the airway tube  22  to reduce the likelihood of kinking at its attachment to the ventilating apparatus (not shown). 
     The portions of the airway tube  22  and external-drain tube  165  in side-by-side tangential relation each have the same outer diameter. The inner diameter of this portion of the airway tube  22  is greater than the inner diameter of the adjoining portion of the external-drain tube  165 . These portions of the airway tube  22  and external-drain tube  165  each have approximately the same stiffness and resistance to longitudinal bending. A metallic cylindrically helical wire  190  is provided between inner and outer surfaces of the airway tube  22  in coaxial relation therewith to increase the kink resistance of the thinner-wall airway tube. The kink resistance of this portion of the airway tube  22  may be further increased by forming it of a material having a harder durometer of silicone. It may also be possible for the chemical compositions of these portions of the tubes  22 ,  165  to be approximately the same if, for example, the helical wire  190  sufficiently increases the stiffness of airway tube. 
     A hard plastic or polycarbonate cylindrical fitting (not shown) is inserted in the end of the airway tube  22  proximal of the triangular wedge  185 . The fitting is inserted into the airway tube  22 , and has a radial flange which abuts the proximal end of the airway tube to longitudinally limit the insertion of the fitting into the airway tube. The fitting facilitates connection to the ventilating apparatus (not shown). 
     In use, an inflation/deflation device is actuated to apply a vacuum, via the tube  55 , to the main-cuff  40  sufficient to fully deflate it prior to insertion of the main-cuff through the mouth of the patient. Such a vacuum extends to the space enclosed by the flexible panel  62  and backplate  27 , via the channel  90  in the main-cuff  40 , deflating the back-cuff  65  to collapse it onto the pharyngeal-side  60  of the backplate  27  and posterior surface of the main-cuff. 
     The main-cuff  40  is preferably deflated into a predetermined shape by using the forming tool disclosed in U.S. Pat. No. 5,711,293, the entire disclosure of which is hereby incorporated by reference herein. 
     The flattened sheet, comprising fully deflated the main-cuff  40 , backplate  27  and internal-drain tube  115 , is passed easily through the mouth  25  of the patient because of the reduced compressible antero-posterior dimension of the part of the LMA-device  20  having the largest anterior-posterior dimension, i.e., the generally proximal region  42  of the main-cuff  40  and the heel  50 . This reduced compressible antero-posterior dimension results from the recess  47  of the heel  50 . The deflated main-cuff  40 , backplate  27  and internal-drain tube  115  is pressed against the hard and soft palates  192 ,  195  as it is pushed inwardly, resulting in the deflated main-cuff being guided distally by the soft palate onto the posterior wall of the pharynx  197 . Such deflection of the main-cuff  40  is normally only reliably achieved if the total stiffness of the LMA-device  20  is within certain predetermined limits. 
     The main-cuff  40  is preferably urged through the throat  32  by placement of either the operator&#39;s index finger or an insertion tool inserted into the strap  200  against the heel  50 , because the side-by-side airway tube  22  and internal-drain tube  115  are normally not sufficiently stiff to be used as a rod to direct the main-cuff through the throat. 
     The main-cuff  40  is preferably positioned in the throat  32  by inserting the a sufficient length of the index finger of the operator through the introducer tool slot  201  such that the finger is placed on the mid-line groove  102  of the tube joint  92  and the end of the finger abuts the heel  50 , as shown in  FIG. 16 . Inserting the finger through the introducer tool slot  201  enables the finger to be partly wedged into the strap  100  to secure the index finger to the mid-line groove  102 . Placement of the index finger on the mid-line groove  102  of the tube joint  92  and against the heel  50  assists in locating and stabilizing the finger against the proximal region  42  of the main-cuff  40 . This reduces the risk of finger slippage from its intended position on the backplate  27  due to the presence of slippery secretions in the mouth  25  and/or the application of lubricant, to assist smooth passage of the LMA-device  20  during its insertion into the patient and to avoid the risk of injury to the patient or of damage to the LMA-device. During such insertion, the proximal region  42  of the main-cuff  40  provides a fulcrum. 
     An alternative and equally preferable way to position the main-cuff  40  in the throat  32  is by an introducer tool (not shown) including a relatively rigid elongate member having a distal end adapted for removable keyed engagement with the heel  50  and strap  200  adjacent to the tube joint  92  for insertional guidance of the main-cuff  40 . During such insertion, as with placement of the operator&#39;s finger against the heel  50 , the proximal region  42  of the main-cuff  40  provides a fulcrum. The introducer tool and LMA-device  20  may both be included in a kit. 
     Preferably, the deflated main-cuff  40  and backplate  27  are sufficiently flexible that they do not overcome the resistance provided by the soft palate  195 . The main-cuff  40  and backplate  27  are preferably flexible similar to a palette knife such that, when the main-cuff and backplate are urged or tensed against the soft palate  195 , the distal region  45  is deflected downward by the soft palate rather than being forcibly driven into it, which may bruise the soft palate. Also preferable is for the deflated main-cuff  40  to itself bend smoothly around (i.e., in the shape of) an arc  196 , as shown in  FIG. 21 , also similar to a palette knife. 
     Further, the deflated main-cuff  40  and backplate  27  resist kinking. Kinking results in the main-cuff  40  and backplate  27 , during their insertion through the throat  32 , collapsing on the tongue  202  rather than arching over it. To avoid kinking, a specific overall stiffness and long-axis gradation of stiffness in the delated main-cuff  40  is required, which in turn depends on the shape of the backplate  27 . The primary factors or considerations to be balanced when designing the backplate  27  are (i) desirability of long-axis gradation of stiffness (i.e., linear tapering-off distally of resistance to flexure), (ii) adequate stiffness and appropriate architecture to prevent anterior herniation from fluid pressure within the inflated back-cuff  65 , and (iii) minimal thickness in the anterior-posterior dimension to reduce overall resistance to flexure. 
     The relative stiffness of the airway tube  22 , external-drain tube  165  and backplate  27  facilitate piloting and guiding of the substantially flattened, deflated main-cuff  40  to smoothly ride or track posterior contours of the throat  32  and pharynx  197  and to assure that the deflated main-cuff enters and locates immediately above the upper oesophageal sphincter  207  and adjacent to the laryngeal inlet  67 , as shown in  FIGS. 1 and 2 . 
     Additionally, the backplate  27 , internal-drain tube  115  and main-cuff  40  are sufficiently flexible to allow anterior and posterior deflection of the distal region  45  in the sagittal plane  97  when the main-cuff is fully deflated, as shown in  FIG. 21 . Such deflection further facilitates riding or tracking of the distal region  45  of the main-cuff  40  over the posterior contours of the throat  32  by allowing the distal region to deflect as necessary to conform to protrusions or recesses in the posterior surface of the throat. 
     The deflated main-cuff  40  further enters into its correct position opposite the laryngeal inlet  67  without colliding with anterior structures such as the posterior surface of the tongue  202 , epiglottis  35 , or arytenoids  205 . Insertion of the deflated main-cuff  40  is facilitated by forming the main-cuff  40  and attaching it to the backplate  27  such that the seam  85  abuts the backplate, as shown in  FIGS. 12 ,  13  and  14 . As a result, when the main-cuff  40  is fully deflated, the anterior surface of the main-cuff is uninterrupted by the seam  85 , i.e., the seam is buried between the backplate  27  and the deflated main-cuff. Accordingly, the likelihood is reduced of the anterior surface of the deflated main-cuff  40  scraping or catching on the anatomical structures of the throat  32 , such as the epiglottis  35  and arytenoids  205 . Further disclosure of insertion of the deflated main-cuff  40  through the throat  32  may be had by reference to U.S. Pat. No. 5,632,271, the entire disclosure of which is hereby incorporated by reference herein. 
     When the LMA-device  20  is fully inserted in the throat  32 , the side-by-side airway tube  22  and external-drain tube  165  extend proximally from the tube joint  92  in contacting relation with the soft palate  195 , and lie against the hard palate  192 , i.e., the roof of the mouth  25 . The tubes  22 ,  165  are spaced inwardly of the sides of the throat  32  to avoid damage to the lingual nerves. The tubes  22 ,  165  rest lightly against the posterior aspect of the upper teeth, usually close to parallel with the inner surface of the upper incisors, and emerge from the mouth  25  between the teeth. 
     The bite-plate  176  is positioned at the emergence of the tubes  22 ,  165  from the mouth  25  such that the bite-plate is disposed between the upper and lower teeth and the tubes. The teeth thereby directly contact the bite-plate  176 , rather than the tubes  22 ,  165 , to provide protection to the tubes. 
     When the main-cuff  40  is correctly positioned, the distal orifice  123  of the internal-drain tube  115  contacts the upper oesophageal sphincter  207  and lies posterior to the cricoid cartilage  210 . The bevelled distal region  45  of the main-cuff  40 , including the distal orifice  123  of the internal-drain tube  115 , forms a wedge-shape of approximately 45 degrees when the main-cuff  40  is deflated. This facilitates insertion of the main-cuff  40  and backplate  27  behind the cricoid cartilage  210  because such insertion requires the cricoid cartilage to be gently forced anteriorly to allow passage of the wedge-shaped distal region  45 , including the distal orifice  123 , behind it. Further disclosure of positioning the LMA-device  20  may be had by reference to U.S. Pat. No. 5,241,956, the entire disclosure of which is hereby incorporated by reference. 
     When the LMA-device  20  is completely inserted, the main-cuff  40  contacts the base of the hypo-pharynx  212  with the distal region  45  being wedged into the upper opening of the upper oesophageal sphincter  207 , a constriction which is however much too small to permit the LMA-device  20  to pass through it. Complete insertion of the LMA-device  20  is thereby detected by the operator as a resistance to insertion of the main-cuff  40  into the upper oesophageal sphincter  207 . The main-cuff  40  is then inflated with sufficient air, via the tube  55 , to obtain a seal against the laryngo-pharyngeal perimeter. The LMA-device  20 , when completely inserted in the pharynx  197 , lies in the sagittal plane  97 . 
     Inflation of the main-cuff  40  causes expansion of the distal region  45  enabling it to lie against and adapt to the pharynx  197  and hypo-pharynx  212 . Additionally, inflation of the main-cuff  40  causes the gas or fluid to flow into the space enclosed by the flexible panel  62  and backplate  27 , for example, via one or more ports in the main-cuff, resulting in inflation of the back-cuff  65 . Inflation of the back-cuff  65  initially causes engagement between the flexible panel  62  and posterior surface of the pharynx  197 . Further inflation of the back-cuff  65  urges the main-cuff  40  anteriorly to press it against the tissue surrounding the laryngeal inlet  67 . This tightens the sealing engagement between the main-cuff  40  and the tissue surrounding the laryngeal inlet  67 , thereby reducing leakage between such tissue and the main-cuff. The sealing engagement is further improved by provision of the increased anterior-posterior space between the oval portion  87  of the backplate  27  and the anterior surface of the main-cuff  40 , permitting accommodation of the posteriorly bulging posterior surface of the cricoid cartilage  210  which is located distally relative to the laryngeal inlet  67 . 
     If the back-cuff  65  is overinflated, the oval portion  87  may bulge anteriorly outward resulting in anterior displacement of the internal-drain tube  115  relative to the main-cuff  40 , and loss of the advantageously increased anterior-posterior space between the oval portion  87  and the anterior surface of the main-cuff  40 , described above. The anterior-posterior dimension a between the anterior tangency of the internal-drain tube  115  and a plane containing the anterior surface of the main-cuff  40 , shown in  FIG. 8 , must not decrease below a minimum level since such may result in the internal-drain tube undesirably impinging against anatomical structures of the throat  32  normally present in the laryngeal-chamber region  110 . For example, if the main-cuff  40  is a standard adult size and is inflated to 40 millimeters Hg (mercury), at a point b contained in the sagittal plane  97  and located 40 millimeters distally from the distal end of the proximal region  42  of the main-cuff  40 , shown in  FIG. 9 , the minimum anterior-posterior distance b must not approach 8 millimeters, is preferably at least 10 millimeters and ideally at least 10.7 millimeters. 
     The transversely arched profile, degree of hardness, and increased anterior-posterior thickness of the distal portion of the oval portion  87  are all factors chosen to offer adequate resistance to such anterior bulging thereby limiting such resulting anterior displacement of the internal-drain tube  115  near the distal region  45  of the main-cuff  40  where the internal-drain tube is nearest to the anterior surface of the main-cuff. Fillets  160 ,  160   a  and tethered panel  62   a , shown in  FIGS. 11 and 12 , also limit anterior displacement of the internal-drain tube  115  relative to the main-cuff  40 . Anterior-posterior dimension a, shown in  FIG. 8 , should be maintained above a minimum amount to avoid anterior displacement of the arytenoids  205  which may obstruct flow of gases through the larynx  37 , and to avoid anterior displacement of anatomical structures relative to the main-cuff  40  which may reduce the tightness of the seal between the main-cuff and the tissues surrounding the laryngeal inlet  67 . Additionally, the backplate  27  is preferably sufficiently flexible to deflect in the anterior-posterior direction during insertion into the throat  32  to follow its contours, e.g., to bend around the soft palate  195 . 
     The backplate  27  is reinforced because the prior LMA-devices (such as is disclosed in U.S. Pat. No. 4,509,514) did not have a back-cuff, such as back-cuff  65 . The back cuff  65  of the LMA-device  20  causes pressure to be applied to the oval portion  87  of the backplate  27 , which may cause the oval portion to herniate anteriorly. The backplate  27  must therefore be designed to resist such herniation, preferably to pressures within back-cuff  65  of up to 100 centimeters of water. Techniques for preventing such herniation of the backplate  27  include arching the backplate  27  such that it has a concavity facing anteriorly, making the backplate of a high durometer silicone or other plastics material, thickening the backplate sufficiently to resist herniation (but not so much that it becomes too stiff to bend easily around the back of the tongue  202 ), and possibly also providing the backplate with a midline longitudinally running groove for accurately locating adhesive to weld to it the back cuff  65 . In addition, the back cuff  65  may be made of a thin elastomeric sheet material capable of considerable elongation in response to the pressure within it, resulting in minimal herniation of the backplate  27 . 
     The anteriorly facing laryngeal-chamber region  110  of the main-cuff  40  is wider than the transverse distance between the edges of the laryngeal inlet  67  as defined by the so-called aryepiglottic folds which bilaterally border the laryngeal inlet thus encouraging a sealing contact between the main-cuff and the pharyngeal tissues as well as the tissues bordering the laryngeal inlet. The main-cuff  40  is thus functionally a pharyngo-laryngeal mask airway forming an end-to-end seal against the larynx  37 . 
       FIGS. 23 and 24  illustrate a third embodiment of the LMA-device  20   c . Parts in  FIGS. 23 and 24  having corresponding parts in  FIGS. 1 to 22  have the same reference numeral with the addition of suffix c. The main-cuff  40   c  may have soft and yielding ridges (not shown) bilaterally disposed on the anteriorly-facing distal region  45   c  of the main-cuff which are suitably contoured to fill the anatomical grooves known as the pyriform fossae to increase the sealing efficacy of the main-cuff. The LMA-device  20   c  exploits the triangular cross-section of the grooves of the pyriform fossae which are roofed over and isolated by the anterior surface of the main-cuff  40   c  bilaterally. The entire length of the grooves of the pyriform fossae are covered by the main-cuff  40   c  such that a respective cavity is defined by each groove and the contiguous portion of the anterior surface of the main-cuff. Incorporation of one or more one-way valves  215 , such as a reed or duck-bill valve, in the anterior wall of the main-cuff  40   c  facing the grooves of the pyriform fossae permits the operator to evacuate residual gas from the cavities by anterior neck pressure so causing the low pressure in the cavities to pull or draw the main-cuff anteriorly enhancing the seal. One-way valves  215  may be duck-bill valves of the type sold by Accusil® Incorporated of Merriville, Ind., U.S.A. 
     Attached to the interior surface of the anterior wall of the main-cuff  40   c  are respective cylindrical housings  217 , shown in  FIG. 24 , each surrounding a respective one of the one-way valves  215 . Deflation of the main-cuff  40   c  draws its posterior wall toward the housings  217  and one-way valves  215 , eventually causing the posterior wall to seat on the open posterior ends of the housings  217 , as illustrated by a portion of the posterior wall being shown in phantom line in  FIG. 24  in dashed lines. Seating of the posterior wall of the main-cuff  40  on the open posterior ends of the housings  217  hermetically seals the respective one-way valves  215  from the remainder of the interior of the main-cuff. Each of the one-way valves  215  thereby becomes isolated from the reduced pressure inside the main-cuff  40   c . This prevents the reduced pressure within the main-cuff  40   c  from drawing gases external of the main-cuff in the vicinity of the one-way valves  215  through the one-way valves into the main-cuff thereby enabling the reduced pressure inside the main-cuff to deflate it. 
     In an alternative embodiment (not shown), one-way valves  215  and their associated housings  217  may be replaced by ports or apertures, the ends of which within the main-cuff  40   c  are each connected to a tube also within the main-cuff. The tubes connected to the ports or apertures communicate via a tube or, less preferably, multiple tubes which extend through the wall of the main-cuff to a point outside of the main-cuff  40   c  such that the ports or apertures, and the tubes connected to them, are isolated from the interior of the main-cuff. A source of suction may then applied to the tube or tubes outside of the main-cuff  40   c  to evacuate residual gas from the cavities defined by each groove of the pyriform fossae and the contiguous portion of the anterior surface of the main-cuff. 
     The sealing efficacy of the main-cuff  40  may be further increased by an optional wedge-shaped crescent (not shown) in sealing contact with the anterior surface of substantially the proximal one-half of the main-cuff. 
     The epiglottis  35 , a leaf-like structure which normally projects proximally and posteriorly, is supported against the anterior surface of the internal-drain tube  115 . The internal-drain tube  115  thereby defines a stop to prevent the epiglottis  35  from interfering with communication between the airway tube  22 , via the airway port  30 , and the laryngeal inlet  67 . This creates adequate space in the laryngeal-chamber region  110  posterior to the epiglottis  35  for passage of gases between the airway port  30  and laryngeal inlet  67 . 
     Such passage of gases between the airway port  30  and laryngeal inlet  67  is mainly in the portion of the laryngeal-chamber region  110  lateral of the sagittal plane  97  and containing the airway port. If, however, the epiglottis  35  slides laterally from its propped position against the internal-drain tube  115  into the lateral portion of the laryngeal-chamber region  110  containing the airway port  30 , gas passage between the airway port and laryngeal inlet in this portion of the laryngeal-chamber region may be obstructed. If so, gases may circulate between the airway port  30  and laryngeal inlet  67  via the radial clearance between the internal-drain tube  115  and well  95 , and through the portion of the laryngeal-chamber region  110  laterally of the sagittal plane  97  offset from the airway port  30 . An alternative circulation flowpath is thereby provided to permit adequate and free gas communication between the airway tube  22  and laryngeal inlet  67 , while simultaneously preventing obstruction to such gas flow by the epiglottis  35 . The contour of the laryngeal-side  81  of the backplate  27  props the internal-drain tube  115  away from the laryngeal-side to facilitate sufficient radial clearance between the internal-drain tube  115  and well  95  thereby to provide the adequate and free gas communication between the airway tube  22  and laryngeal inlet  67 . 
     The oval portion  87  of the backplate  27  has a sufficiently large anterior-posterior depth to contain the internal-drain tube  115  such that the drain tube does not bear against other laryngeal structures and interfere with gas flow. 
     The well  95  also provides a route for drainage of secretions from the trachea, which may enter the laryngeal-chamber region  110  via the laryngeal inlet  67 . Such secretions normally collect in the well  95  since, when the LMA-device  20  is fully installed and the patient is supine, the laryngeal-side  81  of the backplate  27  faces upward. In the absence of the well  95 , such secretions would collect between the laryngeal-side  81  of the backplate  27  and internal-drain tube  115 . 
     The adequately-sized well  95  is provided behind the internal-drain tube  115  to allow gases or secretions to pass between the internal drain tube and the backplate  27 . This improves drainage of secretions emerging from the trachea  36  and improves gas exchange if there is any obstruction due to the epiglottis  35  falling into the laryngeal-chamber region  110  close to the distal-end  72  of the airway tube  22  adjacent to the airway port  30 . 
     Inflation of the main-cuff  40  causes expansion of the distal region  45  including the anterior-facing lip  127 , lateral portions  145 ,  147 , and posterior portion  150  of the invaginated end, as shown in  FIGS. 8 and 14 . The hermetic seal between the oblique distal orifice  123  of the internal-drain tube  115  and the distal region  45  of the main-cuff  40  obstructs communication between the oesophagus  57  and laryngeal-chamber region  110 . Accordingly, leakage, e.g., of contents from the oesophagus  57  into the laryngeal-chamber region  110 , and via the laryngeal inlet  67  into the trachea is obstructed. 
     The invagination and 45 degree angulation of the distal end of the main-cuff  40  reduces the likelihood of leakage between the distal orifice  123  of the internal-drain tube  115  and the laryngeal-chamber region  110  of the main-cuff  40  which may result from the expansion of the main-cuff being hampered at the narrower distal region  45  and distal end by the presence of the distal orifice. Also, the angle formed by the main-cuff  40  when deflated was sufficiently large to impede insertion of the LMA-device  20  to its correct location in the pharynx  197  opposite the laryngeal inlet  67 . The desired insertion characteristics are obtained by invagination by 3.5 millimeters (size 4) of the wall of the main-cuff  40  forming the anterior lip  127  of the distal orifice  123  produced an increased expandable area around the distal orifice of the internal-drain tube  115 , improving the seal and, by drawing proximally only the anterior lip  127 , sufficiently sharpening the angle of the distal tip of the deflated main-cuff. 
     The side-by-side bonded adjacency of the airway tube  22  and external-drain tube  165  conforms to the cross-sectional shape of the mouth  25  and throat  32  facilitating insertion into and displacement through the throat. The side-by-side adjacency of the airway tube  22  and external-drain tube  165  also reduces the likelihood of kinking when they bend. 
     After positioning the main-cuff  40  opposite the laryngeal inlet  67  as described herein above, the ventilating apparatus (not shown) is actuated, as needed, to provide anesthesia gas to the trachea, via the laryngeal inlet, through the airway tube  22 . 
     The evacuation tube  80  has the following functions: 
     (i) the evacuation tube  80  allows gases to be administered to the lungs through the airway tube  22  under positive pressure without the risk of inflating the stomach, via the upper oesophageal sphincter  207 , since gases escaping from the laryngeal-chamber region  110  between the main-cuff  40  and the tissues surrounding the laryngeal inlet  67  into the hypo-pharynx  212  will be ducted out through the evacuation tube instead of being forced through the upper oesophageal sphincter  207  into the oesophagus  57 , the latter of which may occur with other known LMA-devices such as is disclosed in U.S. Pat. No. 4,509,514 which is hereby incorporated by reference herein; 
     (ii) conversely, if there is no evidence of gases being ducted through the evacuation tube  80  during positive pressure ventilation through the airway tube  22 , this indicates proper positioning of the main-cuff  40  with its distal end of the distal region  45  pressed into the base of the hypo-pharynx  212 . The evacuation tube  80  thus provides monitoring of correct placement of the LMA-device  20 ; 
     (iii) In the event of unexpected regurgitation though the upper oesophageal sphincter  207 , gastric contents are likely to follow the path of least resistance and enter into the evacuation tube  80  through the oblique orifice  123  rather than the larynx  37  via the laryngeal inlet  67 , the latter of which may occur with other known LMA-devices such as is disclosed in U.S. Pat. No. 4,509,514 which is hereby incorporated by reference herein; and 
     (iv) If desired, a suction catheter (not shown), probe for monitoring temperature or other parameter (not shown), or endoscope (not shown) may be inserted through the evacuation tube  80  provided the outer diameter of any such inserted device is less than the internal diameter of the evacuation tube. 
     An additional drain tube (not shown) may also be inserted though the airway tube  22  in a distal direction to emerge through the airway port  30  adjacent to the well  95 . A suction may be applied to such additional drain tube to remove secretions which may collect in the well  95 . The different inner diameters of the airway tube  22  and external-drain tube  165  facilitate their respective identifications by the operator so to facilitate insertion into the proper tube of such additional drain-tubes or endoscope. 
     The internal- and external-drain tubes  115 ,  165  have different external but the same internal diameters because the external-drain tube must be soft in order to bend around the tongue  202  without exerting undue pressure on it. For example, a disadvantage of the airway tube of the LMA-device disclosed in U.S. Pat. No. 4,509,514 is that it may be too stiff. If the external-drain tube  165  is too soft, however, it may kink unless it has a sufficient wall-thickness. The airway tube  22  must be of maximum internal diameter for optimal gas flow through it but of minimum outside diameter to reduce its cross-sectional area and consequent bulk. The resulting outer diameter of the airway tube  22 , about 11 millimeters (for #4), is therefore applied to the outer diameter of the external-drain tube  165 . The tubes  22 ,  165  therefore have the same or similar outer diameter, but for different reasons. 
     The portion of the internal-drain tube  115  contained in the laryngeal-chamber region  110 , however, preferably also has a reduced outer diameter to prevent it from interfering with free passage of gases within the laryngeal-chamber region. Additionally, the inner diameter of the internal-drain tube  115  is the same as the inner diameter of the external-drain tube  165  because if the inner diameter of the internal-drain tube is less than the inner diameter of the external-drain tube, the clinician will not know if a catheter inserted through the external-drain tube from outside the mouth will pass through the internal-drain tube. If the inner diameter of the internal-drain tube  115  is less than the inner diameter of the external-drain tube  165 , then a catheter just able to pass through the external-drain tube (e.g., the catheter having an outer cross-sectional area which is slightly smaller than that of the external-drain tube) will become obstructed when it reaches the internal-drain tube having the narrower internal cross-section. 
     Conversely, if the inner diameter of the internal-drain tube  115  is larger than the inner diameter of the external-drain tube  165 , then the outer diameter of the internal-drain tube must be correspondingly larger resulting in the internal-drain tube having a larger outer cross-sectional area thereby occupying additional space in the laryngeal-chamber region  110  (free space within the laryngeal-chamber region is precious). The additional internal cross-sectional area of the internal-drain tube  115  resulting from its larger inner diameter would, however, limited use since, for example, the gastric flow volume through the internal-drain tube would be limited by the smaller internal cross-sectional area of the external-drain tube  165 . 
     Anatomical Structures 
     Ary-epiglottic folds—wings of tissue joining the arytenoid cartilages  205  to each side of the epiglottis  35 . 
     Arytenoid Cartilages  205 —a pair of pyramid-shaped cartilages bordering the posterior rim of the laryngeal inlet  67 . Arytenoid cartilages  205  are attached anteriorly to the vocal cords which they open, close, lengthen and shorten by rotation and sliding actions, pulled by the laryngeal muscles. The most important of the arytenoid cartilages  205  is the posterior crico-arytenoid muscle, which draws the vocal cords open to permit air to enter and leave the lungs. 
     Cervical vertebrae—the neck bones, of which there are seven counting from above downwards. The sixth vertebral body lies opposite the cricoid cartilage  210  and the distal tip of the LMA-device  20  lies between the two when correctly inserted. 
     Constrictor muscles—three cylinders of muscle stacked within each other like plastic cups surround the interior space of the pharynx  197  and act sequentially to squeeze swallowed food into the oesophagus  57 . The lower pharyngeal constrictor muscle is the one which mostly wraps around the inserted LMA-device  20 . The lowest part of this muscle (most distal part) forms a complete ring and defines the upper oesophageal sphincter  207 , also known as the crico-pharyngeus muscle. 
     Cricoid cartilage  210 —a ring of cartilage which acts as the container or chamber of the larynx  37 . Cricoid cartilage  210  is attached distally to the trachea or wind-pipe  36 . From the lateral sides of the cricoid cartilage  210 , the membrane forming the vocal cords stretches upwards and medially. Proximally, the thyroid cartilage surrounds the cricoid cartilage  210  but overlaps it on either side postero-laterally. Posteriorly, the broad flat surface (lamina) of the cricoid cartilage  210  carries the paired posterior crico-arytenoid muscles, which are separated in the mid-line by a ridge. There is normally no space between the muscle-covered lamina an the posterior wall of the pharynx  197 , so when the LMA-device  20  enters this area of the pharynx, the LMA-device  20  must squeeze in between these two normally contiguous surfaces. Hence the need to make the deflated LMA-device  20  form a suitable wedge-shape with sufficient resilience to slip in behind (posterior to) the cricoid  210 . The part of the internal-drain tube  115  which is enclosed by the distal region  45  of main-cuff  40  of the LMA-device  20  lies immediately posterior to the mid-line ridge on the back of the cricoid cartilage  210 . Were the LMA-device  20  to lie to one or other side, it might compress one or other of the vitally important posterior crico-arytenoid muscles. 
     Cricopharingeus muscle—same as upper esophageal sphincter  207 . Part of the inferior constrictor muscle of the pharynx  197 . 
     Epiglottis  35 —a fibro-elastic cartilage often described as leaf-shaped, whose pointed end is firmly attached to the posterior surface of the front of the thyroid cartilage and whose lateral borders are suspended between the ary-epiglottic folds, so that its free posterior surface projects proximally and posteriorly. This free posterior surface acts like a shield preventing food entering the glottis but can also cause obstruction to air-flow especially when the pharyngeal space sags inwardly as surrounding muscles weaken during anaesthesia. If the space available inside the LMA-device  20  is inadequate, the epiglottis  35  potentially causes obstruction, particularly if it is large and floppy as may be the case in elderly males. The epiglottis  35  may be downfolded over the laryngeal vestibule if the distal tip of main-cuff  40  catches it and flips it downwards during insertion. Correct deflation and insertion of the LMA-device  20  minimise this risk, as does a good design permitting the optimal wedge-shape of the deflated LMA-device. 
     Oesophagus  57 —muscular tube which is normally closed, unlike the trachea  36  which lies immediately anterior to it. The muscular coat is thickened to form the upper oesophageal sphincter  207  and lower oesophageal sphincter. Stimulating the upper oesophageal sphincter  207  excessively by insertion of a bulky device or inflation of the LMA-device  20  to too high a pressure may cause the upper oesophageal sphincter  207  and lower oesophageal sphincter to open reflexively, making regurgitation of gastric contents more likely. Also, the esophageal muscles tend to relax during anaesthesia, so if there is any obstruction to inspiration, as caused for example by closure of the glottis or a misplaced LMA-device  20 , the chest movement of inspiration may cause such a high negative pressure within the chest cavity that the thin-walled oesophagus  57  is literally sucked open, encouraging fluids to be drawn up into it from the stomach. A correctly placed LMA-device  20  with a hole in the distal end, e.g., distal orifice  123 , communicating with the oesophagus  57  may prevent this cycle of events from occurring, since it permits air to be drawn into the oesophagus from above. 
     Glottis—the constriction of the airway tube  22  which occurs in the region of the vocal cords. The larynx  37  is the structure which surrounds and controls the movements and shape of the glottic opening. 
     Hard Palate  192 —the dome shaped bony vault which arches over the upper surface of the tongue  202 . The soft palate  195  is attached to it posteriorly and it stretches down to the dental arcades anteriorly and laterally. The anterior surface of the hard palate  192  blends with the gums and is innervated with nerves which trigger deglutition. Hence the importance of stimulating the anterior surface of the hard palate  192  when inserting the LMA-device  20 , which must be designed so that when deflated, its posterior surface forms a smooth broad sheet which imparts a soft, atraumatic feel to the surface of the hard palate  192 , stimulating the acceptance of the LMA-device  20  by triggering deglutition reflexes rather than rejection of the LMA-device, e.g., triggering vomiting reflexes. 
     Hyoid bone—a semicircular ring of bone vital to the mechanical 0.5 functions of swallowing, including opening of the mouth  25 . The hyoid bone lies above, i.e., proximal to, the thyroid cartilage and is attached above to the base of the tongue  202 , the front of the mandible and the base of the skull. The lower part of the hyoid bone is attached to the chest wall, the thyroid cartilage and the pharyngeal constrictor mechanism. The lateral wings of the hyoid bone press into the sides of the inflated main-cuff  40  of the LMA-device  20  near the proximal region  42  of the main-cuff  40 . The hypoglossal nerves pass near the inner ends of the hyoid bone, limiting the pressure which should be safely generated within the main-cuff  40  and the lateral expansion permissible in any device inflated in this region of the pharynx  197 . 
     Hypo-pharynx  212 —the region of the pharynx  197  lying behind the larynx  37 , and normally a closed sack at the level of the cricoid  210 . Adjacent to the base of hypo-pharynx  212  is the closed upper oesophageal sphincter  207 . The hypo-pharynx  212  is surrounded by the middle and lower constrictor muscles. Anteriorly, the distal region of the hypo-pharynx  212  is bordered by the posterior surface of the cricoid cartilage  210 . Also anteriorly, the proximal region of the hypo-pharynx  212  is bordered by the laryngeal vestibule. 
     Inter-arytenoid muscle—the muscle joining the two arytenoid cartilages  205  posteriorly and transversely, and proximal to the upper border of the cricoid cartilage  210 . The inter-arytenoid muscle consists of two parts, a straight transverse part and an “X” shaped part, both of which enable closure of the glottis. The distal end of the bowl which defines the posterior surface of the laryngeal-chamber region  110  of the LMA-device  20  must have adequate depth to avoid interfering with the inter-arytenoid muscle or with the arytenoid cartilages  205  which lie immediately anterior to it. Bruising of the overlying mucosal surface is common with improper insertion of the LMA-device  20 . 
     Larynx  37 —the apparatus responsible for protecting the entrance to the lungs from contamination and for vocalisation. The principle advantage of the LMA-device  20  is that it permits the larynx  37  to retain these functions, of which the first is the most important. Endotracheal intubation prevents effective coughing, which is an airways-cleaning mechanism vital to our survival. 
     Laryngeal inlet  67 —the rim of tissue surrounding the vestibule of the larynx  37 , consisting of the ary-epiglottic folds laterally, the tip of the epiglottis  35  proximally, and the arytenoids  205  and inter-arytenoid notch distally. 
     Laryngeal vestibule—a pocket of space above the vocal cords bounded laterally by the quadrate membranes, proximally by the epiglottis  35  and distally by the vocal cords. The distal tip of the LMA-device  20  may lodge in the laryngeal vestibule if the tip does not pass posterior to the arytenoids  205 . The laryngeal vestibule closes during swallowing, partly by the action of the ary-epiglottic muscle which acts like a sphincter and partly by the elevation of the larynx  37 . This closure of the laryngeal vestibule is observed when the LMA-device  20  is inserted prematurely. 
     Posterior crico-arytenoid muscle—the most important muscle of the larynx  37  because it acts to separate the vocal cords. The posterior crico-arytenoid muscle lies as a pair of muscles on the posterior surface of the cricoid lamina, which is the broad posterior region of the cricoid cartilage  210 . The distal tip of the LMA-device  20  presses against the cartilaginous ridge which separates the two muscles. Excessive pressure in the main-cuff  40  might drive blood out of the muscle, depriving it of the necessary oxygen to function, though such a complication has yet to be reported. 
     Pyriform fossae—gutters lying on either side of the entrance to the larynx  37 , bounded medially by the ary-epiglottic folds and laterally by the membranes stretching between the thyroid horns and the hyoid bones. 
     Quadrate membrane—the side-walls of the laryngeal vestibule. The quadrate membrane is bounded below by the rima glottidis, posteriorly by the ary-epiglottic folds, and anteriorly by the epiglottis  35 . 
     Rima glottidis—the space between the vocal cords. 
     Soft palate  195 —a muscular wedge of tissue extending posteriorly from the posterior edge of the hard palate  192 . The surfaces of the soft palate  195  converge to the mid-line posteriorly and distally to end in a mid-line triangular structure known as the uvula. The soft palate  195  acts like a bridge arching across the space separating the nasal cavity from the rest of the pharynx  197  and completely closes this gap during swallowing. Insertion of the LMA-device  20  relies on the resistance offered by the oral surface of the soft palate  195  to distally guide the distal tip of LMA-device  20 . If the deflated LMA-device  20  is too rigid, or incorrectly deflated, the soft palate  195  cannot guide it downwards, thereby impeding insertion of the LMA-device  20  into the pharynx  197 . 
     Thyroid cartilage—a shield-like structure whose lower border bilaterally overlaps the cricoid cartilage  210 . The thyroid cartilage has two posterior-directed horns, the lower of which articulates with the sides of the cricoid  210 , so that the whole structure can hinge on the cricoid in the manner of a visor of a helmet. This articulation produced by the crico-thyroid muscle serves to lengthen the vocal cords. The epiglottis  35  is attached to the anterior prominence of the thyroid, also known as the “Adam&#39;s Apple”, because it projects more sharply in males. 
     Trachea  36 —the wind-pipe, connected directly to the lower rim of the cricoid cartilage  210 . 
     Upper esophageal sphincter  207 —guards the entrance to the oesophagus  57 . The upper esophageal sphincter  207  is normally closed, even when the LMA-device  20  is in place and pressed into the upper surface of the upper esophageal sphincter. The upper esophageal sphincter  207  can open to approximately 1.5.times.1.0 centimeters. 
     Vocal cords—folds of tissue which represent the upper free borders of a membrane arising from the cricoid  210 , i.e., the crico-vocal membrane. The vocal cords vibrate, lengthen and shorten (for speech), adduct (to prevent soiling of the airway or trachea  36  and to allow coughing), and abduct (to admit air to the lungs). The crico-thyroid muscle lengthens the vocal cords by activating the visor-like hinging action of the crico-thyroid joint. The thyro-arytenoid muscle shortens the vocal cords by pulling the arytenoids  205  anteriorly. The vocalis muscle thickens the vocal cords to affect vibration frequency. The posterior crico-arytenoids abduct the vocal cords. The transverse arytenoids and lateral crico-arytenoids draw the arytenoids  205  together to close the vocal cords. 
     While the invention has been described by reference to certain preferred embodiments, it should be understood that numerous changes could be made within the spirit and scope of the inventive concept described. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that it have the full scope permitted by the language of the following claims.