Guinea-Pig: External Features, Nervous System and Life History

**In this article we will discuss about:**

'*1. **Habitat of*** **Guinea-Pig*.'***

'***2.**External Feature of* Guinea-Pig*.'*

''''***3. Skin. ''''***

''''***4. Body Cavity.''''***

''''***5. Skeletal System.''''***

''''***6. Alimentary System.''''***

''''***7. Vascular System.''''***

''''***8. Respiratory System. ''''***

''''***9. Nervous System.''''***

''''***10. Receptor Organs. ''''***

''''***11. Endocrine System.''''***

''''***12. Urinogenital System.''''***

''''***13. Excretory System.''''***

''''***14. Reproductive System.''''***

''''***15. Breeding and Life-History.''''***

**1. Habitat of *Guinea-Pig*:**

The guinea-pig, ***Cavia porcellus***, is a *well-known* household pet.

The name seems to be a misnomer, because the animal is neither a pig nor does it come from Guinea.

It is a herbivorous rodent belong­ing to the same order as the common rat.

It is timid and inoffensive in the domesticated state.

In the wild state, *guinea-pig* is gregarious and lives in underground burrows or in bushes hiding inside thick vegetation.

It feeds chiefly at dusk and makes a hasty retreat on slightest provocation.

**The *guinea-pig* is a mammal because:**

(*1*) Its body is covered with a coat of hair which serves to keep it warm.

(*2*) It is warm­ blooded or *homoio-thermal* and its body temperature remains con­stant in spite of changes in the temperature of its environment.

(*3*) It is viviparous, i.e. its, young is born alive and fed by the mother with milk produced in her mammae.

**2. External Feature of *Guinea-Pig*:**

The body of the guinea-pig is divisible into head, neck, trunk and limbs.

There is no tail, but a rudimentary *tail-bone* is found at the posterior end of the trunk.

All the parts of the body, excepting the tip of the nose and the foot pads, are covered with hair which may be diversely coloured, ranging from white to brown or even black.

The head is elongated and produced anteriorly into a narrow snout bearing a pair of external nostrils at its tip.

The mouth is a small opening situated beneath the nostrils.

It is bordered by soft lips of which the upper one is cleft in the middle, producing the *so-called hare-lip*; two *chisel-shaped* front teeth are thus exposed.

A number of stiff and sensitive whiskers or vibrissae are found on the snout by the sides of the external nostrils.

The large eyes, one on each side of the head, are protected by movable upper and lower eyelids which bear a few eyelashes.

There is a third eyelid in the form of a whitish nictitating membrane which can be ficked across the eye from the inner corner of the orbit, therefore cleaning the outer surface of the eyeball.

The most prominent feature on the head are a of pair of large pinna or external ears; each pinna is freely movable and can be turned towards the sources of the sound.

At the base of the pinna is an aperture called external auditory meatus which leads through a short tube to the tympanum or eardrum.

The neck is short and flexible.

It connects the head with the trunk and permits free movement of the head.

The trunk is composed of an anterior part, the thorax, pro­tected by ribs and sternum, and a posterior part, the abdomen, having no bony support.

A pair of teats or nipples are seen on the ventral surface of the abdomen near the posterior end of the trunk; they bear the external openings of the mammary glands.

The teats are small and the mammary glands are undeveloped in male *guinea-pigs.*

The anus lies beneath the rudimentary *tail-bone* at the poste­rior end of the trunk.

There is no cloaca; urinary and genital apertures lie ventral to the anus.

The space between the anal and the urinogenital apertures is known as the perineum.

A small pe­rineal gland opens in the perineum on either side of the anus and the secretion of these glands gives the *guinea-pig* its characteristic smell.

In the male, the urinogenital aperture lies at the tip of a muscular organ called penis which is ensheathed by a fold of skin, the prepuce.

A pouch of skin, the scrotum, is found at the root of the penis in the adult male; this pouch encloses the testes.

In the female, the genital aperture is the *slit-like* vulva which is bounded laterally by folds of skin called labia.

A small elevation not larger than the head of a pin, the clitoris, is found in the anterior part of the vulva and the urinary opening is situated close to this.

The clitoris corresponds to the penis of the male.

The limbs are of the pentadactyle type and built on the same plan as in the toad; but the digits end in claws and are not webbed.

The limbs are paired and the forelimbs are shorter than the *hind-limbs.*

Each forelimb consists of arm, forearm, wrist and hand; the hand bears *four-clawed* digits, the one corresponding to our thumb being absent.

Each hindlimb consists of thigh, shank, ankle and foot; the foot bears only three clawed toes, the toes corresponding to our first and second being absent.

The animal walks on all fours.

The short forelimbs help in burrowing underground.

The long hindlimbs are used for leaping.

The planter surfaces of the feet and ankles rest on the ground during locomotion.

**3. The Skin of *Guinea-Pig:***

The mammalian skin has two principal parts, an outer epidermis and an inner dermis.

The epidermis is derived from the ectoderm and the dermis from the mesoderm.

The epidermis of the skin consists of a superficial horny layer, the stratum corneum, and a deeper germinative layer, the stratum germinativum.

The former is composed of dead *scale-like* cells and the latter is formed by living poly­hedral cells.

The cells of the germinative layer divide actively to regenerate the superficial layers of dead horny cells.

The protoplasm of the *scale-like* superficial cells is gradually replaced by a hard protein called keratin due to increased pressure of new cells that are constantly forming under them.

In parts exposed to pressure and friction, such as palm of the hand and sole of the foot, the horny layer is thickened and hardened to form the *so-called* friction tori (*singular—torus*).

The claws at the ends of the digits are much hardened and specially modified pieces of the horny layer.

The dermis is a layer of fibrous connective tissue lying under the epidermis; it contains blood vessels and nerves.

Embedded in the dermis, but formed by downward prolongation of the epidermis, are small pits called hair follicles.

At the bottom of each pit, there is a hair papilla from which arises a hair.

Each hair follicle is provided with a bunch of smooth muscle fibres whose contraction erect the hair.

Opening into the hair follicle by short duct is a group of *flask-shaped* structures called seba­ceous gland; it liberates an oily substance which acts as a natural hair oil and keeps the hair in good condition.

There is another set of epidermal glands called sweat glands.

A sweat gland is a coiled *tube-like* structure opening by a long narrow duct on the surface of the skin.

The sweat glands secrete watery sweat and thereby regulate the body temperature; they also excrete waste products.

In addition to the above, there are other modified glands in the skin.

The perineal glands, opening at the surface of the perineum on either side of the anus, are specialised sebaceous glands.

The mammary glands occur in both sexes on the ventral surface of the abdomen near the posterior end of the trunk.

Each is a much branched tube, secreting milk and opening on the surface of a small promineace called nipple or teat.

The glands are *well-developed* in female *guinea-pigs*, particularly during the period of lactation, and milk is secreted for feeding the young.

In the male, the glands are small and do not function.

**4. Body Cavity of *Guinea-Pig*:**

The skin is loosely attached to the underlying muscles by a thick layer of subcutaneous connective tissue.

On cutting through the skin and muscular body wall, the body cavity is exposed.

It is divided by a transverse partition called diaphragm into an anterior thoracic and a posterior abdominal cavity.

The diaphragm is a dome-shaped muscle which is convex towards the thorax and concave towards the abdomen; its central part is thin and transparent, and radiating from this part are muscle fibres which are attached to the vertebral column, ribs and sternum.

The *dome-shaped* diaphragm is flattened by the contraction of its muscle fibres, so as to increase the size of the thoracic cavity.

The thoracic cavity is bounded posteriorly by the *back-bone*, laterally by the ribs, and ventrally by the *dagger-shaped* sternum.

It is lined internally by a membrane called pleura.

The two lungs, one on each side, are enclosed in the pleural cavities.

Each pleural cavity is a *double-walled* bag.

Between the two pleural bags is a small space called mediastinum which contains the heart enclosed in the pericardium.

The thoracic part of the oesophagus and tra­chea also lie in the mediastinal cavity.

The abdominal cavity is larger than the thoracic cavity, and is lined internally by the peritoneum.

It contains the stomach, intestines, spleen, liver, pancreas, kidneys, urinary bladder and ovaries.

The peritneum is folded to form mesenteries for sup­porting the various abdominal organs.

Thus in mammals the body cavity consists of four coelomic chambers: the peritoneal, the pericardial and the two pleural cavities.

**5. Skeletal System of *Guinea-Pig*:**

The skeleton of guinea-pig is almost entirely osseous with strips of cartilage persisting at the ends of some bones.

Cartilage is also present in a few places such as in the external ears and nose.

The skeleton is divided into axial and appendicular portions.

The axial skeleton is composed of the skull, vertebral column, ribs, and sternum.

The appendicular skeleton consists of the limbs and limb girdles.

*(1*) The cranium, which surrounds the brain,

(*2*) the sense capsules—olfactory, optic, and otic—which enclose the organs of special sense; and

(*3*) the visceral skeleton formed by the jaws, hyoid apparatus and part of the larynx.

Cranium, sensory capsules and the upper jaw are firmly united.

The cranium or *brain-box* is composed of a number of bones rigidly articulated with one another by *well-marked* sutures.

At the posterior part of the cranium, there are two occipital condyles for articulating with the vertebral column, one on either side of the foramen magnum.

Surrounding this foramen there are four bones—a supraoccipital above, a basioccipital below, and two exoccipitals, one on each side.

Each exoccipital is produced ventrally into a pointed and curved paroccipital process which serves for the attachment of muscles.

The roof of the cranium consists, from behind forwards, of the single supraoccipital, paired parietals and paired frontals.

The floor is composed of the basioccipital, basisphenoid, and presphenoid.

Each lateral wall of the cranium is made up of squamosal, alisphenoid and orbitosphenoid.

Arterial wall of the cranium is formed by a cribriform plate pierced by a number of holes.

This plate separates the cranium from the factory capsule.

The upper jaw, on each side, is composed chiefly of a *pre-maxilla*, a maxilla, and a jugal which runs backwards to meet a process of the squamosal, thus forming the zygomatic arch.

The premaxilla bears a cutting or incisor tooth, whereas the maxilla bears four grinding teeth, of which the anterior one is the premolar and the remaining three are molars.

A long space, the diastema, separates the incisor from the premolar.

The anterior part of the floor of the skull is known as the hard palate.

This is formed by inward expansions of the premaxillae, maxillae, and by two pala­tines.

Just behind each palatine is a small pterygoid.

The quadrate is absent.

The lower jaw or mandible consists of two halves united in front by a weak symphysis.

Each half is composed of a single bone, the dentary, which bears teeth similar to those on the upper jaw, and there is a diastema between the incisor and the premolar.

Each dentary has an ascending ramus which forms the condyle for arti­culating with the undersurface of the squamosal at the posterior end of the zygomatic arch.

In front of the condyle is a small curved process called coronoid process.

The posterior end of each dentary is produced behind into an angular process which forms the angle of the mandible.

The olfactory capsule, housing the nose, is roofed over by two nasals, whilst its floor and side walls are formed by the inward expansions of the premaxillae anteriorly, maxillae posteriorly, and the vomer in the midventral line.

Posteriorly, the vomer articulates with the presphenoid.

There is a porous cribriform plate at the posterior part of the olfactory capsule which shuts out the nose from the cranium.

The olfactory nerves enter the nose through the holes on this plate.

A vertical plate of Cartilage, the mesethmoid, separates the two nasal chambers, and a few insig­nificant spongy bones called turbinals are found inside each nose to support the nasal epithelium.

The optic capsule, housing the eye, is represented on each side by a large orbit.

At the anterior part of each orbit is a small lacrimal bone interposed between the frontal above and the maxilla below; this bone is perforated by a minute lacrimal fora­men.

The orbit is bounded dorsally by the frontal, ventrally by the zygomatic arch, and posteriorly by the squamosal.

The inner wall of the orbit is the interorbital septum. It is formed by the orbitosphenoids above, and the presphenoid below.

The otic or auditory capsule, housing the internal ear, is placed between the squamosal and the exoccipital on either side of the cranium.

Each capsule is principally composed of a periotic bone, a tympanic bone, and the auditory ossicles.

The periotic bone is formed by the fusion of three small bones; it is irregular in shape and encloses the membranous labyrinth.

The tympanic bone is firmly applied to the outer surface of the periotic; it is *flask-shaped*—the opening of the flask is the external auditory meatus, and the swollen base of the flask is the tympanic bulla which encloses the tympanic cavity along with the auditory ossicles.

The pinna is attached to the rim of the external auditory meatus.

The auditory ossicles constitute a chain of three small bones, the malleus, the incus and the stapes; they connect the eardrum with the internal ear and are themselves enclosed by the tympanic bulla.

The hyoid is a small bone lying at the root of the tongue on the floor of the buccal cavity.

It consists of a stout median part, the body and two pairs of slender horns or cornua.

The anterior cornua connect the hyoid with the periotics; the posterior cornua are similarly connected with the larynx.

**Vertebral Column:**

The vertebral column is composed of about *thirty-seven* vertebrae, which are separated from one another by pads of cartilage, called intervertebral discs.

**It is divisible into five regions:**

(1) The cervical regions consisting of seven vertebrae,

(2) The thoracic region of twelve vertebrae,

(3) The lumbar region of seven vertebrae,

(4) the sacral region of four vertebrae, and

(5) the caudal region of about seven vertebrae. In each of these five regions the vertebrae possess certain common distinguishing features.

**A typical vertebra, from the middle of the series, consists of the following parts:**

(1) a vertebral body or centrum with a flat surface at the anterior and posterior ends; the centrum is there­fore acoelous;

(2) a neural arch, placed above the centrum, en­closing the neural canal and thereby protecting the spinal cord; the part of the neural arch which joins with the centrum is known as the pedicle, and the remaining part forming the roof of the neural canal is the lamina; the margins of the pedicle are notched in such a way that openings, called intervertebral foramina, are formed bet­ween successive vertebrae serving as outlets for the spinal nerves;

(3) a spinous process or neural spine, arises dorsally from the summit of the neural arch;

(4) two transverse processes, one on each side, arising from the junction of the centrum with the neural arch; they are directed outwards and downwards;

(5) a pair of *upward-feeing* anterior articulating processes or prezygapophyses, borne upon the inner side of a pair of large forwardly directed processes, called metapophyses;

(6) a pair of posterior articulating processes or postzygapophyses facing downwards and outwards from the posterior end of the neural arch; at the base of each postzygapophysis is a backwardly directed small process, called anapophysis (*Fig. 148—F, G).*

The cervical region is composed of seven cervical vertebrae.

All the cervical vertebrae, excepting the first and the second, are characterised by the shortness of the centrum, the smallness of the neural spine, and the presence of a bifurcated transverse process on each side pierced through its base by a small hole, the verte­broarterial canal, for the passage of the vertebral artery.

The first vertebra or atlas (*Fig. 148—A, B)* is a bony ring with no distinct centrum; it has no zygapophyses and its neural spine is very small; its transverse processes are flattened and perforated by the verte­broarterial canal.

Anteriorly, the atlas bears a pair of concave facets for articulating with the occipital condyles.

Posteriorly, there are similar but smaller facets for articulating with the second vertebra.

The large *ring-like* neural canal of the atlas is divided horizontally by a fibrous ligament into an upper and a lower compartment.

The upper compartment is occupied by the spinal cord, and the lower by an anteriorly directed process of the second vertebra, the odontoid process.

The second cervical vertebra or the axis *(Fig. 148—G*) has a broad centrum which is produced anteriorly into a *pig-like* odon­toid process; this is really the centrum of the atlas removed from it and fused with the axis.

The large neural spine is elongated *antero-posteriorly* and compressed from side to side.

There are no prezygapophyses, but a pair of postzygapophyses are present for articulating with the prezygapophyses of the third vertebra.

On each side of the odontoid process is an articulating facet for the atlas.

Trans­verse processes of the axis are small and perforated.

The thoracic region is composed of twelve thoracic vertebrae, each of which typically bears a pair of ribs.

**They are divided into two series:**

(1) an anterior series composed of the first nine vertebrae; and

(2) a posterior series including the last three.

An anterior thoracic vertebra (*Fig. 148—E*) has a short thick oentrum which bears at its sides, concave facets for articulating with the of a rib; its neural spine is tall, slender and backwardly the transverse processes are stout and horizontal; each van a facet ventrally for articulating with the tuberculum of a rib.

A posterior thoracic vertebra has smaller transverse processes which do not bear any facet for articulating with the ribs. Its centrum, how­ever, carries *well-developed* facets for the capitulum of the ribs.

The neural spine is flattened from side to side and is inclined forwards.

Prezygapophyses are supported on forwardly directed metapophyses.

The lumbar region is composed of seven lumbar vertebrae (*Fig. 148—F, G*).

They are the largest in the body, increasing in size from before backwards.

The structure of a lumbar vertebra con­forms to the typical vertebra which has been already described.

In addition to the zygapophyses a lumbar vertebra bears metapophyses anteriorly and anapophyses posteriorly.

The neural spine is flat and points forwards.

The transverse processes are large and expanded.

The centrum of the first and the second lumbar vertebra bears a median ventral ridge, the hypapophysis, which serves for the attachment of the diaphragm muscle.

In others, the hypapophysis is absent.

The sacral region is composed of four sacral vertebrae immo­vably fused together to form a single bone, the sacrum, *(Fig. 148 —H*), which is wedged in between the two halves of the pelvic girdle.

The first sacral vertebra is the largest; its neural spine is upright; its transverse processes are expanded into *wing-like* structures which are attached to the ilia of the pelvic girdle.

Metapophyses are small and anapophyses are absent.

The zygapophyses are greatly reduced.

The remaining sacral vertebrae decrease in size from before backwards.

The caudal region is composed of about seven caudal verte­brae.

They decrease in size from before backwards, gradually losing all their processes and finally the neural arch as well. The last one or two are merely *rod-like* centra.

The ribs and sternum.

The ribs, with the sternum and the vertebral column, form a basket for enclosing the heart and lungs.

There are twelve pairs of ribs, which are articulated to the vertebral column in such a manner that they are capable of movement.

**A typical rib such as the sixth is a curved rod divisible into two por­tions:**

(1) a vertebral portion which is bony and articulates with the vertebral column, and

(2) a shorter sternal portion which is cartila­ginous and therefore known as the costal cartilage; the costal cartilage joins the vertebral portion to the sternum.

The first seven are complete or true ribs, because they extend from the ver­tebral column to the sternum.

The following five are false ribs, because they do not extend up to the sternum.

The costal carti­lages of the eighth and the ninth ribs are attached in front to the costal cartilage of the seventh rib and are thus indirectly connected to the sternum.

The last three ribs remain free at their ventral ends, and are, therefore, called floating ribs.

There are thus seven true ribs and five false ribs, of which the last three are floating.

**The first nine ribs have each two articulations to the vertebral co­lumn:**

(1) the rounded proximal end or heat or capitulum of the rib articulates with concave facets borne on the centra of *ad­jacent* vertebrae;

(2) the tuberculum, which is situated a little distal to the capitulum, articulates with a facet on the undersurface of the transverse process of the following vertebra.

The last three ribs do not articulate with the transverse processes of ver­tebrae and they have no tubercles.

The sternum is an elongated narrow rod, lying in the *mid-ventral* wall of the thorax.

It is composed of a series of segments or sternebrae with which articulate the costal cartilages of the true ribs.

The first sternebra or man­ubrium is the largest and articu­lates with the greatly reduced clavicle.

The last sternebra or xiphisternum extends beyond the attachment of the ribs and expands into a large flat *cartila­ginous* plate, the xiphoid cartilage.

**Pectoral Girdle:**

**The pec­toral girdle of the *guinea-pig* consists of two bones on each side:**

(1) a large flat scapula, and

(2) a small rod-like clavicle.

The scapula is triangular in scape, with its base turned up­wards and the apex pointing for­wards and downwards;’ its flat portion overlies the ribs to which it is attached by muscles.

The apex of the scapula is expanded and bears the glenoid cavity into which fits the head of the humerus.

Overhanging the glenoid cavity is a hook-like coracoid process.

Posteriorly, the broad base of the scapula bears a narrow strip of cartilage, the suprascapula.

On the outer surface of the scapula is a prominent bony keel known as the spine.

The spine ends in a pointed acromion near the apex of the scapula and a long metacromion projects downwards from the acromion.

The clavicle is a short slender bone which is imperfectly developed.

It is merely embedded in muscles and has no articulations.

Extend­ing from the sternum to the acromion is a strong band of fibrous tissue; the clavicle occupies the middle third of this band.

In some mammals such as man the clavicle is a *well-developed* bone connecting the scapula with the manubrium of the sternum.

Compared with the toad, the pectoral girdle of the *guinea-pig* is relatively small; it is com­pletely ossified, except for the greatly reduced cartilaginous suprascapula.

Two halves of the girdle are not connected by *any-hard* structures; the precoracoid and coracoid are fused with one another to form a slender cora­coid process which is permanently fixed to the scapula.

The clavicle is also reduced and imperfect.

The sternum of the guinea-pig is formed by the fused ventral ends of the ribs and does not correspond to the sternum of the toad.

**Forelimb:**

The arm is sup­ported by a stout humerus.

The humerus has a rounded head at its proximal end, articulating with the glenoid cavity of the scapula.

Close to the head is a greater tuberosity on the outer side, and a lesser tuberosity on the inner side; between the two tuberosities is the bicipital groove for lodging the tendon of the biceps muscle.

Anteriorly, the shaft of the humerus bears a deltoid ridge.

Distally, the humerus has a *pulley-like* surface, the trochlea, for articulation with radius and ulna of the forearm.

Above the trochlea are two deep depressions, one in front and the other behind.

The front one is the coronoid fossa, into which fits the radius and the back one is the olecranon fossa, into which fits the olecranon process of the ulna.

The two fossae are joined by a small hole through the bone, the supratrochlear foramen.

The forearm is composed of two separate bones, the radius and the ulna, which are immovably articulated to each other.

The radius is the shorter of the two bones and lies towards the inner side of the forearm.

As the two bones are fixed in a prone position with the thumb pointing inwards, the *guinea-pig* cannot rotate its palm like a man.

The ulna is longer than the radius; it bears a sigmoid notch at the proximal end for fitting into the tro­chlea of the humerus.

The proximal end of ulna is continued backwards as the olecranon process which forms the point of the elbow and acts as a level for the arm.

The ulna and the radius form a *hinge-joint* with the trochlea of the humerus.

Distally, both the bones bear articular surfaces for the carpals.

The wrist or carpus is composed of seven carpal bones which are arranged in two rows.

The proximal row includes the ra­diate, the intermedium and the ulnare.

The distal row con­sists of four small bones. In addition to these, a very small sesa­moid bone, the pisiform, is found on the underside of the wrist.

The palm includes four matacarpal bones.

There are four fingers, each of which is composed of three phalanges.

The ter­minal phalanx ends in a claw.

**Pelvic Girdle:**

The pelvic girdle of the guinea-pig consists of two halves, the innominate bones, which are united with one another in the midventral fine by a strip of cartilage.

Each innominate bone is composed of three parts—the ilium, the ischium, and the pubis, which are completely fused into one.

It bears on its outer side a *cup-shaped* cavity, the acetabulum, in which fits the head of the thigh bone of the same side.

The acetabulum marks the point of union of ilium, ischium, and pubis.

The ilium is a *blade-like* bone which lies anterior to the acetabulum; it is joined to the sacrum by a rough articular surface on its inner side.

The is­chium forms the posterior third of the acetabulum, and is conti­nued backwards along the line of the ilium; it is then continued to meet the pubis of the same side.

The pubis, the smallest of the three bones, lies ventral to the acetabulum; it joins the ilium anteriorly and the ischium posteriorly.

A large obturator foramen is enclosed between the pubis and the ischium.

The pubes of the two sides meet in the midventral line to form the symphysis pubis.

Compared with that of the toad, the pelvic girdle of the *guinea-pig* is more massive.

The ilium is relatively short and forms nearly *two-third* of the acetabulum; the remaining part of the aceta­bulum is formed by the ischium, and the pubis has practically no share in this.

In toad, all the three bones take part in the for­mation of the acetabulum.

In toad, the innominate bones are fuse together more or less completely at their posterior ends; only the two ilia remain free.

In *guinea-pig*, the two innominates are united only by the pubic symphysis; ilia as well as ischia re­main free.

**Hindlimb:**

The thigh is supported by a long bone, the femur, which bears medially a rounded head for articulation with the acetabulum, thereby forming a ball and socket *hip-joint.*

**Close to the head are three rough elevations:**

(1) a greater trochanter externally,

(2) a lesser trochanter internally, and

(3) a third tro­chanter externally just below the greater trochanter.

Distally, the femur has two rounded condyles for articulation with the tibia.

The condyles are separated by an intercondylar groove which is continued upwards for a short dis­tance for accommodating the patella or *knee-cap*.

The patella is a small sesamoid bone which develops in the tendon of a muscle in front of the *knee-joint.*

The leg is composed of two bones, the tibia and the fibula.

The tibia is a stout bone bearing, in front, a sharp ridge called cnemial crest.

The fibula is a thin *splint-like* bone lying on the outer side of the, tibia.

Proximally, the tibia and fibula articulate with the condyles of the femur; distally, they form articular surfaces for the tarsals.

The ankle or tarsus includes six bones arranged in two rows: a pro­ximal row of two bones called astragalus and calcaneum, a centrally placed bone called navi­cular, and a distal row of three small tarsals.

The foot is composed of three metatarsals.

There are three toes, each consisting of three phalan­ges.

The terminal phalanx is clawed.

**6. Alimentary System of *Guinea-Pig:***

The alimentary system of guinea-pig consists of the alimen­tary canal and the glands associat­ed with it.

Alimentary canal is complete with the mouth in front and the anus behind.

Mouth is a transverse slit bounded by soft and movable lips, of which the upper a slightly cleft in the middle to expose the incisor teeth.

The mouth leads into the buccal cavity, the roof of which is the palate.

The palate is divisible into two parts, an anterior hard palate sup­ported by bones, and a posterior soft palate without bony support.

The palate separates the buccal cavity from the nasal cavities which open by the internal nostrils into the pharynx.

The tongue is a mobile muscular organ which is attached to the floor of the buccal cavity.

Its free anterior end is just visible in the gap caused by the cleft upper lip.

The surface of the tongue is covered by numerous white spots, the papillae, some of which bear taste buds.

The *guinea-pig* has teeth for cutting and chewing food.

Teeth are implanted in sockets on the margins of both the jaws and hence the dentition is thecodont.

It may be recalled that in the fish the teeth are not implanted into sockets and are, therefore, pleurodont.

Moreover, in *guinea-pig*, all the teeth are not alike; that is the dentition is heterodont.

In the fish the dentition is homodont (*same shape*).

Lastly, the *guinea-pig* produces two generations of teeth of which the first set, known as the milk teeth or deciduous teeth, is temporary and is replaced later by the second set of permanent teeth.

As there are only two sets of teeth, the dentition is diphyodont.

Thus, the dentition is thecodont, heterodont, and diphyodont.

This is characteristic of mammals.

**In most mammals, there are four kinds of teeth:**

(1) The incisor or cut­ting tooth with *chisel-like* cutting edge and situated in front;

(2) The canine or biting tooth with pointed edge, situated behind the incisor;

(3) Premolar; and

(4) Molar, with flat edges for grinding and situated behind the canine.

The premolars and molars are practically identical—the only difference being that die molars are not present in the temporary set.

The number and arrange­ment of the various kinds of teeth differ according to the feeding habit, and is expressed in the form of a dental formula.

The types of teeth are denoted by their initial letters.

Thus, the letters i, c, pm and m stand for incisors, canines, premolars, and molars respectively.

Each letter is followed by a fraction, the numerator of which stands for the number of that kind on one side of the upper jaw, and the denominator indicates the number of the same kind on same side of the lower jaw.

Thus the dental formula of i[1] is , c-[2], pm[3], m[4] this indi­cates that there are two incisors, one canine, two premolars, and three molars, on each side of each jaw, and there are in all *thirty-two* teeth.

In *guinea-pig,* the dental formula is i[5], c [6] pm[7],m[8] indicating that there are in all twenty teeth.

There are one incisor, one premolar, and three molars in each side on each jaw, and the canine is absent.

The incisors are long and curved and they gross- indefinitely as their edges wear away.

There is a long gap or diastema between the incisor and the premolar on both sides of the two jaws.

In carnivorous mammals the diastema is occupied by the canine teeth (*Fig. 147*).

A tooth consists of two parts: the crown projecting above the gum, and the root lodged in a socket of the jaw.

The bulk of the tooth is composed of dentine which is enveloped by a coat of hard enamel at the crown, and by a substance called cement in the region of the root.

The central cavity of the tooth is occupied by a *jelly-like* pulp, blood vessels, and nerves.

The buccal cavity leads into the pharynx, which consists of a dorsal part or nasopharynx and a ventral part or buccopharynx.

The nasopharynx is continuous in front with the nose through the in­ternal nostrils and bears on each side a small Eustachian aperture leading into the middle ear.

The posterior margin of the soft palate extends into the naso-pharynx to form the velum.

On either side of the velum is a mass of lymphoid tissue called tonsil.

The buccopharynx is continuous in front with the buccal cavity and bears on its floor a *slit-like* glottis, situated behind the tongue.

The glottis opens into – the voice box or larynx and is guarded by a flap of cartilage, the. epiglottis, which shuts the glottis during the act of swallowing.

Posteriorly the pharynx is continued into the oesophagus, and the oesophageal opening lies dorsal to the glottis.

The oesophagus is a long cylindrical tube which runs back­ward, along the neck, just ventral to the vertebra] column. The trachea or windpipe lies in front of the oesophagus.

Passing through the thoracic cavity, it pierces the diaphragm muscle and finally opens into the cardiac portion of the stomach.

The stomach is a large sac which lies transversely on the left side of the abdominal cavity.

Its concave anterior border is known as the lesser curvature, whereas the convex posterior border is the greater curvature.

As usual, the stomach is divided into two parts: a cardiac portion lying towards the left side of the oesopha­geal opening, and a narrow pyloric portion situated on the right side.

The pyloric stomach is continued behind into the small intestine, the first part of which is the duodenum.

There is a pyloric constriction between the pylorus and the duodenum con­taining a circular sphincter, the pyloric valve.

The mucous mem­brane fining the interior of the stomach bears innumerable gastric glands which produce gastric juice.

The duodenum extends backwards and is then continued forwards to a *U-shaped* loop.

The remaining part of the small intestine is the much coiled ileum, the loops of which are held together by mesentery.

The mucous membrane of the small in­testine is very much folded to form *finger-like* projections called villi.

Each villus is richly supplied with blood capillaries and lymphatics.

There are Brunner’s glands opening in between the bases of the villi for secreting intestinal juice (*succus entericus)*.

The ileum is directly continuous with the colon which is the proximal part of the large intestine; but there is, at the junc­tion of the two, a huge, sacculated, blind, diverticulum known as the caecum.

The aperture between the ileum and the colon is guarded by the ileocolic valve.

Commencing on the right side, the colon forms several coils before it is continued into the rectum, which lies on the left side of the abdomen.

The rectum communi­cates with the exterior by the anus which is guarded by a sphincter.

**Glands associated with the alimentary canal are:**

(1) The sali­vary glands,

(2) The liver, and

(3) The pancreas.

There are four pairs of salivary glands, opening into the buccal cavity by separate ducts and secreting saliva for moistening food.

The parotids are the largest of the salivary glands and are situated in the cheek near the angle of the mandible.

The subma­xillary glands are compact white masses opening on the floor of the buccal cavity.

The sublinguals lie below the tongue.

The infra­orbitals are small, pale yellow bodies lying beneath the eyes.

The liver is the largest gland in the body.

It is subdivided into five lobes.

The liver lies behind the *dome-shaped* diaphragm to which it is attached by a band of peritoneum, called falciform ligament.

Its convex anterior surface fits into the concavity of the diaphragm and the concave posterior surface lies close to the stomach.

The *gall-bladder* is pyriform in shape; it is attached to the posterior surface of the liver between the third and the fourth lobes.

The cystic duct from the *gall-bladder j*oins the hepa­tic ducts from the liver to form the common bile duct which opens into the duodenum behind the pyloric sphincter.

The pancreas is an elongated mass of irregular shape situated in the mesentery between the two limbs of the duodenum; its narrow tail extends under the dorsal surface of the stomach and ends near the spleen.

The pancreatic duct opens distally into the interior of the duodenum.

**Feeding and Digestion:**

The young are fed with their mo­ther’s milk.

There is a milk-curdling enzyme, called rennin, in the gastric juice.

Rennin coagulates *milk-protein* before it is digested by proteolytic enzymes.

The adults is herbivorous.

The vegetable food is thoroughly chewed by the teeth and mixed with saliva.

The latter contains an amylolytic ferment, known as ptyalin, which splits starch into malt sugar.

The masticated food passes into the stomach via the oesophagus.

In the stomach, the food is acted upon and partially digested by the enzymes of the gastric juice in the presence of free HCI.

The pyloric sphinc­ter relaxes after the gastric digestion is over.

The acid chyme now enters the duodenum and is immediately neutralised by the Na2COa which is present there.

In the duodenum, the partially digested food is simultaneously treated with pancreatic juice, bile and succus entericus.

Bile is an antiseptic and helps in the digestion and absorption of fats.

The pancreatic and intestinal juices contain several kinds of enzymes.

Proteins are acted upon by proteolytic enzymes such as trypsin and erepsin, and con­verted into amino acids.

Fats are split into fatty acids and gly­cerol by the lipase; and carbohydrates, such as starch, are con­verted into monosaccharides such as glucose and fructose by amy­lase.

The digested food is absorbed by the villi of the ileum.

The residue passes into the caecum and colon where it is solidified by the absorption of water.

The solid faecal matter is extruded through the anus as pellets.

**7. Vascular System of *Guinea-Pig'''***:

This is built on the same plan as that of the toad, and consists of heart, arteries, veins, blood and lymphatics.

**(A) Heart:**

The heart of *guinea-pig* is ovoid in shape.

It lies completely enclosed by a membranous pericardium, in a space called mediastinum, between the two pleural cavities.

The broad anterior end of the heart is known as the base and the narrow pos­terior end is the apex.

The apical region is slightly projected towards the left side.

The heart is four-chambered and consists of two auricles, right and left, and two ventricles, right and left.

The auricles are placed anterior to the ventricles.

They form the broad base of the heart.

Each auricle communicates with the ventricle of its own side, but the left and right auricles are completely shut off from each other by a strong partition, the *inter-auricular* septum.

Compared with the ventricles, the auricles are relatively *thin-walled.*

The right auricle receives deoxygenated blood from the two venae cavae which open separately into its lumen.

There is no sinus venosus, because it is completely fused with the right auricle.

The left auricle receives oxygenated blood from each other by the inter-auricular septum, there is no chance of admixture of the two kinds of blood.

There is a small thin area, the fossa ovalis in the centre of the interauricular septum.

It marks the position of an embryonic aperture between the two auricles, the foramen ovale, which, however, is completely closed before the animal is born.

The ventricles are very *thick-walled*.

Strong muscular ridges, known as columnae cameae, project from the inner wall into the cavity of both the ventricles.

The left ventricle is lightly larger than the right, and is more muscular.

The *inter-ventricular* septum is strongly built and shuts off the cavities of the two ven­tricles from one another.

The right ventricle receives deoxygenated blood from the right auricle through the right auriculo ventricular aperture.

Similarly, the left ventricle receives oxygenated blood from the left auricle through the left *auriculo-ventricular* aperture.

The auriculo-oentricular apertures are guarded by *flap-like* valves, the edges of which are attached to papillary muscles by means of cords, known as chordae tendineae.

The right auriculo-ventricular valve has three cusps and hence it is known as the tricuspid valve.

The left *auriculo-ventricular* valve has only two cusps; it is, therefore, known as the bicuspid or mitral valve.

The main arterial arches arise from the bases of the ventri­cles—the systemic or aortic from the left, and the pulmonary from the right.

The entrance from the ventricles into both the arterial arches is guarded by a set of three *pocket-like* semilunar valves.

The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs for aeration.

It is firmly attached to the under surface of the aortic arch by means of. a band of fibrous cord known as the ductus arteriosus.

The aorta distributes oxygenated blood from the left ventricle to different parts of the body.

**(B) Arteries:**

The main arteries of the guinea-pig are the aorta and the pulmonary artery together with their branches.

The aorta arises from the left ventricle.

It curves to the left and then turns sharply backward to run behind the heart, where it lies ventral to the vertebral column.

From this point onwards it is designated as the dorsal aorta, whereas the curved anterior portion is known as the arch of aorta.

The dorsal aorta runs through the thorax in the middorsal line.

Passing into the abdo­men through the diaphragm, it finally ends by dividing into two common iliac arteries at the posterior part of the abdominal cavity.

During its long course the aorta gives out several branches to supply oxygenated blood to the entire body.

A pair of very slender coro­nary arteries arise from the root of the arch to rim into the tissues of the heart itself.

A stout innominate artery arises from the arch proper and at once divides into the right subclavian, and the right and left common carotid arteries.

A little beyond the innominate and slightly on its left side, arises the left subclavian artery as an independent branch from the arch of aorta.

Each common carotid artery runs forward through the neck along the outer side of the windpipe, and near the level of the voice box divides into two branches—an external carotid supplying the outer side of the head, and an internal carotid supplying the brain and its men­inges.

**Each subclavian artery divides into three branches:**

(1) A vertebral artery supplying to the vertebral column,

(2) An internal mammary to the ventral body wall including the mamma, and

(3) A brachial to the forelimb of the same side.

The thoracic part of the dorsal aorta supplies five to six pairs of intercostal arteries to the chest wall and the inter­costal muscles be­tween the ribs.

The abdominal part of the dorsal aorta gives out a stout coeliacomesenteric artery from its commencement.

**It breaks up into two branches:**

(i) A coeliac artery supplying to the liver, stomach and spleen, and

(ii) An anterior mesen­teric artery to the small intestine, pan­creas and the mesen­teric folds.

A pair of renal arteries origi­nate behind the *coeliaco-mesenteric* for supplying to the kid­neys.

This is followed by a pair of slender genital arteries to the gonads and three to four pairs of lum­bar arteries to the dorsal body wall.

A median posterior mesenteric artery arises in between the roots of the first and second pair of lumbar arteries. It is distributed mainly to the large intestine and its mesentery.

A small coccygeal artery is given out from the distal end of the dorsal aorta which supplies the sacral and coccygeal parts of the vertebral column.

The abdominal aorta terminates by dividing into a pair of common iliac arteries near the posterior end of the trunk.

Each common iliac divides into an external and an internal iliac arteries.

The external iliac is continued into the hindlimb of the same side as the femoral artery, whilst the internal iliac supplies to the ventral body wall, urinary bladder and the uterus.

The pulmonary artery arises from the base of the right ventri­cle.

It divides into two branches for distributing deoxygenated blood to the two lungs.

(i) Pulmonary veins,

(ii) Systemic veins, and

(iii) Portal veins.

(i) Two pulmonary veins convey oxygenated blood from each lung back into the left auricle.

(ii) The systemic veins include the two venae cavae which return deoxygenated blood to the right auricle.

The anterior vena cava or precaval vein is formed by the union of the right and left innominate veins behind the first segment of the sternum.

Each innominate vein is formed by the union of five tributaries: a subclavian from the forelimb, an internal jugular from die brain, an external jugular from the outer side of the head and face, an anterior intercostal from the first and second intercostal spaces and an internal mammary from the ventral body wall.

The azygos vein, from the posterior thoracic wall, drains directly into the precaval or rarely into the right innominate vein. The coronary vein from the heart drains into the precaval vein just before the latter opens into the right auricle.

The posterior vena cava or postcaval vein is formed by the union of a pair of common iliac veins at the posterior end of the trunk.

It runs forward along the ventral aspect of the vertebral column by the right side of the dorsal aorta.

It enters the liver and passing through the diaphragm muscle ends in the right auricle of the heart.

An external iliac vein from the hindlimb and an internal iliac vein from the pelvic viscera meet each other to produce the common iliac vein of the side.

The posterior vena cava in its long course receives two or three pairs of lumbar veins from the dorsal body wall, a pair of genital vein (spermatic in males and ovarian in females) from the gonads, a pair of renal veins from the kidneys, a pair of hepatic veins from the liver, and phrenic veins from the diaphragm.

The left spermatic vein of the male guinea-pig usually drains into the left renal vein.

**(iii) Portal Veins:**

There is no renal portal system but well- developed hepatic portal system is present.

Blood is conveyed from various parts of the alimentary canal by a large portal vein (Fig. 156) which in formed by the union of gastric, splenic, intestinal, pancreatic and mesenteric veins.

The portal vein on entering the Hver, breaks up into sinusoids.

A part of the food is thus extracted out of the blood and stored in the liver for future use.

Finally, this blood is diverted through the hepatic veins into the postcaval.

'*(D) '***Blood:**

The blood consists, as usual, of blood plasma and the blood cor­puscles.

The ery­throcytes are cir­cular, biconcave, non-nucleated cells charged with haemo­globin.

They are better known as the red blood corpuscles.

The leucocytes or white blood corpus­cles are nucleated, amoeboid cells de­void of haemoglobin.

Blood platelets are small non-nucleated cytoplasmic bits occurring in groups and derived from broken cells in the bone-marrow.

They correspond in function to the thrombocytes of the toad’s blood.

'*(E) '***Lymphatics:**

Associated with the main venous vessels of every organ and limb, there are slender lymphatic vessels which ultimately drain into two large trunks situated within the body cavity.

There is a stout thoracic duct on the left side of the dorsal aorta, communicating with the left subclavian vein at its junction with the external jugular vein. Similarly there is a right lympha­tic duct on the right side, opening into the right subclavian vein.

**Course of Circulation:**

The blood is kept circulating through the lungs and the body by the alternate contraction (systole) and expansion (diastole) of the heart.

The systole starts simultaneously in the two auricles, thereby driving the blood into the corresponding ventricles.

When the ventricles are filled, the blood attempts to flow back, but leakage is prevented by the closure of the tricuspid and mitral valves.

This is followed by a sharp contraction of the two ventricles expelling the blood through the aorta and the pulmonary artery, because the auriculo- ventricular valves (bicuspid and tricuspid) are tightly closed bf the contraction of the chordae tendineae.

The vessels expand with the inflow of blood, but with the commencement of the ven­tricular diastole their elasticity tends to drive the blood back into the ventricles, a process which is prevented by the rapid closure of the pocket-like semilunar valve.

Deoxygenated blood is brought back to the right auricle by the vanae cavae.

This blood is then pumped into the right ventricle and thence through the pulmonary artery into the lungs.

The blood is aerated in the lungs and brought back to the left auricle by pulmonary veins, thus completing the pulmonary circuit.

From the left auricle, the aerated blood is pumped into the left ventricle and then through the aorta it is pumped into every part of the body.

It is deoxygenated after circulating through the tissue capillaries and finally brought back to the right auricle by the venae cavae, thus completing the systemic circuit.

As there is no communication between the two sides of the heart, there is no possibility of admixture between the two kinds of the blood.

Thus, in guinea-pig, any portion of the blood has to pass through the heart twice before it returns to a particular part of the body here we find an example of complete double circulation—a pulmonary circuit through the lungs and a systemic circuit through all other parts of the body, excepting the lungs.

The sys­temic circuit includes a hepatic-portal system, in which blood loaded with food is diverted through the liver via the portal vein.

The latter breaks up into capillaries in the liver substance, so as to enable the liver cells to extract food for storage and other purposes.

These capillaries ultimately drain into the hepatic veins which join the postcaval.

There is no renal-portal system.

'*8. Respiratory System of Guinea-Pig'***:**

The respiratory system of guinea-pig consists of the lungs and the respiratory passages leading from them into the exterior.

**Its parts are best described under the following headings:**

(1) The accessory respiratory tract.

(2) The true respiratory tract, and

(3) The paired lungs.

The accessory respiratory tract includes the external nares, the nasal cavities, the internal nares and the nasopharynx.

Dur­ing breathing air rushes in­wards through the external nares into the nasal cavities which are separated from one another by the nasal septum.

The nasal cavities are separated from the buccal cavity by the palate.

The indrawn air passes into die nasopharynx through the internal nares.

The nasopharynx opens into the buccopharynx behind the soft palate and the air crosses the buccopharynx to enter into the true respiratory tract through the glottis.

The glottis is guarded by a hood-like cartilaginous flap, the epiglottis.

The true respiratory tract includes the larynx, the trachea and the bronchi.

The larynx or voice-box is composed of four cartilaginous plates.

Its ventral and lateral walls are strength­ened by the saddle-shaped thyroid cartilage, and the dorsal wall by a pair of small arytenoid cartilages.

A ring-like cricoid cartilage supports the posterior part of the larynx which is continued into the windpipe or trachea.

Stretched across the cavity of the larynx there are two elastic bands of membrane.

These are known as the vocal cords.

The vocal cords lie side by side, being separated from one another by a nar­row chink, the rima glottidis.

When air is driven out through the rima, the vocal cords vibrate and the.se vibrations produce sound waves.

The intensity and pitch of the sound depend mostly upon the degree of elastic tension of the cords and the size of the rima glottidis.

The trachea is a long tube running along the median line of the neck and lying ventral to the oesophagus.

It is stiffened by cartilaginous rings, each of which is incomplete dorsally.

Ex­tending into the thorax, the trachea divides into right and left bronchi.

The bronchus enters the lung of its own side, within which, it divides again in a tree-like fashion.

The lungs (Fig. 159) of guinea-pig are enclosed within the tho­rax, one on either side of the heart.

They are not simple hollow sacs like those of the toad.

The mammalian lungs are highly elastic, spongy, structures with three main lobes—anterior, middle and posterior.

The right lung is slightly larger than the left, and has an additional lobe called azygos lobe.

Each lung is surrounded by a pleural cavity which is composed of two thin membranes, an outer parietal pleura lining the thoracic wall, and an inner visceral pleura invetsing the outer surface of the lung.

The two layers of the pleura are continuous with each other near the root of the lung through which the bronchus enters.

On entering the lung, the bronchus divides repeatedly.

The very small terminal branches of the bron­chus are called bronchioles.

Each terminal bronchiole divides into a number of finer tubes called alveolar ducts, which give out sac-like alveoli from their walls.

The neighbouring alveoli lie pressed against one another, so that they are separated by very thin parti­tions.

The pulmonary artery forms capillary networks which are distributed on these partitions.

While the blood is circulating through the pulmonary capillaries, gaseous exchange occurs between the blood and the alveolar air.

**Mechanism of Respiration:**

The mechanism of breathing is entirely different from that of the toad, and the buccal cavity takes no active part in the process.

The elastic lungs invested by the airtight pleural cavities are located within the thorax.

This is a cage surrounded by the vertebral column, ribs and sternum.

The thoracic cage is closed posteriorly by the dome-shaped dia­phragm, a strong muscular partition with its convexity turned forwards.

During respiration, the size of the thoracic cage is increased by the raising of the ribs and the flattening of the dia­phragm.

The ribs are raised by a set of intercostal muscles (inter= between; costa—rib), thereby increasing the transverse diameter of the chest.

The antero-posterior diameter of the thorax is increased by the flattening of the diaphragm.

When the thoracic cage increases in size, the pleural cavities are enlarged.

Thus, the pressure within the pleura’ cavities is diminished and the lungs expand.

Consequently, air rushes into die lungs through the res­piratory tracts.

This process is known as inspiration.

After this, the thoracic cage reverts to its normal size by the relaxation of diaphragm and intercostal muscles. Intrapleural pressure is thus increased and air is forced out of the lungs.

This is expiration.

'*9. Nervous System of Guinea-Pig'***:**

The nervous system consists, as in toad, of a cerebrospinal axis, the peripheral nerves and the autonomic nervous system.

It is the brain which has undergone marked specialisation, so as to exhibit very considerable advance over that of the fish and that of the toad.

Nevertheless, the general relationship of the parts are the same.

**(A) Brain:**

This may be subdivided into the same five parts as in toad.

The telecephalon consists of a pair of greatly en­larged cerebral hemispheres, in front of which there is a pair of relatively small club-shaped olfactory lobes.

The cerebral hemispheres (Fig. 160) are so enormous that they completely over­lap the diencephalon and the mesencephalon.

The two hemispheres are separated from each other by a deep groove, the median fissure.

Laterally, there is another groove called Sylvian fissure which divides each hemisphere into two parts.

The larger anterior part is faintly subdivided into frontal and parietal lobes, whereas the posterior part into temporal and occipital lobes.

The superficial part of each hemisphere is composed of nerve cells and is known as the cerebral cortex or pallium.

The great deve­lopment of pallium chiefly distinguishes mammals from fishes and amphibians.

The ventral and lateral walls of each hemisphere is thickened to form the corpus striatum.

The two hemispheres are connected by a transverse band of nerve fibres, the corpus callosum, which is situated on the floor of the median fissure and can be seen from above by pressing the hemispheres apart.

The diencephalon is completely hidden by die cerebral hemis­pheres.

Its thick side walls form the optic thalami (singular— thalamus).

Epiphysis, ending in the pineal body, arises from its dorsal surface, whereas the optic chiasma and the hypophysis are found on its ventral surface.

The hypophysis (infundibulum) bears the pituitary body.

Posterior to the pituitary, there is a pair of small rounded masses, the mammillary bodies.

The roof of the diencephalon is non-nervous and includes a vascular fold, called anterior choroid plexus.

The floor of the diencephalon, including the optic chiasma, pituitary body and mammillary bodies, is known as the hypothalamus.

The roof of the mesencephalon is thickened and expanded to form four rounded bodies, the optic lobes or corpora quadrigemina; of the four, the anterior pair form the superior colliculi and the posterior pair, the inferior colliculi.

They are covered by the cere­bral hemispheres.

Ventral to the optic lobes are the thickened cerebral peduncles or crura cerebri which connect forebrain with hindbrain.

The metencephalon is formed by the much enlarged cerebe­llum which is subdivided into a median part called vermis and two lateral lobes, each of which terminates in a small flocculus.

It is marked dorsally by numerous transverse folds.

Ventrally the two halves of the cerebellum are connected by a broad band, the pons varolii.

The myelencephalon or medulla oblongata, gradually tapers behind to join the spinal cord.

The roof of the medulla is non-nervous and includes the posterior choroid plexus.

The ventricles of the brain are named as in other vertebrates.

The cavities of the cerebral hemispheres are known as the lateral ventricles.

They communicate with the cavity of the diencephalon, that is third ventricle, and with one another by a transverse slit, the foramen of Monro.

The narrow third ventricle passes backwards to the equally narrow iter, which connects with the cavity of the medulla oblongata or fourth ventricle.

The wide fourth ventricle communicates behind with the central canal of spinal cord.

All the cavities are filled with cerebrospinal fluid.

The brain is enclosed in membranes, which are collectively known as the meninges.

The tough outermost covering is the dura mater.

The highly vascular innermost covering is the pia mater, between the pia mater and the dura mater, there is a spongy layer known as arachnoid.

(B) The spinal cord is fundamentally similar to that of the toad.

It passes backward without any visible line of demarcation from the posterior end of the medulla oblongata.

It is situated within the neural canal and protected by meninges which are continuous with those of the brain.

There is a deep longitudinal groove running along its midventral line, forming the ventral fissure.

The cavity of the cord, the central canal or neurocoel, is continuous with the fourth ventricle of the brain and filled with cerebrospinal fluid.

The spinal cord is slightly swollen in the branchial and lumbar regions and ends posteriorly in a narrow cone, the conus terminalis from which arise a tuft of nerves, forming the filum terminate.

'*(C) '***Cranial Nerves:**

There are twelve pairs of cranial nerves, of which the first ten are similar in name, origin and distribution to those of the toad.

**The additional nerves are:**

(XI) the spinal accessory and

(XII) the hypoglossal.

Both are motor nerves, arising from the posterior part of the medulla.

XI. The spinal accessory is a short nerve which passes almost vertically downwards to innervate certain muscles of the neck region.

A ramus internus from spinal accessory joins the vagus of the same side, to be distributed to the muscles of the pharynx and larynx.

XII. The hypoglossal (Fig. 162) is a stout nerve which arises by a number of roots.

Near the angle of the jaw, die nerve crosses the internal carotid artery and iplits into branches for supplying the muscles of the tongue.

The distribution of the vagus nerve is slightly different in the neck region of the guinea-pig (Fig. 162).

The vagus comes out of the skull and expands to form a large vagus ganglion.

Just beyond this, it gives off a depressor branch to the heart, and an anterior laryn­geal branch to the larynx.

The main trunk of the vagus now runs ventral to the aortic arch on the left side and the subclavian artery on the right side.

It then gives off a recurrent laryngeal branch, curves round the aorta on die left side, and round the subclavian artery on the right side.

Both recurrent laryngeals ultimately turn forwards to enter the larynx.

Supplying a branch to the lung of its own side, each vagus now swings backwards into the abdominal cavity and is distributed to the visceral organs.

**(D) Spinal Nerves:**

These are built on the same plan as those of the toad, but are much more numerous.

There are about 32 pairs of spinal nerves, each of which arises by two roots.

The dorsal root is ganglionated.

The last four cervical nerves and the first thoracic nerve unite to form the brachial plexus for supply­ing the forelimb.

Similarly the last two lumbar nerves and the sacral nerves form the sciatic plexus for supplying die hindlimb.

The fourth and fifth cervical nerves unite on each side to form phrenic nerve for innervating the diaphragm muscle (Fig. 162).

**(E) Sympathetic or Autonomic Nervous System:**

Run­ning on each side of the aorta, and lying on the ventral surface of the vertebral column there is a ganglionated sympathetic nerve trunk.

It receives, at each ganglion, a small ramus communicans from the spinal nerve of that segment.

It traverses through the thorax and in the root of the neck swells up to bear a posterior cervical ganglion.

Further forward each sympathetic trunk bears an anterior cer­vical ganglion, which lies just dorsal to the vagus ganglion of the side (Fig. 162).

It then accompanies the inter­nal carotid artery and enters the skull to end in the Gasserian ganglion of the V nerve.

The main trunk on each side is connected with the’ trigeminal glosso­pharyngeal vagus and hypoglossal nerves.

The sympathetic nervous system innervates all the viscera and regulates the activity of the involuntary muscles.

It sends communicating branches to the sympathetic plexus on the heart as well as on other viscera.

'*10. Receptor Organs of Guinea-Pig'***:**

The receptors for touch, pain and temperature occur in the superficial layers of the skin, either as free nerve endings or as encapsulated corpuscles.

Chemoreceptors for taste are found on the papillae of the tongue as groups of cells forming the taste buds.

Receptors for smell are distributed as olfactory cells in the mucous membrane of the nose.

**Eye:**

The eye is the organ of vision. It is essentially camera type and is built on the same plan as that of the toad.

**The eyeball is more or less rounded and is composed of three coats:**

(1) Sclerotic or sclera,

(2) Choroid and

(3) Retina.

The sclera is com­posed of dense fibrous tissue and forms a pro­tective covering for the eyeball.

On the exposed outer surface of the eye, the sclera is transparent and is known as the cornea.

The cornea covers the pupil and the iris which lie jump behind.

The externally visible part of the eye­ball is covered by a delicate transparent membrane, the con­junctiva, which is continuous with the internal lining of the eyelids.

A lacrimal gland is found on the upper part of each eyeball, which secretes tear for lubricating and washing the surface of the eye.

The excess of tear drains into the nose through a small foramen on the lacrimal bone.

The choroid is a darkly pigmented membrane which lies internal to the sclera.

It makes the eyeball light-proof. In front of the eye and a little behind the cornea, the choroid forms a circular disc, the iris, which is perforated by the pupil.

The pupillary aperture may decrease and increase like the iris diaphragm of a microscope, thereby regulating the amount of light entering the eye.

The action is effected by radial and circular muscle fibres which are present in the iris.

The biconvex lens lies behind the pupil.

It is held in position by a circular fold, the suspensory ligament.

Accommodation or focussing is brought about by ciliary muscles which ran alter the curvature of the lens.

The retina is the innermost coat which lines the interior of the eyeball.

It extends anteriorly up to the ciliary muscles with which it forms the ciliary body.

It is the light-sensitive screen of the eye and contains photoreceptors, the rods and cones.

There are three kinds of cone cells for perceiving red, green and violet colours.

The rods perceive varying intensity of light by producing a peculiar substance called visual purple.

The rod cells and cone cells are connected to the fibres of the optic nerve, which spread out as a thin layer on the inner surface of the retina.

The point through which the nerve enters the eye is known as the ‘blind spot’, because it is devoid of rods and cones.

The eyeball encloses a more or less spherical cavity which is divided into two chambers by the lens and iris.

The chamber between the cornea and the lens is filled with a thin watery fluid called aqueous humor, and that between the lens and the retina by a thick jelly-like substance called vitreous humor.

Bodi the humors are transparent.

Light is focussed upon the retina in the same manner as in toad.

When the rods and cones are stimulated, impulses are set up in them which are transmitted along the optic nerves to the brain.

Vision is monocular.

Accommodation or focussing is effected by altering the curvature of the lens and not by changing its position.

**Ear:**

The ear is the organ of hearing and balancing. It includes the external ear, the middle ear and the internal ear.

The external ear is composed of the pinna and the external auditory meatus.

The eardrum or tympanic membrane is stretched at the far end of the auditory meatus.

The pinna is movable; it serves to collect sound waves and transmits the same down to the eardrum. The middle ear lies behind the tympanic membrane.

It is enclosed within the tympanic cavity of the tympanic bulla the Eustachian tube connects the middle ear to the pharynx, thus equalising the pressure on the two sides of the eardrum.

The middle ear includes the three auditory ossicles, namely the malleus, the incus and die stapes.

Of these the malleus is connected to the tympanic membrane and die incus.

The incus is articulated to the stapes, and the latter is attached to an opening in the wall of the internal ear.

This opening is known as the fenestra ovalis.

The audi­tory ossicles serve to conduct the sound waves from the tympanic membrane to die internal ear.

The internal ear is enclosed within the tympanic, bone.

Its essential part is the membranous labyrinth which is surrounded by the corresponding bony labyrinth.

The cavity of the bony labvrindi is filled by a fluid called perilymph, whereas the membranous labyrinth itself is filled with endolymph.

The membranous labyrinth of guinea-pig is essentially built on die same plan as that of the toad.

It consists of: the utriculus, the semicircular canals, the sacculus and the cochlea.

The three semicircular canals, anterior, posterior and horizontal, are perpendicular to one another.

Each semicircular canal opens into the utriculus by both ends, but bears an ampulla at only one end.

The utriculus and the sacculus are separated by a narrow constriction.

The sacculus is connected with a long, spirally coiled cochlea which contains the organ of Corti.

The latter is composed of receptor cell specially modified for perceiving sound waves.

The auditory nerve breaks up into branches which are distributed to the organ of Corti and to other receptor cells within the sac­culus, utriculus and ampullae of die semicircular canals.

Changed position of the body in any of the three planes causes flow of the endolymph in one or more semicircular canals.

The re­ceptors within the ampulla perceive these movements and the im­pulse is transmitted to the brain via the auditory nerve.

The animal at once assumes correct posture.

Receptor cells of the utriculus and sacculus probably perceive the direction of the gravitational force.

Sound waves are caught by the pinna and transmitted to the tympanic membrane.

The vibration of the latter is transmitted to the internal ear through the auditory ossicles and cause correspond­ing vibrations of the perilymph.

These are perceived by the organ of Corti and transmitted to the brain where they are translated as sounds.

The ear of the guinea-pig differs from the amphibian ear in three important particulars.

First, there is an well-developed pinna for collecting sound waves.

Secondly, the middle ear is traversed by three ossicles, the malleus, the incus and the stapes; in toad, there is a single columella which serves the same purpose.

Thirdly, the sacculus gives out a spirally coiled cochlea containing organ of Corti.

'*11. Endocrine System of Guinea-Pig'***:**

The principal ductless glands are the thyroid, parathyroid, thymus, pituitary, adrenal and spleen.

The pancreas and the gonads produce endocrine secretions in addition to their exocrine functions.

The thyroid is a soft bilobed structure situated at the anterior end of the trachea, behind the thyroid cartilage of the larynx. It consists of a collection of vesicles enclosed in connective tissue.

The vesicles are lined by a single layer of cubical epithelium.

The thyroid vesicles secrete the hormone thyroxine which has now been successfully isolated and synthesised.

It is an iodine-containing derivative of amino acid.

The thyroxine stimulates growth and metabolism. If the gland is removed from a young animal, the development is defective and the individual becomes a dwarf.

The parathyroids are two pairs of globular bodies closely associated with the thyroid, from which they are difficult to sepa­rate.

It is believed that the parathyroid hormone regulates bone formation.

It has a controlling influence over the metabolism of calcium and phosphorus both of which are important constituents of bone.

The thymus is a fairly large soft pinkish mass, lying within the thorax encircling the base of the aorta.

It is relatively larger in the young and may then partly cover the heart.

It is composed mainly of lymphoid tissue and its secretion retards the onset of premature sexual maturity.

The pituitary body is attached to the floor of the diencephalon and lodged in a pit at the base of the skull.

Its function has been fully discussed when considering this organ in the toad.

The adrenals are conspicuously yellow-coloured bodies com­posed of two distinct zones—the cortex and the medulla.

Each adrenal lies in front of a kidney.

The cortical hormone is essential for maintaining life, whereas the medullary hormone, adrenaline, controls the activities of the sympathetic nervous system.

The islet of Langerhans are microscopic patches of insulin- secreting tissues embedded in the pancreas.

The gonads, in addition to producing germ cells, act as duct­less glands. The functions of these are the same as the correspond­ing structures in the toad.

The spleen, though not secreting any hormone, is a ductless gland. It is a dark-red, elongated body attached to the dorsal sur­face of the stomach by a fold of mesentery.

Spleen is mainly com­posed of lymphoid tissue which forms a 20ft mass, the splenic pulp.

The spleen is a reservoir for the blood and manufactures blood corpuscles.

Worn-out corpuscles are destroyed in the spleen.

'*12. Urinogenital System of Guinea-Pig'***:**

As in toad, the excretory organs are closely related to the re­productive organs, and the two sets of organs are described together as the urinogenital system.

In the male guinea-pig, urine and germ cells are voided through a common passage, the orethra.

But unlike toad, this urethra is never associated with the gut to form a cloaca, and opens to the exterior separately.

'*13. Excretory System of Guinea-Pig'***:**

The excretory organs consist of a pair of kidneys, a pair of ureters, the urinary bladder and die urethra.

The kidneys are situated in the anterior part of the abdominal cavity, attached to the dorsal body wall, and behind the peri­toneum.

Each kidney is bean-shaped, having on its inner border a concavity called hilus.

Projecting into the hilus is a small protu­berance, the pyramid.

The renal artery enters and the renal vein leaves die kidney at die hilus.

The cavity of the hilus is continued backwards as the ureter.

Secretion of a kidney reveals that the kidney substance is distinguishable into two parts, an outer dark-red cortex, and an inner striated medulla.

The kidney is essentially composed of uriniferous tubules and collecting tubules.

The collecting tubules arc straight and they open into the hilus on the summit of the pyramid.

They run radially through die medulla to the cortex where each receives a number of uriniferous tubules.

Each uriniferous tubule beign sin the cortex as a cup-shaped Malpighian body re­sembling that of the toad.

It then runs a tortuous course to join a collecting tubule in the medulla.

The urine is form­ed in the same manner as in the toad.

It passes through the uriniferous and collecting \* tubules into the cavity of the tiilus which is continu­ous with the proximal end of the ureter.

The ureter runs backward as a white tube to open dorsally into the neck of the urinary bladder.

The uri­nary bladder lies ventrally in the posterior part of the abdominal cav­ity.

It is a pear- shaped muscular sac in which the urine is held until its evacuation is convenient.

The broad anterior end or fundus of the urinary bladder gradually tapers back into the narrow neck which is continued into the urethra.

The urethra is a narrow canal which, in the male guinea-pig, receives the genital ducts dorsally.

It then traverses through the penis opening to the exterior at the tip of the copulatory organ.

In the female guinea-pig, the urethra is short and opens into the vulva very close to the clitoris.

The female urethra is solely urinary in function, whereas the male urethra is a channel for the voiding of urine as well as germ cells.

'*14. Reproductive System in Guinea-Pig'***:**

The reproductive system of guinea-pig includes the organs of reproduction, namely, the testes in the male and the ovaries in the female, and their ducts.

**Male Reproductive Organs:**

There is a pair of ovoid testes which in the young guinea-pig lie close to the kidneys in the ante­rior part of the abdominal cavity.

As the animal approaches mature age, the testes pass backwards and ultimately come to lie within a pouch-like fold of skin at the root of the penis.

This pouch is the scrotal sac.

It communicates with the abdominal cavity by narrow passages, the inguinal canals, through which blood vessels and nerves from the abdomen pass into the testis.

Each testis is suspended from the scrotal sac by a short cord called gubemaculum.

A vas deferens arises from each testis.

The first part of the vas deferens is much-coiled and is bound up together to form the epididymis.

The epididymis encircles the inner side of the tests.

The spermatozoa which develop in the testis are released through the convoluted tubules of the epididymis into the vas deferens.

From the posterior end of the epididymis the vas deferens leads away as a straight tube.

It runs through the inguinal canal where it lies enclosed along with the spermatic vessels and nerves to form the spermatic cord.

Entering the abdomen, the vas deferens curves round the ureter of its own side and runs to the dorsal sur­face of the neck of the bladder.

Here the two vasa deferentia lie side by side.

Finally, they open into the base of a thin-walled, bifurcated diverticulum, the uterus masculinus, which is a hollow, median sac arising from the commencement of the urethra.

The uterus masculinus corresponds to the vagina of the female guinea- pig

The terminal part of die urethra, traverses through the penis openings outside at its tip.

The penis is the male copulatory organ.

It is enclosed in a loose fold of skin, the prepuce and pro­jects ventrally in front of the anus.

It consists of two corpora caver­nosa and the single corpus spongiosum.

All the three are composed of highly vascular erectile tissue.

The urethra runs through the corpus spongiosum.

There are a few accessory glands the secretion of which nou­rishes the male germ cells.

Surrounding the neck of the bladder and the base of the uterus masculinus, there is a compact pros­tate gland which opens into the urethra.

Behind the prostate are a pair of small Cowper’s glands which open into the urethra as it enters the penis.

Two perineal glands are found near the root of the penis, one on each side.

They produce a secretion which gives the animal its characteristic smell.

**Female Reproductive Organs:**

The ovaries are small ovoid bodies, one lying behind each kidney.

They are attached to the dorsal body wall by folds of peritoneum and retain their abdo­minal position throughout life.

The surface of the ovary is covered with raised vesicles of different size, the Graafian follicles, each of which encloses an egg or ovum.

Near each ovary lies the funnel-shaped open­ing of an oviduct.

Mature eggs burst out from the follicles and are caught by the adjacent oviducal funnel.

The first part of the oviduct is narrow and twisted.

It is known as the Fallo­pian tube.

The second part of the oviduct is wide and muscular.

This is the uterus, where early development of the embryo occurs.

The two uteri unite to form a median tube which continued back- is wards behind the neck of the urinary bladder as the vagina.

This is the third part of the oviduct.

The uteri and the fallopian tubes are attached to the body wall and held in place by a fold of peritoneum known as the broad ligament.

The vagina communicates with the exterior through a slit-like vulva which is bounded laterally by raised folds, called labia.

Just internal to the vulval outlet there is a very short vestibule containing the urinary opening in its antero-median part and the vaginal opening at the postero-median part.

There is a small rod-like clitoris closely associated with the urinary opening.

The clitoris corresponds to the penis of the male guinea-pig.

**15. Breeding and Life-History of Guinea-Pig:**

There is a periodic ripening of the eggs during the breeding season.

When ripe, the eggs are discharged by the bursting of the Graafian follicles.

These are caught by the oviducal funnels and led into the Fallopian tubes.

This process is known as ovulation.

It occurs spontaneously after copulation.

During copulation sperms are injected into the body of the female by the male.

The sperms swim up the vaginal canal into the uteri and thence into the Fallopian tubes where they meet the ova.

Fertilization now takes place—a sperm uniting with an ovum.

The fertilized over descend into the uteri, getting attached to the uterine wall by a peculiar tissue called placenta.

In the meantime, a blood clot develops inside each Graafian follicle from which the ovum has been extruded.

Each blood clot is transformed into an endocrine gland, the corpus luteum, secreting a hormone, known as the progestin, which prevents further ovulation and brings about certain somatic changes associated with conception.

But if the ova are not fertilized the corpus luteum dries up and is replaced by scar tissue.

Development proceeds within the uteri, each embryo becoming attached by a stalk-like umbilical cord to the placenta.

The placenta is a highly vascular mass of tissue derived partly from the embryo and partly from the mother.

It is the organ by means of which nutriment and oxygen are com eyed from the mother to the developing embryo, and CO2 and nitrogenous waste products are removed.

Development is completed in the course of about ten weeks—-this interval being known as the period of gestation.

It is followed by parturition during which the young is expelled to the exterior through the vulva by the rhythmic contraction of the wall of the uteri and the vagina.

This type of reproduction, wherein the young are retained within the body of the mother, is known as viviparous reproduction.

Compared to this, the toad and the fish are oviparous, because they lay eggs which subsequently develop into the young, outside the body of the mother.

Usually two or three young are born in a litter.

They are nourished by a milky secretion produced in the mammary glands of their mother for a few days, after which they commence nibbling at green vegetables.

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