Cavia: Habitat, Structure and Sense Organs | Mammals

In this article we will discuss about cavia:- 1. Introduction to Cavia 2. Habit and Habitat of Cavia 3. Structures 4. Sense Organs.

Introduction to Cavia:

Cavia porcellus is the scientific name of guinea-pig. Guinea-pig is a misnamed animal. Neither it hails from the island of Guinea nor it is a pig belonging to the family Suidae. The place of origin of guinea-pig is obscure. It comes under the Order Rodentia. Guinea-pigs are much in demand as useful laboratory animals.

Systematic Position [According to Simpson, 1945]:

Phylum Chordata

Subphylum Vertebrata (= Craniata)

Superclass Gnathostomata

Class Mammalia

Subclass Theria

Infraclass Eutheria

Cohort Glires

Order Rodentia

Suborder Hystricomorpha

Family Caviidae

Scientific Name Cavia porcellus

Habit and Habitat of Cavia:

Guinea-pigs have been made domestica­ted. Wild guinea-pigs are almost cosmopolitan in distribution. Little information is available of its habits in the wild. They are crepuscular, avoiding bright sunlight and darkness. They generally prefer marshy land and live on wild fruits and fresh green vegetation. Guinea-pigs are in the habit of passing food twice through the alimentary canal (Caecotrophy).

Faecal pellets passed in the daytime are of dry and hard consistency. But those passed at night are soft with much mucus in them. The guinea- pigs are in the habit of taking these nocturnal faeces and as such they are called coprophagus animals. They ate terrestrial in true sense and cannot climb.

Being poor burrowers they take shelter in naturally formed crevices or burrows dug by other animals. They maintain a territory within the limits of which; live a dominant male, several females and: young ones. The average life-span of guinea-pigs is less than eight years.

Structures of Cavia:

External Structure:

The length of the Cavia is about eight inches from mouth to vent. The body is cov­ered over with variously tinged soft hairs. Certain areas of the body are devoid of hairs. The body is divisible into three distinct parts, head, neck and trunk (Fig. 10.2). The tail is absent, but the tail bone is perceptible at the posterior end of the trunk.

Following structures are found in the three different parts of the body:

Head:

The head is short and more or less trian­gular in appearance. Anteriorly, it ends in a pointed snout. The snout is devoid of hairs and bears at its tip a pair of apertures, called exter­nal nostrils. Round the nostrils there are seve­ral stiff and sensitive whiskers called, vibrissae. The vibrissae are special long hairs that serve as tactile organs. According to locations they are arranged in seven groups.

The aper­ture of mouth is located beneath the nostrils. It is bounded by upper and lower lips. The upper lip has a cleft in the middle through which the upper incisors protrude.

Round and large eyes are present, one on each lateral side of the head. Each eye is provided with three eye­lids—upper, lower and the third or nictitating membrane which is vestigial. Upper and lower eyelids are movable and are provided with short hairs except at their margins.

Nictitating membrane is a whitish transparent movable curtain oriented obliquely in front of the eyeball. On each posterior lateral side of the head an elongated flap-like movable fold of skin, called pinna, is present. The pinna is cartilaginous. It stands erect in young animals but droops forward in older ones.

Neck:

The neck is short and flexible. It merges imperceptibly with the trunk. It is a sort of bridge between the head and trunk. It permits independent movement of the head.

Trunk:

The cylindrical trunk is divisible into two parts, thorax and abdomen. Thorax is sup­ported by ribs and sternum but abdomen has no such bony support on the ventral side. The trunk bears paired mammary glands at its posterior ventral part. Each gland opens externally through a teat or nipple (Fig. 10.3A). In males the nipples are poorly developed.

Anus is present as a small aper­ture on the posterior end of the trunk. Beneath the anus the urinary and genital apertures are located. In males there is a common opening for both the systems and the opening is present at the tip of a muscu­lar organ, called penis. In the female the gen­ital and urinary openings are separate.

Limbs:

The trunk is provided with two pairs of limbs—forelimbs and hind limbs. The hind limbs are larger than the forelimbs. There are four digits in each forelimb and three dig­its in each hind limb. The digits of the manus correspond with the digits II to V of the typi­cal pentadactyl appendage while the digit of the pes corresponds with digits II to IV.

The digits end in claws. The claws of the fore­limbs are short but those of the hind limbs are long, slightly curved and extend beyond the distal phalanges. The hairless soles are pro­vided with well-defined foot pads.

The manus is provided with a three-lobed palmar pad behind which lies a carpal pad (Fig. 10.3B). The pes houses a bilobed plantar pad and a cresentic tarsal pad proximal to it (Fig. 10.3C). Forelimbs are used in burrowing and hind limbs are used in leaping during loco­motion.

Skin:

The skin on Cavia is made up of two layers—epidermis and dermis (Fig.10.4).

a. Epidermis:

Epidermis is the outer layer and is ectodermal in origin while the inner dermis is mesodermal in origin.

The epidermal part of the skin is made up of the following layers:

Stratum corneum:

It is the outer horny layer and is made up of dead cells. The horny tex­ture is due mainly to the formation of a protein, called keratin.

Transitional layer:

Beneath the stratum corneum, a layer, called transitional layer, is present. It is made up of flat cells and the cells are derived from the layer situated beneath it.

Stratum germinativum:

The inner­most layer of the epidermis is known as stra­tum germinativum or Malpighian layer. The cells of this layer are polyhedral in appearance and contain conspicuous nucleus and gran­ules. These cells divide mitotically to form the upper layers.

b. Dermis:

The dermis is made up of loose connective tissue, blood vessels and nerves. In the dermis the following structures are present: Hair follicle: Epidermis pene­trates into the dermis and forms small pocket, called hair follicle. Hair papilla is located in the inner end of the follicle and the hair emerges from the papilla. Smooth muscles, called erector muscles, are associated with the papilla. These muscles raise the hair.

Sebaceous glands:

These are flask-shaped glands and they open into the hair papilla. The secretion of these glands is oily in nature and the name of the secretion is sebum. Sebum keeps the hairs oily.

Sweat glands:

These are much coiled tube-like glands. The glands open to the out-side through the epidermis. Secretion of these glands is known as sweat. Sweat is watery in nature. It helps in regulation of the body tem­perature and excretion.

Mammary glands:

These tubular and branched glands are well-developed and func­tional in adult females. The secretion of these glands is the milk which is taken by the suck­ling ones.

Coelom:

The body cavity or coelom of Cavia is transversely separated into the compartments by a muscular partition, called diaphragm. The anterior compartment is known as tho­racic cavity and it houses the heart and lungs.

The posterior cavity is known as abdominal cavity which contains stomach, liver, intestine, kidneys, gonads, etc. The abdominal cavity is lined internally by peritoneum which becomes folded to form mesenteries to sup­port the abdominal organs. The diaphragm is convex at its anterior surface and concave at its posterior surface.

Skeletal Structures:

Exoskeleton:

Cavia possesses well-developed exoskeletal structures in the form of hairs and claws. The hairs cover the body and the claws are present at the digital tips. As already described, hair develops in hair-follicle. Each hair is a slender rod-like structure having a central core {pith) and an outer solid layer (cortex). The claws are typically reptilian in construction.

Endoskeleton:

The entire internal skeleton of Cavia except certain parts in the nose and ear is fully ossified. Cartilage is present at the tip of cer­tain bones. The skeletal system is divided into (1) axial, (2) appendicular and (3) heterotopic skeleton (skeletons involving bones of unusual location).

The number of bones varies with age because some sort of osteologic fusion occurs in the adult. In a mature individual there are 256 to 261 bones.

Axial skeleton:

The axial skeleton includes the:

(a) Skull with jaws, hyoid bones and tympanic ossicles,

(b) Vertebral column,

(c) Sternum and

(d) Ribs.

Skull:

Three firmly united parts constitute the skull.

These parts are:

(a) Cranium,

(b) Sense capsules and

(c) Visceral skeleton.

Cranium:

Cranium or the brain box is made up of several well-defined skeletal pieces. These pieces are united with each other and distinct sutures are present between these pieces. The skeletal pieces that make up the cranium are:

(i) Occipital bone:

This bone encircle the foramen magnum. This unpaired bone is formed by the supraoccipital bone located at the top and below it lies the basiocapital. There are two exoccipital bones loca­ted one on each lateral side. Each exoccipital is drawn back as pointed and curved paroccipital process,

(ii) Occipital condyle:

There are two occipital condyles (Fig. 10.5B) located one on either side of the foramen magnum. The condyles act as articulators between the skull and first vertebra,

(iii) Parietal bones:

Anterior to the supraoccipital there are paired rectangular parietal bones forming the roof of the cranium. A triangular inter-parietal bone remains present in young stage of the guinea- pig. This, however, becomes absent in adults by being fused with the parietals.

(iv) Frontal bones:

The anterior most portion of the roof of the cranium is formed by a pair of frontal bones (Fig. 10.5A). The outer border of each frontal bone is concave. The dorsal wall of the orbit is formed by it.

(v) Sphenoid bone:

Unpaired and roughly butterfly shaped bone forms the floor of the brain case. Its anterior end is called presphenoid and posterior end is called basis phenoid’. The lateral Walls of the cranium are formed by alisphenoid and orbitosphenoid (Fig. 10.6).

(vi) Squamosal:

Two squamosal’s, one on either side of the lat­eral wall of cranium are present. The squamosal’s take part in the formation of zygo­matic arch,

(vii) Cribriform plate:

The inner anterior wall of the cranium is formed by a cribriform plate. The plate is perforated and separates the cranium from the olfactory cap­sule.

Sense capsules:

Three different sense cap­sules are present in the skull. The capsules are paired and house eyes, ears and nostrils.

The architecture of the sense capsules is discussed below:

(i) Olfactory capsule:

The roof of the olfactory capsule is formed by a pair of nasals. The posterior lateral wall of the capsule is made up of maxilla and the anterior lateral wall is formed by premaxilla. Both maxilla and premaxilla are parts of the visceral skeleton.

A vomer is present along the mid-ventral line and posteriorly it articulates with the presphenoid. The two nasal cavities are separated from each other by a vertically placed cartilaginous plate, called mesethmoid. As supporting structures for the olfactory epithelium there are small and spongy bones, called turbinals present inside the nasal cavity,

(ii) Optic capsule:

The anterior wall of the capsule is formed by a small bone, called lacrymal. It is perforated by a small lacrymal foramen. The ventral wall of the orbit is formed by the zygomatic arch.

The maxilla, jugal and squamosal bones take part in the formation of the arch. An inter-orbital septum forming the inner wall as a partition between the two orbits is present. The upper part of the septum is formed by orbit osphenoid and the lower part is formed by presphenoid.

(iii) Otic capsule:

Covering the membranous labyrinth, a bone, called periotic bone, is pre­sent. The shape of the bone is irregular and is formed by the fusion of three small bones. A tympanic bone remains firmly attached with the outer surface of the periotic.

The tympanic bone is flask-shaped and base of the flask is called tympanic bulla while the neck is called external auditory meatus. Three small bones malleus, incus and stapes are jointly known as auditory or ear ossicles. These are housed inside the tympanic bulla.

Visceral skeleton:

The first and second pair of visceral arches becomes modified to form the upper jaw, lower jaw and also help in the articulation of the jaw with the skull. Each half of the upper jaw is composed of three pieces of bones. They are premaxilla, maxilla and jugal. Teeth are borne by the premaxilla and maxilla. The roof of the mouth cavity is known as palate. The palate is formed by the inward projection of premaxilla, maxilla, a pair of palatines and a pair of pterygoids.

Lower Jaw:

The lower jaw or mandible is the largest bone of the skull and is made up of two incom­pletely united halves. Each half of the mandible is composed of a single piece of bone, called denary, having a body and ramus (non-tooth- bearing part). Teeth are present on the jaw bone.

The posterior broad part or ramus of the mandible is provided with three projections— called coronoid, condylar and angular process­es (Fig. 10.6). The mandible is articulated with the squamosal of the upper jaw at the posteri­or region of the zygomatic arch.

Hyoid Apparatus:

The hyoid apparatus is embedded in the base of the tongue. It consists of a median body and two pairs of projections, called cornua (greater and lesser cornu). The body is ‘U’-shaped (Fig. 10.7).

It consists of craniodorsally oriented basihyoid and two bony sheets form­ing wings or alae. The greater cornu extends from the dorsolateral process of basihyoid and articulates with the tympanic bulla.

It consists of the following parts craniocaudally—epihyoid which is oval in shape; stylohyoid which is long and slender, and tympanohyoid which is cartilaginous. The lesser cornu extends as car­tilaginous thyrohyoid dorsomedially from the basihyoid and articulates with the thyroid car­tilage of larynx.

Vertebral Column:

The vertebral column is made up of vertebrae arranged in a linear fashion. The vertebrae are divisible into five distinct types. They are: (a) cervical, (b) tho­racic, (c) lumbar, (d) sacral and (e) caudal. A cartilaginous pad, called intervertebral disc is present between the successive vertebrae.

There are 36-38 vertebrae, some of which are fused in the sacrum. The caudal vertebrae are reduced. The number of vertebrae is: cervicals = 7, thoracics = 13-14, lumbars = 6, sacrals = 3-4 and caudals = 7.

Structure of a typical vertebra:

A typical vertebra is built upon the same plan as in other vertebrates.

The specialised features are:

(i) Centrum is acoelous, that is flat on both sur­faces.

(ii) Neural arch is with distinct upper lamina and lower pedicel. The pedicel is notched to form the intervertebral foramen through which emerges the spinal nerve,

(iii) Prezygapophysis and postzygapophysis are mounted on specially developed metapophysis and anapophysis.

Cervical vertebrae:

Cervical vertebrae are seven in number. In general, all the cervical vertebrae are with small centrum and short neural spine (Fig. 10.8D). The craniocaudal base of the transverse process is perforated by a vertebrarterial foramen through which the carotid artery passes.

It is also pierced dorsally by the alar foramen. The first cervical vertebra is known as atlas (Fig. 10.8.A, B.) It is ring-like and its centrum is insignificant. Its neural spine is very small and the transverse process is small and flat. The zygapophyses are absent. The anterior surface is provided with a pair of facets for receiving the occipital condyles (Fig. 10.8 A).

The neural canal is divided horizon­tally by a fibrous ligament. The second cervi­cal vertebra is known as axis. The centrum of the axis is broad and drawn anteriorly into a peg-like odontoid process or dens of axis (Fig. 10.8C). Its neural spine is compressed and the spine extends anteroposteriorly.

The transverse processes are small with perforated base. The axis articulates anteriorly with the atlas through a stout elongated odon­toid process (Fig. 10.9). The odontoid process rests in the dens fossa of the atlas. The remaining cervical ver­tebrae vary slightly. The transverse processes are stubby, those of C4, C5 and C7 are bifid while those of C6 bearing three terminal prominences.

Thoracic vertebrae:

Thoracic vertebrae are 13-14 in number and have been grouped into two series:

(i) Anterior thoracic vertebrae and

(ii) Posterior thoracic vertebrae.

Anterior thoracic vertebrae:

The first ten thoracic vertebrae (T₁-T₁₀) are called anterior thoracic vertebrae. These vertebrae are pro­vided with small and compact centrum and concave facets on the sides for the articulation of the head of capitulum of the rib.

The neural spine of the vertebra is elongated, narrow and directed backward. The transverse process is strongly built and bears a facet on the ventral surface for articulation with the head or tuberculum of rib (Fig. 10.8E).

Posterior thoracic vertebrae:

The remai­ning (T₁₁-T_13/14) thoracic vertebrae are called posterior thoracic vertebrae. The transverse processes of these vertebrae are small and lack facets. The centrum has lateral facets for artic­ulating the ribs. The neural spine is flat side- wise and directed forward.

Lumbar vertebrae:

Lumbar vertebrae are six in number. The vertebrae are large and the size increases from anterior to posterior end. The vertebrae are with well-developed zygapophyses, metapophysis and anapophysis (Fig. 10.8F, G). The neural spine is flat and directed anteriorly.

The first and second lum­bar vertebrae are provided with ventromedian ridges. The ridge is called hypapophysis. The muscles from the diaphragm rest on it. The transverse processes are large and expanded.

Sacral vertebrae:

The 3-4 sacral vertebrae are fused and have formed the sacrum (Fig. 10.8 H). The sacrum is placed between the two halves of the pelvic girdle in a compact fashion. The first sacral vertebra is the largest and the size decreases from anterior to poste­rior end.

The neural spine of sacral vertebra is straight. Its transverse process is flat and later­ally expanded. The zygapophysis is reduced, anapophysis is absent and metapophysis is much reduced. The adult male has four sacral vertebrae, the adult female has three.

Caudal vertebrae:

Caudal or coccygeal vertebrae are seven in number and the size of the vertebrae decreases gradually from anteri­or to posterior end. There is a gradual reduc­tion of the associated parts of the vertebrae and the last two vertebrae have nothing but rod-like centra.

Sternum:

Sternum is present in the mid- ventral line of the thorax as an elongated and narrow rod. It consists of 6 or 7 separate bony elements joined by intersternebral cartilage. The bony elements are—Presternum, Manubrium, Sternebrae (3-4) and Xiphisternum in craniocaudal arrangement.

The first presternum is the largest and it articulates with the ill-developed clavicles. All but the last sternebrae are provided with articulating surfaces for the ribs. The tip of the xiphisternum is provided with a cartilaginous plate, called xiphoid cartilage.

Ribs:

There are thirteen pairs of ribs. Occasionally, fourteenth pair presents. The first six pairs of ribs are extended between the vertebral column and sternum. They are called true ribs. Ribs (7-9) are false ribs which arti­culate with the 6th rib cartilage. The remain­ing ribs are either with indirect or without any connection with the sternum. They are called floating ribs.

A typical rib is made up of two parts—a bony vertebral part and a short and cartilaginous sternal or costal part. The first nine ribs articulate with the vertebral column by two heads—the capitulum is articulated with the centrum and the tuberculum is arti­culated with the transverse process of the ver­tebrae.

The tuberculum is absent in the last three ribs and, as such, they cannot articulate with the transverse process. Fig. 10.10 shows the structure of true rib of cavia.

Appendicular skeleton:

The pectoral and pelvic girdles together with forelimb and hind limb bones make up the appendicular skeleton (Figs. 10.11-10.13).

Pectoral Girdle:

The pectoral girdle is small and no hard structure is present in between the two halves of the girdle. Ossification of the girdle is almost total. Each half of the girdle is made up of scapula and clavicle. The scapula is large and triangular in shape (Fig. 10.12A). Its apex is provided with glenoid cavity which articulates with the head of humerus.

A small coracoid process is pre­sent in front of the glenoid cavity. Suprascapula is present as a strip of cartilage along the broad margin of the scapula. On the outer region of the scapula, there is a bony projection or spine. The spine extends as acromion. Acromion gives out a downward process, called metacromion. The clavicle is slender, rod-like and reduced. It remains embedded within the muscles.

Forelimbs:

From proximal to distal, end, the forelimbs are made up of humerus, radius and ulna, carpals, metacarpals and phalanges (Figs. 10.11 A and 10.13 A, B). The humerus is strongly built. The proximal end of the humerus is round and has formed the head.

Greater and lesser tuberosities and bicipi­tal groove are located at the proximal end. A deltoid ridge runs along the side of the humerus. The distal end of the humerus is pro­vided with a pulley-like trochlea. The trochlea articulates with the radius and ulna. There are two depressions just above the trochlea. The depression located in front is called coronoid fossa and the one at the back is called olecra­non fossa.

A supratrochlear foramen runs between the depressions. Radius and ulna are two completely separated rod-like bones but the two bones lie close together. Radius is shorter than ulna and is located on the inner side. The proximal end of the ulna is provided with a sigmoid notch which fits with the trochlea of humerus.

The tip of the ulna is extended backward as olecranon process which forms the elbow point. The distal ends of both radius and ulna are provided with articulating surfaces. The carpal bones are nine in number and they are arranged in two rows.

Carpal bones are irregular, nodular and are with flat surfaces. The proximal row, when seen from the dorsal side, consists of radiate at the centre, ulnare at the left and centrale at the right of it.

The distal row contains four small bones—(Carpale l-IV). Carpale I is located at the right margin on the dorsal side. There are two sesamoid bones—Falciforme and Pisiforme. There are four metacarpal bones and three sesamoid bones forming the palm region of the forelimb. There are four digits and each digit is made up of three phalanges. The terminal phalanx in each digit ends in a claw.

Pelvic Girdle:

The pelvic girdle is made up of two strongly built halves (Fig. 10.12 B- C), each half being known as os innominatum. Each half is composed of three bones—ilium, ischium and pubis (Fig. 10.12C). Symphysis or union between the two halves occurs at the region of pubis. The ilium and ischium remain free. The symphysis is called pubic symphysis.

Ilium is long, blade-like and articulates with the sacrum along the side. Ischium is the pos­terior continuation of the ilium and forms the posterior one-third of the girdle. Pubis is small and curved.

One end of it is united with the ilium and the other end is united with the ischium. It occupies the ventral position. A large gap, called obturator foramen, is present between pubis and ischium. A socket, called acetabulum, is present. It is formed by the con­tribution of both ischium and pubis.

Hind limb:

The hind limb is made up of the following bones along its proximal to distal axis.

The bones are:

(i) Femur,

(ii) Tibia and fibula,

(iii) Tarsals,

(iv) Metatarsals and

(v) Pha­langes (Figs. 10.11 B and 10.13 C, D).

Femuris a long and stout bone. The proximal end of the femur has become round to form the head. The head fits in the acetabulum of the pelvic girdle. Three elevated areas, called greater trochanter; lesser trochanter and the third trochanter, are located near the head.

Two round condyles mark the distal end of the femur. The condyles are separated from each other by a patellar groove. Tibia and fibula are long rod-like bones.

They are united at their proximal and distal tips but are separated in the middle. Tibia is strong and well-developed and fibula is ill-developed and thin. A ridge, called cnemial crest, is present on the tibia.

Tarsals are eight in number and they are arranged in three rows. The proximal row houses two bones, called astragalus or Talus and calcaneum. The middle row consists of three bones—tibiale at the right, centrale at the middle and cuboid at the left (all seen from the dorsal side).

The distal third row consists of three Tarsale bones (Tarsale l-lll). The distal row houses three small tarsal bones. There are three long metatarsal bones forming the foot. Three digits are present in the hind limb. Each digit is made up of three phalanges. The ter­minal phalanx is provided with claw. Besides these there are two tarsal sesamoid, six metatarsal sesamoid and six inter-phalangeal sesamoid bones.

Heterotopic Skeleton:

Heterotopic skeleton involves bones of unusual locations. The different sesamoid bones present in the limbs are examples of heterotopic skeleton. Besides this the os penis within the glans is a heterotopic skeleton. It is a small, thin and dorsoventrally flattened bone.

Sense Organs in Cavia:

To receive different types of stimuli, there are different categories or receptors. The stim­uli in the form of touch, pain and temperature are received by numerous free nerve endings or encapsulated corpuscles which remain scattered within the superficial layer of the skin. Taste is determined by group of spe­cialised cells which remain within the papillae on the surface of the tongue.

These sensory papillae are called taste buds. Smell is per­ceived by specially sensory olfactory cells which are distributed in the mucous mem­brane of nasal cavity. Eyes and ears are much specialised receptor organs for receiving stim­uli in the form of light and sound respectively. Ears, in addition to hearing, are also responsi­ble for maintaining balance.

Eyes:

Eyes are built up in typical vertebrate plan (Fig. 10.38). The upper and lower eyelids are provided with small fine hairs. The edges of the eyelids are devoid of eye lashes.

Special features in the eyes of guinea-pig are:

(a) Presence of lacrymal gland on the upper side of each eyeball. It secretes a fluid, called tear which cleans and lubricates the surface of the eye.

(b) Lens is biconvex and focusing is done by changing the curvature of lens through the ciliary muscles.

(c) Retina pos­sesses both rod and cone cells. The former determines the intensity of light while the lat­ter adjudge colours. Fig. 10.39 gives an idea of the microscopic structures of retina.

Ear:

Each ear is divisible into:

(a) External ear,

(b) Middle ear and

(c) Internal ear (Fig. 10.40).

External ear:

It consists of a movable, flap­ like pinna and a canal, called external audi­tory meatus. Pinna is responsible for collect ting sound waves and sending it inside the ear through external auditory meatus.

Middle ear:

The external ear is separated from the middle ear by a tightly stretched membrane, called tympanum. The middle ear is in communication with buccal cavity by a canal, called eustachian tube. Within the mid­dle ear, tympanum is connected with the inter­nal ear by three ear ossicles—elongated malleus, slightly bent incus, and triangular, ring-like stapes.

Malleus is attached with the tympanum and stapes is attached with the opening in the wall of internal ear, called fenestra ovalis. The incus is present in between the two. The ear ossicles are respon­sible for carrying the sound waves to the inter­nal ear and the Eustachian tube is for regulat­ing the equilibrium of atmospheric pressure.

Internal ear:

It consists of a bony labyrinth which is filled up with a fluid, called peri­lymph. Within the perilymph a membranous labyrinth is suspended which contains another fluid, called endolymph. The membranous labyrinth consists of following parts— lower sacculus, at the top of which lies the utriculus with which three semicircular canals are con­nected.

The sacculus is drawn into a spirally coiled cochlea which contains special recep­tor cells, called organ of Corti. Fig. 10.41 shows the details of cochlea in cross-section. The bony cochlear canal encloses three cavi­ties. The scala vestibuli is bounded by the vestibular membrane and located above the cochlear duct. The scala tympani is situated below the cochlear duct.

The scala media is the cavity of the cochlear duct itself. The scala vestibuli and scala tympani are the perilym­phatic spaces, while the scala media is filled with endolymph. The Reissner’s membrane and basilar membrane are the demarcating partitions between the three cavities as shown in figure.

The organ of Corti, the receptor apparatus for hearing, is supported on the basilar membrane inside the cochlear duct. The organ of Corti is composed in differentia­ted cells arranged in orderly rows. The semicircular canals, utriculus and sacculus are responsible for balancing.