# A Laboratory Manual of the Anatomy of the Rat/The Skeletal System

THE SKELETAL SYSTEM
Animals possess two kinds of skeleton. The exoskeleton, found among the Arthropods, covers the outside of the body, protecting the delicate internal organs, and assisting in locomotion by providing surfaces for the attachment of muscles. The disadvantage of an exoskeleton, like that of a crayfish, is that growth must be accompanied by periodic molts to permit of increase in body size. Vertebrates possess an endoskeleton, which is completely inclosed by the soft parts of the body. The endoskeleton supports the body and thereby preserves its general form. It facilitates locomotion by providing levers which are moved by the contraction of the muscles. The skull protects the delicate structures of the brain, while the ribs prevent injury to the organs of the thorax. One important advantage of the endoskeleton is that it does not interfere with growth, for the bones of the vertebrate body increase in length, width, and thickness as the soft parts grow.
The axial skeleton of a vertebrate comprises the skull, backbone, ribs, and sternum. The appendicular skeleton consists of the pectoral and pelvic girdles, the fore legs, hind legs, and feet.
THE SKULL
The skull of the higher vertebrates is a bony case completely surrounding and protecting the delicate tissues of the brain. It incloses and protects the olfactory organ, ​and the inner and middle ears. It offers some protection to the eyes also. The ventral part is modified to form the upper and lower jaws which are used in grasping, killing, and chewing the food. The skull may be divided into two parts, the cranium, which surrounds the brain, and the visceral skeleton. The latter includes some of the bones of the face, of the upper and lower jaws, some of the bones on the lateral surface of the head in the ear region, and the small sound transmitting bones of the middle ear. The visceral skeleton of the rat is doubtless a highly modified heritage from the fish-like ancestors of the mammals. The chief function of the visceral skeleton of the fishes is to support the gills and lend strength to the jaws. The student should familiarize himself, either through works on evolution, or with the aid of the instructor, with the probable changes by which the visceral skeleton, epiglottis, and the laryngeal and tracheal cartilages of the mammals have evolved from the visceral arches of fishes. Wilder's "History of the Human Body" discusses this question.
The parts of the skull develop in two ways. The floor, and a portion of the sides and back of the cranium consist of bones which have developed from centers of ossification in the embryonic cartilage of these regions. These are the cartilage bones. The roof of the cranium, the remainder of the sides, the hard palate, and the bones of both jaws are intramembranous in origin. They are therefore called membrane bones.
Dorsal aspect of skull. The skull is long and narrow, gradually decreasing in width from the posterior to the anterior end. The width of the human skull is relatively greater, as compared with the length, than the width of the rat skull. The ratio of cranial width to length, or the cephalic index, is an important characteristic in the classification of the races of man. The irregular lines on the ​surface of the rat's skull are the sutures where the adjacent bones are joined to one another.
The extreme posterior, or nuchal, surface of the skull is nearly perpendicular to the long axis of the animal, and joins the dorsal surface almost at a right angle. The lambdoidal ridge occupies the region of union of these two surfaces. It is semicircular in form. Anterior to the lambdoidal ridge is a second ridge extending ventrally to the region of the ear. Dorsally this second ridge joins the temporal line at right angles. This line extends forward laterally on the dorsal surface of the cranium to the orbit, where it turns medially and disappears. Compare the rat with the horse and cat, for instance, where the lambdoidal ridge is well developed, and with the human skull where it is absent.
The orbito-temporal fossa is a deep depression on each side of the mid-region of the cranium, about one-third as long as the whole cranium. It contains the eye, eye muscles, and muscles, which help close the jaw. The zygomatic arch bounds it laterally. This arch is a bow-shaped bar of bone which extends forward from the articulation of the lower jaw. At its anterior end this arch broadens and is pierced by the large infraorbital fissure. Viewed from the anterior end this fissure is shaped somewhat like a palm-leaf fan, broad dorsally and narrow ventrally. In other vertebrates the orbito-temporal fossa is more or less completely separated by bony processes into two distinct cavities, the orbit for the eye and its muscles, and the temporal fossa.
The anterior nares [piriform aperture in man] are located at the anterior extremity of the cranium. Air enters through these openings and is conveyed through the nasal passages to the lungs.
The following bones may be seen from the dorsal side: ​the occipital, interparietal, parietals, frontals, nasals, pre-maxillaries, maxillaries, zygomatics, squamosals, and lacrimals. The position and structure of each of these bones will be described in detail later.
Exercise I. Draw and label the dorsal view of the skull.
Lateral aspect of the cranium. The nuchal plate bears ventrally the occipital condyle, by which the skull articulates with the backbone. The jugular process is a long, pointed, triangular structure lateral to and slightly in front of the occipital condyle. Between the two is a semi-circular depression, the jugular fossa. The mastoid portion of the petrosal bone is immediately anterior to the jugular process, and is located in a pronounced depression of the lateral part of the occipital bone. The tympanic bulla is a hollow spheroidal capsule anterior to the jugular process, opening to the outside through the external acoustic meatus. This opening is closed in life by the tympanic membrane, which transmits sound waves from the outer ear to the three small bones of the middle ear.
Where the mastoid portion connects anteriorly with the squamosal bone there is a ventral extension of the latter, the post-tympanic hook, which helps to hold the tympanic bulla in place.
The orbito-temporal fossa and zygomatic arch are seen from the lateral side also. Within the fossa are foramina through which the nerves emerge from the brain. The foramen ovale is located directly ventral to the posterior end of the zygomatic arch, and anterior to the tympanic bulla. A branch of the fifth cranial nerve emerges here. The foramen communicates by a perforation of the external pterygoid process with the ventral surface of the skull. Another small foramen is immediately in front of the foramen ovale. A few millimeters anterior to the foramen ​ovale is the large anterior lacerated foramen. The optic foramen for the passage of the second cranial nerve is anterodorsal to the anterior lacerated foramen.
There is a pronounced groove along the ventral side of the orbito-temporal fossa. The small postpalatine foramen communicates near the center of this groove with the ventral surface of the palatine bone. Anterior to this foramen another foramen establishes a connection between the orbit and the nasal cavity. A small foramen dorsal to these openings perforates the cranium just posterior to the cribriform plate, which is described later.
The maxillary bone is incompletely ossified medial to the infraorbital fissure. Consequently in cleaning the skull the nasal cavity is usually opened at this place.
The convex lateral surface of the premaxillary bone reveals the course of the root of the enormous curved incisor tooth. The root occupies a semicircular canal (alveolus) which extends back to the level of the infraorbital fissure. Compare the arrangement of the rat's teeth with that of the cat, dog, and man, for instance. The rat has no canine teeth, which are so prominent in carnivores, and no premolars. The three molar teeth are inserted in the maxillary bone a considerable distance posterior to the incisors. The toothless space between the incisor and molar teeth is the diastema. A similar arrangement occurs in the horse, for instance.
Exercise II. Draw and label the lateral view of the cranium.
Ventral view of the skull. The following bones are seen from this view: the occipital, which is flanked on either side by the petrosal bone and the tympanic bulla, the basisphenoid with an alisphenoid fused to each side, the presphenoid and the paired squamosals, palatines, maxillaries, and premaxillaries. ​At the posterior end of the skull is the foramen magnum, through which the brain connects with the spinal cord. On each side of this foramen is the occipital condyle. Two pairs of bony plates, the pterygoid processes, arise at the base of each alisphenoid bone. The outer plates are nearly horizontal in position. They extend from the posterior end of the maxillary bone caudad and laterad to the lateral surface of the tympanic bulla. The two inner pterygoid processes are vertical to the base of the cranium. Each extends posteriorly to the opening of the bony canal through which the Eustachian tube passes from the cavity of the middle ear (within the tympanic bulla) to the pharynx. The single posterior naris, or choana lies between the anterior extremities of the inner pterygoid processes. It leads forward into the nasal cavity, which opens at the anterior end of the head through the two anterior nares.
The roof of the mouth in front of the choana consists of three pairs of bones (palatines, maxillaries, and premaxillaries), which taken together constitute the hard palate. At the extreme posterior end of the hard palate are the two small palatine bones, which form the ventral boundary of the choana. The maxillary bones are considerably larger than the palatines and are located laterally and anteriorly to the latter. The lateral, or alveolar, portion of the maxillary bears the three molar teeth, which are used for grinding food. The maxillaries articulate in front with the pre-maxillaries. These bear the two large curved incisor teeth. Canine and premolar teeth are absent in the rat.
Several pairs of foramina appear on the ventral surface. The twelfth (hypoglossal) cranial nerve emerges through the hypoglossal canal at the base of the occipital condyle. The posterior lacerated foramen lies immediately in front of the hypoglossal canal, at the boundary between the ​petrosal and occipital bones. This foramen transmits the ninth (glossopharyngeal), tenth (vagus), and eleventh (accessory) cranial nerves. The carotid foramen is anterior to the jugular foramen, between the tympanic bulla and the basioccipital bone. Near the posterior ends of the pterygoid processes is a pair of foramina which communicate with one another by a horizontal space in the basisphenoid bone. A large oval foramen lies immediately anterior to the one just mentioned. Note the communication of this oval foramen with the superior orbital fissure. Ventral to the foramen ovale another large oval foramen perforates the posterior part of the lateral pterygoid process. Note the two slits in the cranium, dorsal and posterior to the choana, communicating with the anterior lacerated foramen.
The prepalatine foramina are two conspicuous slits on the roof of the mouth in the region of the diastema. They are surrounded by the maxillary and premaxillary bones. The postpalatine foramina perforate the roof of the mouth near the lateral margin of each palatine bone. A bristle inserted through the foramen will show its communication with the groove on the floor of the orbito-temporal fossa.
Exercise III. Draw and label the ventral view of the cranium.
Interior of the cranium. Bisect the skull vertically in a plane slightly at one side of the sagittal plane. Use a sharp bone saw with small teeth. Care should be exercised to injure the bones as little as possible.
Two cavities will be observed, the cranial cavity for the brain, and the nasal cavity. The cranial cavity is subdivided further into the cerebellar, cerebral, and olfactory fossae.
The cerebellar fossa holds the cerebellum. It occupies the posterior part of the cranium and is partially sepa​rated from the cerebral fossa by an incompletely ossified lateral partition, the tentorium. The ossified portion of the tentorium is a dorsal extension of the petrosal bone, which forms a part of the lateral wall of the cerebellar fossa. The floccular fossa is a relatively large cavity occupying the posterodorsal region of the petrosal bone. Its opening into the cranial cavity is an oval aperture considerably narrower than the cavity itself. The cerebral fossa is the largest division of the cranial cavity. It incloses the cerebrum. The foramina observed in the study of the lateral view may be identified easily by thrusting a bristle through each foramen from the outside. The olfactory fossa is a laterally compressed cavity opening behind into the cerebral fossa and bounded in front by the cribriform plate. This cavity contains the olfactory lobes of the brain. The branches of the olfactory nerve enter the nasal cavity through the numerous foramina in the cribriform plate. In a cranium from which the roof has been removed, the cribriform plate will be seen to be heart-shaped.
Though the nasal cavity can be satisfactorily studied in a longitudinal section of the skull, it can be seen to the best advantage if the dorsal and lateral bony wall of the cavity be removed. In doing this care should be taken to preserve intact the nasal bones and their delicate ventral processes, and while chipping away the lateral wall to injure as little as possible the delicate turbinal bones at the back and sides of the nasal cavity. The nasal cavity is tubular in form posteriorly (naso-pharyngeal duct), and terminates at the single posterior naris, or choana. Its external opening is through the two anterior nares at the anterior end of the snout. The nasal cavity is bounded dorsally by the nasal bones, posteriorly by the cribriform plate and maxillary bones, ventrally by the premaxillary, ​maxillary, and palatine bones. The two long prepalatine foramina lie in its floor, and it communicates by a foramen with each orbito-temporal fossa. A perpendicular median partition, the nasal septum, divides the nasal cavity into two nasal fossae. The septum is partly cartilaginous.
Each nasal fossa contains three very delicate, folded turbinal bones, which probably correspond to the maxillo-turbinal, nasoturbinal, and ethmoturbinal bones of other mammals. The first of these is an elongated thin sheet of bone attached ventrally to the inner surface of the pre-maxillary bone. The second is a process of the nasal bone. Its ventral edge turns outward and upward like the rolled edge of a stiff piece of paper. The ethmoturbinal bone is much more extensively convoluted than the other turbinals. The convolutions inclose spaces known as ethmoidal cells, whose long axes are, in general, directed antero-posteriorly. The ethmoturbinals are attached posteriorly to the cribriform plate. Examination of a skull in which the bone has been removed from the side of the nasal fossa shows that in the posterior part of the fossa the turbinal bones extend from the top to the bottom of the cavity and posteriorly almost to the optic foramen. The convolutions of the turbinal bones, together with the flesh covering them, are sometimes called the nasal labyrinth. In mammals the folds supported by the turbinals warm the air and extract foreign bodies from it. The epithelium covering the ethmoturbinals contains olfactory cells which are stimulated by odorous substances in the air.
Consult a text on comparative anatomy to determine the variations among vertebrates in the bones of the nasal cavity. Compare the nasal cavity of the rat with the same part in the turtle, frog, dogfish, etc. Assisted by a text on embryology outline the stages in the evolution of the nasal cavity.
Bones of the Cranium
Having examined the general topography of the skull, the structure of each bone should be studied in greater detail.
Occipital Bone (Os Occipitale)
This bone first appears in the mammalian embryo as four centers of ossification in the chondrocranium (cartilaginous cranium). One of these centers is dorsal (supraoccipital), one ventral (basioccipital), and two are lateral (exoccipitals) to the foramen magnum. These four bones fuse in mammals to form the occipital. The supra- and ex-occipital regions in the rat are the posterior wall of the cranium. The basioccipital is the floor of the cerebellar fossa.
The basioccipital region is trapezoidal in shape, broader behind than in front. Anteriorly it joins the basisphenoid bone by a suture at right angles to the long axis of the skull. Posteriorly it forms the ventral boundary of the foramen magnum. Laterally it comes in contact with, but is not fused to, the tympanic bulla and the petrosal bone. Note the two marked depressions on the ventral surface of the bone which are separated from one another by a median ridge. The inner surface of the basioccipital, where it comes in contact with the brain, is concave.
The occipital condyles and the triangular jugular processes previously mentioned, are parts of the exoccipital regions. The condyles bound the foramen magnum ventrolaterally. They articulate with the atlas, or first neck vertebra. This articulation permits the head to be raised ​and lowered vertically without a corresponding movement of the neck. The hypoglossal canal is anterior to the condyle. The exoccipitals are fused dorsally with the supraoccipital, ventrally with the basioccipital, and are in contact anteriorly with the mastoid process of the petrosal bone.
Viewed from above the dorsal edge of the supraoccipital region resembles the segment of a circle with its center anterior to the bone. The ends of the segment terminate in a dorsoventral ridge where the supraoccipital connects with the squamosal bone. This ridge is continuous with the post-tympanic hook. A median vertical ridge (the posterior occipital protuberance) bisects the nuchal surface.
Sphenoid Bone (Os Sphenoidale)
The sphenoid is a complicated structure consisting of two distinct parts, the anterior sphenoid and posterior sphenoid. Each of these, in turn, consists of three elements, separated in some of the lower vertebrates, but fused in the higher.
The three fused elements forming the posterior sphenoid are the single median basisphenoid (post-sphenoid) and the two lateral alisphenoids. The basisphenoid is bounded caudally by the basioccipital, anteriorly by the anterior sphenoid, and laterally is fused with the alisphenoid bones. Anterior to its union with the alisphenoids, the basisphenoid increases slightly in width up to its attachment with the presphenoid. The pterygoid processes (one pair on each side), described in the discussion of the ventral view of the skull, are attached to the ventral surface of each alisphenoid bone. Dorsal to the lateral pterygoid process is a depression into which the foramen ovale and two other foramina open. The alisphenoid expands, dorsal ​to these foramina, into a flat process which extends toward the base of the zygomatic arch. This process runs posteriorly to the tympanic bulla but does not fuse with it. The process joins dorsally the squamosal and frontal bones. It forms a part of the posterior wall of the orbito-temporal fossa, sending a process forward to the palatine bone and alveolar process of the maxillary. The failure of the alisphenoid bones to fuse with the presphenoid and anterior part of the basisphenoid, leaves a pair of long slits dorsal to the choana. Observe the position of the anterior lacerated foramen.
The three fused constituents of the anterior sphenoid bone are the single median presphenoid and the two lateral orbitosphenoids. The presphenoid bone extends anteriorly from the basisphenoid. In the cerebral fossa its smooth surface decreases in width anteriorly until it is reduced to a thin septum between the two optic foramina. The ventral surface bears a keel which appears as a low ridge on the roof of the naso-pharyngeal duct. Each optic foramen is bounded medially by the presphenoid, laterally by the wedge shaped orbitosphenoid bone. The base of the latter is fused with the presphenoid. Its apex is thrust outward between the alisphenoid bone, posteriorly, and the orbital process of the frontal bone, anteriorly. The shape and size of the orbitosphenoid can be readily observed from the inside of the cranial cavity.
Squamosal Bone (Os Squamosum)
The squamosal bone resembles a very short-handled claw hammer if viewed from the lateral surface of the skull. The "handle" widens posteriorly, and with a spur from the occipital bone forms the posttympanic hook. The "handle" is bordered dorsaUy by the parietal bone, ​ventrally by the petrous and tympanic bones, and by an unossified region of the skull. The "claw" of the hammer is bounded ventroanteriorly by the alisphenoid bone; posteriorly its tip approaches the tympanic bone. Dorsally the "hammer" joins the frontal and parietal bones; it is bounded anteriorly by the orbital process of the frontal bone.
The squamosal bone bears a pronounced longitudinal ridge which runs nearly its whole length. Dorsal to the "claw" the ridge is extended anteroventrally as a curved spur, the zygomatic process. This process forms a part of the zygomatic arch. The mandibular fossa, for the articulation of the lower jaw, lies ventral to the base of the zygomatic process.
Tympanic Bone (Os Tympanicum)
The tympanic bulla contains the tympanic cavity, or cavity of the middle ear, which opens laterally through the external acoustic meatus.
Petrosal Bone (Os Petrosum)
The petrosal bone incloses the inner ear, and is seen on the inner surface of the skull. The mastoid portion of the petrosal is inserted into the notch at the side of the occipital bone.
Interparietal Bone (Os Interparietale)
The interparietal is an ovoid bone of membranous origin inserted between the supraoccipital and the paired parietals. It meets the nuchal surface nearly at right angles.
Parietal Bone (Os Parietale)
The paired parietal bones are of membranous origin. They constitute the greater part of the roof and a considerable part of the sides of the cerebral fossa. Each bone, were it flattened out, would be nearly rectangular. The true shape is obscured externally by a process of the squamosal bone which covers the anteroventral part of the parietal. Note the connections, posteriorly with the interparietal, anteriorly with the frontal, and laterally with the squamosal bones. The curved inner surface is smooth, but externally the temporal line separates the dorsal from the lateral region of each bone.
Frontal Bone (Os Frontale)
The two frontal bones are of membranous origin. They are united in the mid-dorsal line by the frontal suture. Each bone consists of a frontal and an orbital part. The former roofs the skull in front of the parietals, the latter forms the greater part of the side wall of the orbito-temporal fossa. The temporal line sharply separates these two portions of the bone externally. The frontal part joins the parietal bone posteriorly at the coronal suture. This part decreases in width anteriorly and articulates with the nasal and premaxillary bones. The orbital part is concave. Its posterior region, partly overlaid by the squamosal bone, forms a portion of the back wall of the orbit. Ventrally it joins the alisphenoid, orbitosphenoid, and maxillary bones—anteriorly the lacrimal and maxillary bones.
The frontal bones, internally, are the anterior wall of the cerebral fossa, the lateral wall of the olfactory fossa, and they partly inclose the nasal cavity posteriorly. An ​internal ridge marks the boundary between the cerebral and olfactory fossae. The cribriform plate is attached to the frontal bones between the olfactory fossa and nasal cavity.
Ethmoid Bone (Os Ethmoidale)
This highly complicated bone, of cartilaginous origin, is the chief representative of the embryonic nasal capsule. It includes the perpendicular plate of the nasal septum, the cribriform plate, and the ethmoturbinal bones.
Maxillary Bone (Maxilla)
The maxillary bone consists chiefly of five processes—the frontal, zygomatic, alveolar, palatine, and orbital. It is a membrane bone. The frontal process is divided by the large infraorbital foramen into a lateral and median portion. The latter forms part of the lateral wall of the nasal cavity and is comparatively thin. The skull is unossified posterior to this part of the frontal process, which joins the premaxillary bone dorsally and anteriorly. The lateral portion of the frontal process is a laterally concave plate connected with the dorsal surface of the cranium by a bony arch. To the dorsal end of this plate is attached the zygomatic process, which forms the anterior half of the zygomatic arch. The alveolar process (so called because it contains the alveoli of the teeth) is a strong bony mass holding the three molar teeth. It is bounded posteriorly and dorsomedially by the sphenoid bones, medially by the palatine process and palatine bone. The palatine process joins its fellow on the opposite side by a median suture in the roof of the mouth. It forms part of the hard palate. In front it surrounds the posterior half of the prepalatine foramen. It joins the pala​tine bone posteriorly. The orbital process overlaps the anteroventral portion of the frontal bone's orbital process. It forms but a small part of the wall of the orbito-temporal fossa. A pronounced longitudinal groove on the floor of the fossa partly separates the orbital from the alveolar process.
Lacrimal Bone (Os Lacrimale)
The lacrimal bone is a thin inconspicuous element, of membranous origin, lying on the dorsomedial wall of the infraorbital foramen. It is immediately below the bony arch which connects the dorsal surface of the skull with the lateral part of the maxillary bone's frontal process. The bone is easily overlooked if its sutures are not exposed by careful scraping.
Premaxillary Bone (Premaxilla)
The premaxillary bone, of membranous origin, forms the anterior part of the upper jaw, and the lateral and ventral walls of the front part of the nasal cavity. With the nasal bone it surrounds the anterior nares. Its frontal process is thrust dorsally between the nasal and maxillary bones, extending caudally to the frontal bone. This process is partially overlaid laterally by the maxillary bone. On the ventral side the palatine process extends horizontally across the roof of the mouth, surrounding the anterior part of the prepalatine foramen and meeting its fellow at a median suture. This suture extends backward between the two maxillary, as well as between the palatine bones. The premaxillary bears an enormous incisor tooth. Carefully chip away the bone surrounding this tooth, and note the size and shape of the alveolus. The palatine process joins the palatine process of the ​maxillary bone. The process is attached to the vomer along the median suture of the hard palate. The vomer forms part of the nasal septum. The elongated maxilloturbinal bone attaches to the inner surface of the premaxillary. It lies lengthwise of the nasal cavity and extends upward from its line of attachment.
Zygomatic Bone (Os Zygomaticum, Malar, Jugal)
The zygomatic bone is a small bow-shaped element of membranous origin, joining the zygomatic processes of the maxillary and squamosal bones, thus completing the zygomatic arch.
Vomer, or Ploughshare, Bone
This fragile bone, which is membranous in origin, comprises the ventral portion of the nasal septum. It unites ventrally with the premaxillary bone. It splits dorsally into two longitudinal lamina, between which the cartilaginous part of the nasal septum is inserted. The posterior part of the bone is without a ventral attachment.
Nasal Bone (Os Nasale)
The nasal bones roof the anterior nares and the nasal cavity back to the frontal bones. The ventral surface is concave, both the lateral and the median borders bearing a flange. The median flanges of the two nasals unite to form a ridge for the attachment of the nasal septum. The lateral flange articulates with the frontal process of the premaxillary bone. This flange is widest where the nasoturbinal bone (see longitudinal section of the skull) is attached to it. The line of attachment of the nasoturbinal runs forward and medially from this point. The nasal bone is a membrane bone.
Palatine, or Palate, Bone (Os Palatinum)
The palatine bone, membranous in origin, consists chiefly of three elements—the horizontal part, and the orbital and sphenoidal processes. The horizonal part constitutes the posterior region of the hard palate, and therefore is the ventral boundary of the choana. It meets its fellow of the opposite palatine bone at the median suture along the dorsal side of the mouth cavity. Laterally it articulates with the alveolar process of the maxillary bone. The orbital process, which joins the horizontal part nearly at right angles, is exposed to the orbito-temporal fossa. It constitutes the greater part of the nasopharyngeal duct's lateral wall. The dorsal edge articulates with the presphenoid bone. The sphenoid process extends backward to the pterygoid process, and joins dorsally an anterior extension of the alisphenoid bone.
The Mandible (Mandibula)
The mandible comprises two lateral elements united in front by connective tissue at the mandibular symphysis. Each element contains five parts: the body, the ramus, and three processes attached to the latter posteriorly. The body of each half of the mandible is prolonged anteriorly as a strong, curved process bearing one large incisor tooth. This tooth projects anterodorsally a considerable distance beyond the bone. Behind the incisor is the diastema, a toothless region corresponding to the diastema of the upper jaw. A row of three molar teeth is posterior to the diastema. The two rows of molars are approximately parallel to each other, but each row forms an acute angle with the long axis of the bone in which it is inserted. There are transverse ridges on the grinding surfaces of the molars. The ramus is the laterally flattened region immediately ​behind the body. Three processes are attached to its posterior end. The coronoid process projects dorsally into the orbito-temporal fossa. Just behind this is the condyloid process, bearing an elongated rounded knob which articulates with the mandibular fossa of the squamosal bone. These articular surfaces in the rodent permit anteroposterior motion of the jaw. The angular process is directed posteriorly. The anterior mandibular incisure is a pronounced depression separating the coronoid from the condyloid process. The posterior mandibular incisure likewise separates the condyloid and angular processes. The mandibular foramen lies at the base of the condyloid process. The mental foramen lies ventroanteriorly to the molar teeth. Other foramina occur on the surface of the mandible. A ridge extends on the lateral surface of the mandible from the mental foramen backward to the border of the angular process. The medial surface of this process is concave. The incisor tooth of the lower jaw, like that of the upper, is sickle-shaped, and inserted in a large dental alveolus which extends caudally, ventral to the molar teeth, nearly to the base of the condyloid process. The mandible is largely a membrane bone.
Hyoid Bone (Os Hyoideum)
The hyoid is a u-shaped bone at the base of the tongue composed of a slender transverse body and two pairs of horns (cornua), one pair extending anteriorly, the other pair posteriorly.
Exercise IV. Make a labeled sketch of each of the bones of the head.
THE VERTEBRAL COLUMN
The vertebral column, or backbone, is the second division of the axial skeleton. It is a linear series of numerous ​vertebrae, each one fastened to its neighbor by connective tissue. The back-bone supports the neck, trunk, and tail, and to it are attached the ribs, pelvic girdle and numerous muscles. It comprises seven cervical, thirteen thoracic, six lumbar, four sacral, and about twenty-eight caudal vertebrae.
A typical vertebra possesses a solid, more or less cylindrical ventral portion, the centrum, or body. The vertebral, or neural, arch is attached to the upper side of the body, the two inclosing the vertebral foramen, through which the spinal cord passes. The roots of the vertebral arches (pedicles) are the two lateral vertical plates of the vertebral arch. The two horizontal plates forming the roof of the arch are the laminae. The neural spine, or spinous process, projects upward from the junction of the laminae. The vertebra bears on each side a transverse process. The spinous and transverse processes serve for the attachment of spinal muscles. Each vertebral arch bears two pairs of articular processes, or zygapophyses. Of these the anterior one, or prezygapophysis, articulates with the preceding vertebra ; the posterior surface, or post-zygapophysis, articulates with the succeeding vertebra.
The essential features of the vertebral column may be brought out best by describing vertebrae from each region of it. The seven cervical vertebrae are quite similar to one another, except the first two, the atlas and epistropheus (axis). The atlas is essentially a ring of bone. It lacks a centrum, this element having been separated from the atlas and attached to the epistropheus as the dens (odontoid process). This process rests in a semi-circular depression, the fovea dentis, on the ventral floor of the atlas. The dorsal surface of the atlas bears a tubercle comparable to the spinous process of an ordinary vertebra. The ventral surface bears a similar tubercle. The atlas articulates ​26 ANATOMY OF THE RAT
with the occipital condyles of the skull by means of two lateral concave surfaces, the superior articular foveae, one on each side. It bears posteriorly a pair of lateral ovoid surfaces, inferior articular foveae, for articulation with the epistropheus. A third articular surface connects these facets ventrally. In man, and presumably in the rat, the chief movement permitted between the superior articular foveae and the occipital condyles is a "nodding" of the head. In "shaking" the head the atlas revolves around the dens as a pivot. The elongated transverse process of the atlas is flattened dorsoventrally. Its dorsal surface slopes backward to the posterior end of the vertebra, where it joins a vertical plate. Between this plate and the inferior articular fovea is the posterior opening of the foramen transversarium, through which the vertebral artery passes. This foramen extends anterodorsally, and communicates with the dorsal surface of the atlas by an oval opening. The artery then turning medially traverses the atlantal foramen to the inner surface of the atlas.
The centrum of the epistropheus is flattened dorsoventrally and prolonged anteriorly by the dens. The dens articulate with the fovea dentis by the anterior articular facet on its ventral side. The cranial articular facets, one on each side of the epistropheus, impinge upon the inferior articular foveae of the atlas. The postzygapophyses, one toward the upper end of each pedicle, articulate behind with the prezygapophyses of the third cervical vertebrae. The laterally flattened neural spine is considerably higher than that of any other cervical vertebra. It extends forward over the atlas and backward over the third vertebra. The foramen transversarium pierces the base of the short transverse process.
In the fifth cervical vertebra the centrum and vertebral arch form in side view an obtuse angle with each other. ​The centrum is dorsoventrally flattened. The spinous process is a low pointed crest. The prezygapophysis, borne by the pedicle, is ovoid when viewed from above. Its articular surface is directed dorsomediad. The postzygapophysis is directly behind it, is similar in shape, and its articular surface is ventrolaterad. The transverse process, attached at the base of the pedicle, is pierced in the region of attachment by the foramen transversarium. The vertebrarterial canal comprises the transverse formina of the cervical vertebrae. It terminates with the sixth cervical vertebra.
The centrum of the fifth thoracic vertebra is nearly cylindrical. The spinous process is long and tapering; its point is directed caudally. There is a notch on the anterior edge of the laminae. This notch is flanked on each side by the oval prezygapophyses whose articular surfaces face dorsally. The postzygapophyses lie behind the prezygapophyses, but their articular surfaces are directed ventrally. Each pedicle is deeply notched posteriorly, forming an intervertehral foramen, for the passage of a spinal nerve. The transverse process of each side is situated between the anterior and posterior zygapophyses. The articular surface of the transverse process, facing laterally, supports the tubercle of a rib. A depression, the costal demifacet, will be observed at each end of the centrum near its dorsal border. The adjacent demifacets of two successive vertebrae combine to form the articular surface for the head of a rib. The tubercle of this rib articulates with the posterior vertebra of the pair under consideration.
The fourth lumbar vertebra's centrum is cylindrical ventrally, but flattened above. The spinous process is a laterally compressed plate which runs the whole length of the vertebra, is somewhat triangular in shape, and leans ​cranially. The articular surface of the prezygapophysis faces mediodorsad. The facet of the prominent postzygapophysis faces laterally. The transverse process projects cranioventrally from the base of the pedicle.
The sacrum consists of four vertebrae more or less completely fused together. The transverse processes project horizontally from the centra. They are fused at their distal ends. The transverse processes of the first two sacral vertebrae are considerably thicker than the others. They are expanded laterally, forming the auricular surface, which articulates with the pelvic girdle. Since this girdle is attached to the hind legs, the weight of the posterior part of the body falls on these two sacral vertebrae — hence their stout construction. The intervertebral formina are located, as usual, between the vertebrae. The prezygapophyses of the first sacral vertebra are well developed, for they are part of a movable joint. The remaining functionless zygapophyses of the sacrum are rudimentary. They lie close to the bases of the high spinous processes.
The caudal, or tail, vertebrae show gradual and progressive stages of simplification from the sacrum to the posterior end of the tail. The neural canal is small in the anterior part of the tail, and soon completely disappears. The anterior caudal vertebrae have ventral pairs of spines between which run the caudal blood vessels. There is a chevron bone in some of the vertebrae which partly incloses these vessels on the under side.
Exercise V. Make a labeled sketch of each vertebra which has been described.
The sternum, or breastbone, comprises seven bony segments (sternebrae) united end to end. The first, or manubrium, is keeled ventrally, and has a broad anterior region, concave above. The first rib is attached to it. The suc​ceeding six pairs of ribs are attached between the segments. The seventh segment (xiphoid process) is not associated with a pair of ribs. It terminates in a round plate of cartilage.
Exercise VI. Draw and label the sternum.
There are thirteen pairs of ribs. A rib contains a dorsal and a ventral segment. The anterior seven pairs are the true ribs, for they are attached to the sternum. The six posterior pairs are the false ribs. The eighth, ninth, and tenth pairs attach to the posterior border of the seventh, and by cartilage to the fifth sternebra. The three posterior ribs are floating ribs, lacking ventral segments. The ribs are bow-shaped in general, the angle, or region of greatest convexity, being dorsal. Most of the ribs are distinctly flattened anteroposteriorly. From the fifth to the tenth pairs, inclusive, the distal end of the rib is twisted, so that the broad surface is posterolaterad. Compare the shapes of the different ribs.
The fifth rib may be used to illustrate the articular surfaces. The capitulum costae (head) is a slight swelling at the proximal end. Its circular medial surface articulates with the demifacets of two adjacent vertebrae, as previously described. The tuberculum costae (tubercle) is a dorsal enlargement two or three millimeters lateral to the head. It articulates with the transverse process of the vertebra, and is separated from the head by the collum costae (neck) of the rib.
The thoracic cavity, surrounded on the sides by the ribs, dorsally and ventrally by the backbone and sternum respectively, resembles an ice cream cone with the apex cut off. The cranial thoracic aperture is in front, the larger caudal thoracic aperture behind. Both are circumscribed by ribs, sternum, and backbone. The thoracic cavity in​creases in size, and air enters the lungs, when the ribs are pulled anteriorly in breathing.
Exercise VII. Sketch one of the longer ribs.
THE ANTERIOR APPENDAGES
The pectoral girdle includes the scapula (shoulder blade), and the clavicle (collar bone). The former is a flattened triangular bone with its apex pointing cranioventrad and its broad inner surface facing the vertebral column. A cartilaginous disc connects one end of the clavicle with the acromion process of the scapula. A similar disc unites the other end with the anterior end of the sternum. The clavicle is the only bony connection between the scapula and axial skeleton.
The concave thickened superior margin of the scapula is directed cephalad. The convex vertebral margin is dorsal. The third margin, the axillary faces ventrocaudally, and is nearly straight. The anterior, posterior, and ventral corners of the bone are designated the medial, inferior, and lateral angles respectively. At the lateral angle is the pear-shaped glenoid cavity for articulation with the humerus bone of the upper arm. The short coracoid process (homologous to the bone of the same name in the lower animals) projects medially from the cranial end of the glenoid cavity. The scapular spine is a thin, prominent plate (forming nearly a right angle with the scapula) which divides the lateral surface of the scapula into the supraspinous fossa (anterior) and the concave infraspinous fossa (posterior). The spine tapers dorsally to a point at the vertebral border; it ends ventrally in a free projection, the acromion process or acromion. The inconspicuous projection on the caudal surface of the latter is probably the metacromion process. There are two longitudinal ridges on the inner surface of the scapula. ​The humerus is the bony constituent of the upper arm. Its proximal end articulates with the scapula; it articulates distally with the radius and ulna of the fore arm. Between these two extremities is the shaft, or body, of the bone. The head is a prominent oval enlargement at the proximal end, which fits into the glenoid cavity, furnishing the articulation with the scapula. The lesser tuberosity is a low knob located immediately in front of the head on the medial surface of the bone. The greater tuberosity is a dorsoventrally elongated projection on the cranial surface of the humerus, immediately in front of the head, extending also on to the lateral surface of the bone. The intertubercular groove separates the two tuberosities. The deltoid tuberosity is a very prominent rough-edged ridge running, on the ventral surface, from the greater tuberosity about half way to the distal end of the humerus.
The articular surface at the distal end of the bone is saddle-shaped to receive the corresponding surfaces of the radius and ulna. This end of the bone is flattened. It bears two depressions immediately above the articular surface — the radial fossa on the ventral side to receive the radius during flexion of the arm, and the olecranon fossa on the dorsal side to accomodate the olecranon process during extension. The medial epicondyle is a prominent cylindroid process medial to the saddle-shaped articular surface. The lateral epicondyle is a broad process located laterally near the distal end of the bone.
The radius and ulna support the fore arm, the former lying anterior to the latter. The two are firmly bound together by the interosseus ligament. The body of the radius bows forward and outward. Dorsally it expands abruptly, forming the head, which bears an oval surface for articulation with the humerus. The posterior side of the head is flattened, and rests against a similar surface ​just below the semilunar notch of the ulna. The radius is expanded distally to articulate with the wrist, or carpal bones. Longitudinal grooves cross this expansion; medially it bears the pointed styloid process.
The ulna is laterally compressed, the lateral surface bearing a pronounced groove extending from the level of the head of the radius two-thirds of the way to the distal end. Like the radius, the ulna bows forward and out- ward, so that an articulated skeleton looks "bow-legged." The ulna terminates distally in the conical, blunt styloid process for articulation with the wrist bones. The semi-lunar notch is the saddle-shaped depression near the proximal end, which, with the head of the radius, articulates with the humerus. The flat surface at its ventral border corresponds to the radial notch of other animals ; it articulates with the head of the radius. The olecranon is the stout projection behind the semilunar notch for the insertion of the extensor muscles of the upper arm.
The joint between the humerus and scapula is of the "ball and socket" type, permitting of lateral as well as anteroposterior movement. The elbow joint, however, is of the hinge type, which permits only of extension and flexion of the arm.
There are nine carpal or wrist bones. The metacarpal bones are the relatively long bones in the palm of the hand. Counting from the medial side of the manus, the third and fourth are the longest, the fifth is considerably shorter, while the first is so short as to be readily mistaken at first glance for one of the carpal bones. Each digit contains three phalanges, except the thumb (pollex), which contains two. Each terminal phalanx bears a claw, that of the thumb being flattened like a nail.
Exercise VIII. Draw and label the lateral view of the scapula
THE POSTERIOR APPENDAGES
The two coxal bones (os coxae), sometimes called the innominate bones, comprise the pelvic girdle. Each bone is firmly united dorsally to the two anterior sacral vertebrae. Ventrally the two are held together by connective tissue at the pubic symphysis. The lateral surface of each bone bears a deep pit, the acetabulum, for the reception of the head of the thigh bone (femur). Unlike the pectoral girdle the direct attachment of the pelvic girdle to the backbone furnishes a rigid support for the strong and active hind legs of the animal. The coxal bone consists of three fused elements, the ilium, ischium, and pubis, which radiate from the acetabulum. The distal ends of the pubis and ischium are also fused, so that these two bones surround an oval space, the obturator foramen. Though the sutures are completely obliterated in the adult, they can be very easily seen in a very young animal. The cartilaginous boundaries between the three bones in such an animal all lie in the acetabular wall, though the end of the pubis forms only a very small part of this wall. The distal connection of pubis and ischium will be observed at the ventral extremity of the obturator foramen. The acetabular wall of the adult is well supported with bone. The acetabulum itself is funnel-shaped. The inner, or constricted portion, of this funnel expands medially to form a dorsoventrally elongated pit.
The ilium (os ilium) is the anterior branch of the triradiate coxal bone. It consists of two parts, the body and the wing. The former is somewhat flattened laterally, forms the anterior part of the acetabulum, and bears the anterior inferior spine on its lateral surface just in front of the acetabulum. The greater sciatic notch is a depression along the dorsal border of the ilium, extending back ​on to the ischium. Its anterior border is the posterior inferior spine. The iliopectineal line on the ventral side of the body connects the sharp anterior border of the pubis with the wing of the ilium. The wing is the broad, flat, anterior part of the ilium. The posterior part of its concave medial surface is the auricular surface, which articulates with the sacrum. Anterior to this surface the ilium bends outward rather abruptly, terminating in a pronounced tuberosity at right angles to the long axis of the bone. The thickened anterodorsal margin of the bone is the iliac crest.
The ischium (os ischium), the most posterior component of the coxal bone, has three parts: a body, a superior ramus, and an inferior ramus. The anterior end of the bone forms the posterior side of the acetabulum. The body expands posteriorly, giving off dorsally the superior and ventrally the inferior ramus. This part of the ischium is concave medially, and convex laterally. The inferior ramus joins the pubis at the ventral extremity of the obturator foramen. There is a longitudinal depression along the dorsolateral surface of the ischium. This depression is bounded ventrally by a longitudinal ridge, terminating posteriorly in a thickening, the sciatic tuber. The upper edge of the ischium is convex, while the posterior edge is nearly a straight line. These two edges meet at almost a right angle.
The pubis (os pubis) is the ventral branch of the coxal bone. Its superior ramus extends ventrally from the acetabulum. The inferior, or symphyseal, ramus passes back along the symphysis pubis to join the inferior ramus of the ischium. The body is located at the region of union of the two rami. The sharp anterior edge of the bone bears the iliopectineal eminence. From this the iliopectineal line passes forward. ​The femur, or thigh bone, consists of a long body with processes at each end for attachment of muscles, or for articulation with other bones. The proximal end is Y-shaped. The medial branch of the Y is capped with a hemispherical knob, the head, which fits into the acetabulum. The head is held in place by the round ligament (ligamentum teres femoris) of the femur, attached to the center of the articular surface. The head is joined to the shaft by the narrow cylindrical neck. Head and neck together resemble a small "toad stool," the neck corresponding to the stalk of the ""stool." The massive lateral branch of the Y, the great trochanter, is continued as a high thin ridge down the lateral surface of the femur. This ridge is sometimes referred to as the third trochanter, and the lateral branch of the Y as the first trochanter, the two being considered components of the greater trochanter. The lesser or second trochanter is a knob ventral to the base of the neck on the flexor surface of the femur. The intertrochanteric crest connects the greater and lesser trochanters. The trochanteric fossa is a deep depression at the base of the medial side of the greater trochanter. The lateral and medial condyles are large ventral swellings at the distal end of the femur. They are separated from each other by the deep intercondyloid fossa, which is continued on to the extensor surface of the bone as the grooved patellar surface for articulation with the patella. The two condyles articulate with the proximal end of the tibia.
The tibia extends from the knee to the ankle. The fibula is a slender bone posterior to the tibia. The two are anchylosed in the lower third of the tibia. These two bones resemble an archer 's bow, the fibula representing the cord, the forward bending tibia the wooden bow. The shaft or body of the tibia decreases in thickness distally. Beyond the anchylosis it is cylindrical in cross section, but ​triangular proximal to this point. Thus the bone presents three surfaces: a concave anterolateral surface, a convex anteromedial surface, and a posterior surface with two pronounced longitudinal grooves. A ridge, the anterior crest, is located proximally on the anterior side of the bone at the angle formed by the lateral and medial surfaces. The bone articulates with the femur proximally. The articular surface is divided into two large concave condyles, separated from each other by a longitudinal groove, the intercondyloid fossa. Each of these is opposed to the convex surface of one of the femoral condyles. The tibia and fibula articulate distally with the talus (astragalus) bone of the heel. The tibial part of the articular surface contains two longitudinal grooves, separated by a ridge. The medial malleolus forms the medial boundary of the inner groove. The fibula bears the lateral malleolus upon its lateral surface. The slender fibula expands dorsally, where it is attached by a ligament to the external condyle of the tibia.
The patella is a sesamoid bone lying in front of the knee joint.
The tarsus, or ankle, contains eight bones arranged in two rows, one distal and one proximal, with one bone between the rows. The proximal row contains the talus and calcaneus. The talus articulates with the tibia and fibula. The calcaneus, or heel bone, is shovel-shaped and is the largest bone in the tarsus. The "handle," or heel process, is directed posteriorly, forming the support for the heel. When the toes are depressed the trochlea of the talus acts as a fulcrum, the heel process as a lever moved by the posterior leg muscles. The length of the heel process makes it a relatively efficient lever. There are also four sesamoid bones in the tarsus. There are five long, cylindrical metatarsal bones in the foot, those of the great toe, or ​hallux, and of the little toe being shorter than the remaining three. Each toe, or digit, possesses three phalanges, except the hallux, which has two. Each terminal phalanx ends in a curved claw.
Exercise IX. Make a labeled drawing of the lateral view of the innominate bone and the bones of the hind limb.