Cavia: Digestive, Respiratory, Circulatory and Nervous Systems in Cavia | Mammals

The following points highlight the top eight types of system in cavia. The types are: 1. Digestive System 2. Respiratory System 3. Circulatory System 4. Nervous System 5. Endocrine System 6. Excretory System 7. Reproductive System.

Cavia: Type # 1. Digestive System:

The digestive system is constituted by the alimentary canal and the digestive glands. The basic plan on which the digestive system rests is same as in other vertebrates but individual variations are present.

Alimentary canal:

It is a long tube star­ting from mouth to anus. But the tube is demarked into different regions. The function of each region is different.

Description of the different regions of the alimentary canal is given below:

Mouth:

The alimentary canal begins from the mouth. It is a transverse aperture and is guarded by two soft and movable lips. The upper lip is provided with a cleft in the middle.

Buccal cavity:

Mouth leads to the buccal cavity (Fig. 10.14). The roof of the buccal cav­ity is formed by a palate. The anterior portion of the palate is called hard palate. The sup­porting bones present at this portion make it hard. As no supporting bone is present, the posterior portion of the palate is soft and is known as soft palate. The palate separates the mouth cavity from the nasal passage.

The floor of the buccal cavity houses the tongue. The anterior end of the tongue is free and the pos­terior end is attached with the floor. The tongue is muscular and movable. The upper surface of the tongue is rough and contains numerous papillae or taste buds. Both the jaws are provided with teeth.

The teeth help in ingesting food. The teeth are thecodont, heterodont and diphydont. Dental formula is 1.0.1.3/1.0.1.3. The incisors are long, chisel-shaped and can be seen from outside. Canines are absent and, as a result, a gap, called diastema, has appeared between the incisor and premo­lar. A tooth has two parts, crown and root.

The crown is the visible part and the root remains embedded in a socket on the jaws. The tooth is made up of a substance, called dentine. The dentine of the root region remains covered by cement while the crown region of the tooth remains covered by hard and shiny enamel. Each tooth bears an inner pulp cavity. This cavity remains filled up with jelly-like pulp, blood vessels and nerves.

Pharynx:

Buccal cavity leads to another chamber, called pharynx. The dorsal part of the pharynx is called nasopharynx and the ventral part is called buccopharynx. Paired internal nostrils and Eustachian tubes enter into the nasopharynx region. The posterior margin of the soft palate extends into the naso­pharynx as velum. The two sides of the velum are with a peculiar lymphoid tissue, called tonsil.

A slit, called glottis, is present on the floor of the buccopharynx just posterior to the tongue. The tongue is a large, elongated mus­cular organ covering most of the floor of the mouth cavity (Fig. 10.14). The glottis commu­nicates with the respiratory tube and is guard­ed by a cartilaginous flap, called epiglottis. Posteriorly, the buccopharynx opens into the oesophagus through an aperture, called gullet.

Oesophagus:

Oesophagus is long tube running along the mid-ventral line of the neck region. It runs through the thoracic region and after passing through the diaphragm opens into the stomach.

Stomach:

Stomach is a highly muscular and glandular sac (Fig. 10.15). The inner con­cave side of the stomach is called lesser cur­vature and the outer convex surface is called greater curvature. The end of the stomach towards the oesophagus is called the cardiac end and its opposite end is called pyloric end.

Because of a twisting, the cardiac end has taken a position towards the left and the pyloric end has taken a position towards the right in the abdominal cavity. The opening at the pyloric end is guarded by a valve, called pyloric sphincter.

Intestine:

The remaining part of the ali­mentary canal is known as intestine. It is divi­sible into duodenum, ileum and large intestine. The duodenum begins from the pyloric end of the stomach and forms a ‘U’-shaped loop. The ileum is much coiled continuation of the duo­denum. The coiled loops of the ileum are held in position by folds of mesenteries.

The ileum opens into the large intestine and the opening is guarded by an ileocoelic valve. The large intestine is wide and is divisible into proximal colon and distal rectum. The colon is coiled and beaded in parts while the rectum is straight. A large blind sac, called caecum, is present at the point of opening of the ileum into the colon.

Anus:

The terminal part of the alimentary canal is represented by an aperture, called anus. The anus is guarded by sphincter mus­cle

The different digestive glands that help the process of digestion are listed below:

Salivary glands:

There are five pairs of salivary glands (Fig. 10.16). Parotid glands are located beneath the cutaneous muscles at the junction of mandible and neck. Mandibular glands are located on the ventral surface of the neck. Major sublingual glands lie ventromedi­al to the mandibular gland.

Minor sublingual gland is located between the last two lower molars and the tongue. Zygomatic or infra­orbital gland lies in the orbit along the dorsomedial rim of the zygomatic arch. All the glands have separate openings into the buccal cavity through ducts. The secretion of the sali­vary glands is known as saliva. Saliva helps in moistening food and contains an enzyme called ptyalin.

Liver:

It is a massive gland located beneath the diaphragm and above the stom­ach. It is a four-lobed structure and remains attached to the diaphragm by falciform liga­ment. The largest and square lobe of the liver has been termed quadrate lobe. It lies more ventral than other lobes and is divided into two equally sized left and right sub-lobes.

A rectangular left lobe lies dorsal to the quadrate lobe and to the left of the midline. A right lobe which is oval lies to the right of the midline. The smallest of all the lobes is the caudate ‘ lobe that lies dorsomedially in association with the lesser curvature and in the angular notch of the stomach.

The secretion of the liver is bile. Bile is kept temporarily stored in a pyriform gall bladder lodged in the quadrate lobe of the liver. A common bile duct formed by the union of hepatic duct (from liver) and cystic duct (from gall bladder) carries bile to the duodenum.

Pancreas:

It is a whitish, elongated and irregular-shaped gland located between the limbs of the duodenum. The secretion of the pancreas is known as pancreatic juice. The juice is carried to the distal part of the duode­num by a pancreatic duct.

Gastric glands:

Innumerable gastric glands are present along the inner lining of the stomach. The juice produced by these glands is known as gastric juice.

Intestinal glands:

Numerous glands are present in the inner lining of the duodenum and intestine.

Mechanism of digestion:

The mechanism of digestion may be divided into three steps: ingestion, digestion and egestion.

Ingestion:

Young guinea-pigs live on the milk secreted from the mammary glands of the mother. They suck the glands to get the milk. Adults are vegetarian and herbivorous. The food is taken with the help of the teeth and is chewed.

Digestion:

Digestion begins in the buccal cavity. The amylolytic enzyme, ptyalin, pre­sent in the saliva breaks down starch into maltase. The masticated food inside the stomach comes into contact with the gastric juice con­taining hydrochloric acid in low concentra­tion, pepsin and renin. HCI hydrolyses the food and the enzyme pepsin converts protein into peptone.

The enzyme renin coagulates the protein of the milk. The partially digested food within the stomach is known at this stage as acid chyme. It passes slowly into the duo­denum. The flow is regulated by the pyloric sphincter. Within the duodenum the chyme comes in contact with bile and pancreatic juice.

The bile and pancreatic juice being alkaline in nature neutralize the acidity of the chyme. The chyme then passes to the intestine and becomes subjected to the secretion of the intestinal glands. The intestinal glands secrete trypsin, erepsin and lipase. They convert peptones into amino acids. The enzyme lipase breaks the fats of the food into fatty acid and glycerol.

Another enzyme amylase converts starch into glucose. Thus the final products are the amino acids, fatty acid, glycerol and glu­cose. They are taken into the blood stream from the lining of the small intestine. Water is absorbed from the large intestine.

Egestion:

After water being absorbed, the residual matter is kept stored temporarily in the rectum. Finally, the stored matter, called faeces, is voided through the anus in the form of small pellets. The faecal pellets passed in daytime are dry and hard but those passed at night are soft with mucus. The guinea-pigs are in the habit of taking the soft nocturnal faeces (coprophagy).

Spleen:

It is an elongated brown coloured organ. It is morphologically connected with the ali­mentary canal, being situated on the dorsal side of stomach by a fold of mesentery. Spleen is devoid of any duct, and does not produce any hormone. It is believed that it destroys old red blood corpuscles.

Cavia: Type # 2. Respiratory System:

Respiration in Cavia is aerial. Lungs are the site where gaseous exchanges occur. The air from the environment passes through a long tract before entering the lungs.

Following are the different regions of the respiratory tract:

i) External nares:

These are paired open­ings situated at the tip of the snout.

ii) Nasal cavities:

These are paired cavities separated from each other by a nasal sep­tum.

iii) Internal nares:

These are paired open­ings of the nasal cavities.

iv) Naso-pharynx:

It is the part of the phar­ynx where the internal nares open.

v) Glottis:

It is an aperture on the floor of the buccal cavity. A cartilaginous flap, called epiglottis, forms a cover over the glottis.

vi) Larynx:

It is a chamber formed by four cartilages. A thyroid cartilage (Fig. 10.17) forms the ventral and lateral sides. A pair of small arytenoid cartilages forms the dorsal wall and a circular cricoid carti­lage forms the posterior part. The cavity of the lamyx bears a pair of elastic bands, called vocal cords. The vocal cords are kept “separated by a narrow rima glottis. Sound production is done with the help of the vocal cords.

vii) Trachea:

It is a relatively noncoliapsible long tube. It emerges from the larynx and runs mid-ventrally through the neck. It is encircled by series of 35-40 transverse cartilaginous rings. The rings are incom­plete on the dorsal side and their free ends are joined together by fibrous and muscular tissue.

viii) Bronchus:

The trachea, after entering the thorax, bifurcates into two branches forming the right and left principal bronchi. Each bronchus enters into the lung of the corresponding side and then breaks up into finer bronchioles.

ix) Lungs:

The lungs are housed in the tho­racic cavity. They are spongy and elastic. Each lung is enclosed by a two-layered pleural sheath. The left lung is three lobed, The lobes have been named as cranial, middle and caudal. The cranial lobe of the left lung has an intra-lobar fissure partially dividing it into a smaller cranial segment and a larger caudal seg­ment. The right lung is larger than the left lung.

It is four-lobed. Besides cranial, middle and caudal lobes it has an extra lobe, called accessory or azygos lobe (Figs. 10.18 and 10.19). The cranial lobe of the right lung is not subdivided into lobes. The outer layer of the pleural sheath is called parietal pleura and the inner layer is called visceral pleura.

The two layers of the pleura are united at the apex of the lungs. The bronchioles, with­in the lungs, break up to form finer ducts, called alveolar ducts. The wall of the alveolar ducts forms alveolus. The pul­monary blood vessels form network on the wall of the alveoli.

Mechanism of respiration:

Gaseous exchange takes place on the surface of the lungs. The entire process of physical respira­tion may be resolved into Inspiration and Expiration. In inspiration the intercostal mus­cles contract to raise the ribs. This act increas­es the thoracic cavity.

The diaphragm at the same time flattens to increase the chest cavity anteroposteriorly. Increase of thoracic cavity permits the lungs to expand. By expanding the lungs the air from outside is drawn in. The air from outside rushes through the respiratory tract and ultimately reaches the alveoli for aer­ating blood.

In expiration, the retraction of intercostal muscles and diaphragm brings the thoracic cage back to its normal state and thus exerts pressure on the lungs. This pressure drives the air from the lungs to the outside. Some air, called residual air, always remains captive inside the lungs.

Cavia: Type # 3. Circulatory System:

Two distinct circulatory fluids, namely, blood and lymph, circulate through the circu­latory system of the body. Blood flows through well-defined blood vessels and is pumped by an organ, called heart. The lymph flows through lymph vessels and intercellular spaces. Several contractile lymph hearts force the flow of lymph.

The blood vascular system is formed by:

(a) blood,

(b) heart and

(c) blood vessels, while the

(a) lymph

(b) lymph vessels and

(c) lymph hearts constitute the lymphatic system.

Blood vascular system:

Blood:

Blood consists of liquid plasma and many corpuscles floating in the plasma. The plasma is a pale yellow fluid and contains various inorganic salts, vitamins and hor­mones. There are three different types of blood corpuscles. They are erythrocytes, leucocytes and thrombocytes. Erythrocytes or red blood corpuscles or RBC are round and biconcave.

Mature erythrocytes are non-nucleated and contain a red pigment, called haemoglobin. Haemoglobin renders red colour to the blood. It has a strong but loose affinity for oxygen. The red blood corpuscles carry oxygen to dif­ferent parts of the body and carry back carbon dioxide from different parts of the body to the lungs through the heart.

Leucocytes or white blood corpuscles or WBC are larger than RBC and their number is much less than RBC. The leucocytes are of different types and their classification is dependent upon the structure of nucleus and stain-ability of the cytoplasmic granules.

These cells move in amoeboid fashion and act as scavengers of the body. Thrombocytes or blood platelets are small and non-nucleated. They occur in groups. In case of injury these cells break down and produce an enzyme which coagulates the blood and thereby prevents loss of blood.

Heart:

The heart is located in a space within the thorax between the two pleural bags. The space is known as mediastinum. The heart is covered over by a thin peritoneal membrane, called pericardium. The pericardi­um is two layered. The outer layer is known as fibrous pericardium and the inner layer is called serous pericardium.

The serous peri­cardium, in reality, is again made up of two layers. The innermost layer adherent to heart is balled visceral layer or epicardium and the other (outer) one is called parietal layer. The space between the fibrous layer and serous layer is known as pericardial cavity.

The heart is a muscular, internally hollow, four-chambered and cone-shaped structure that occupies most of the thoracic cavity. It lies roughly in the midline of the cavity with its base located cranially and the apex caudally.

On the surface of the heart there are three grooves or sulci. The coronary sulcus encir­cles the heart transversely and externally represents the line of separation between the auricles and ventricles. The sulcus is fitted up with coronary vessels and little amount of fat.

The dorsal and ventral inter-ventricular sulci mark externally the line of separation between the two ventricles (Figs. 10.20 and 10.21). There are shallow and small vessels that travel’ within them.

The heart is four-chambered consisting of two auricles (left and right) and two ventricles (left and right). The sinus venosus is absent. The right auricle lies cranial to right ven­tricle and receives the venous systemic blood and most of the coronary venous return. An internally located ridge within the right auricle divides it into two regions—sinus venarum cavarnum and right auricula.

The sinus venarum cavarnum is smooth walled but the auri­cula is lined by five muscular ridges, called pectinate muscles. There are four main open­ings in the right auricle. Blood centers in the right auricle through three of these openings while through the other, blood passes on to the right ventricle.

The three openings through which blood comes to the right auricle are:

(a) Coronary sinus:

It drains blood of the heart and is located between the opening of caudal vena cava and atrioventricular opening.

(b) Larger ostium of caudal vena cava:

It is located in the caudal aspect of the auricle near the inter-arterial septum,

(c) Smaller ostium of the cranial vena cava:

It is located on the dorsocranial aspect of the auricle. Blood from the right auricle flows to the right ventricle through the right atrioventricular ostium. The dorsolateral wall of the right auricle is demarked by the inter-auricular septum.

This septum at its right auricular face bears a small crescent-shaped depression, called fossa ovalis. During embryonic stage an aperture, called foramen ovale, remains present at this spot. This aperture, however, becomes closed before the birth of the animal.

The left auricle is smaller in size than the right auricle and is separated internally from it by the inter-auricular septum (Fig. 10.22). Inside the left auricle, there is the left auricula (auricula sinistra) which is very similar to that of the right auricula.

It receives blood through a large pulmonary opening (ostium venerum pulmonalium) located on its dorsal wall. It opens into the left ventricle through the left atrioventricular ostium.

The right ventricle lies caudal to the right auricle. It is thick-walled. It receives blood from the right atrioventricular ostium. The ostium is an oval ring surrounding the right atrioventricular valve. The valve is known as tricuspid valve because it is formed by three delicate and transparent cusps.

According to their location the cusps are known as ventral angular cusp, dorsal parietal cusp and medial septal cusp. The bases of these cusps remain attached to the border of the atrioventricular ostium and their apexes project into the ostium.

Each cusp is held in position by chor­dae tendineae and papillary muscles to pre­vent backflow of blood to the right auricle. The chordae tendineae are strong, white and delicate fibres attached to the cusps and anchored to the muscular wall.

The conus arteriosus is fused with the right ventricle and is present as the funnel-shaped cranial portion of the right ventricle leading to the ostium of the pulmonary trunk internally and bordered externally on the right by the right auricula.

The lumen of the ventricle is provided with a number of muscular ridges, called trabeculae carnae. The pulmonary trunk arises from the right ventricle. The round ostium of the pulmonary trunk lies close to the inter-ventricular septum.

The ostium is provided with pulmonary valves. Three semilunar transparent cusps constitute the valves. The cusps are known as right, left and intermediate cusps. The valves prevent the backflow of blood to the right ventricle.

The left ventricle is larger than the right ventricle. It is provided with thick walls. The left atrioventricular ostium is provided with bicuspid or mitral valve. This valve is com­posed of two unequal-sized cusps—a larger septal cusp and a smaller parietal cusp.

The cusps are held in position by chordae that remain anchored to the ventricular wall. The valves prevent the backflow of blood. The lumen of the left ventricle is provided with tra­becule carneae resembling that of the right ventricle.

The left aortic arch arises from the left ven­tricle. The aortic ostium is the opening of the aorta and it lies near the centre of the base of the heart. The ostium is provided with aortic valves. Three semilunar cusps similar to those of the pulmonary cusps are present. The valves prevent the backflow of blood.

Mechanism of circulation through heart:

The heart works day and night by alternate contraction (systole) and relaxation (diastole). The two auricles begin their systole at the same time and blood from both the auricles is forced into both the ventricles. Deoxygenated blood goes to the right ventricle and oxygenated blood comes to the left ventricle. The back flow of blood is prevented by the auriculoventricular valves.

The ventricles thus filled up with blood, start systole. In this phase the auriculoventricular valves become closed and blood from the right ventricle is forced through the pulmonary arch while that from the left ventricle is forced through the left aor­tic arch.

The ventricular systole is followed by a phase of diastole of the whole heart. At this stage the semilunar valves remain closed, deoxygenated blood from the caval veins enters the right auricle and oxygenated blood from pulmonary veins enters the left auricle.

Blood vessels:

Blood is conveyed from the heart to the different parts of the body through well-developed blood vessels. These vessels form a well-knit circuit inside the body. This type of circulation is known as closed circula­tion. Oxygenated blood is carried away from the heart by arteries (excepting pulmonary arteries) and deoxygenated blood is carried to the heart by veins (excepting pulmonary veins).

The arteries break up into arterioles which, in turn, break up into arterial capillar­ies. The arterial capillaries unite with venous capillaries and make up a capillary network. The venous capillaries unite and form venules. The venules form the veins and thus a close circuit is built.

Arterial system:

Two main arches come out from the heart. These are pulmonary and aortic arches (Fig. 10.23). The pulmonary arch or trunk emerges from conus arteriosus of the right ventricle and the left aortic arch emerges from the left ven­tricle. Semilunar valves are present at the base of each of the openings.

The pulmonary arch carries deoxygenated blood. Soon after its emergence from the right ventricle it bifurcates into right and left branches. The right pulmonary artery passes dorsal to the ascending aorta and then divides into four main branches. Each lobe of the right lung receives one branch. The left pulmonary artery passes ventral to the descending aorta and divides into three branches to enter the three lobes.

Aortic arch:

Only the left aortic arch is present. It originates from the left ventricle. It is the largest artery in the body and the main trunk of the systemic arterial system. For the sake of clarity of description it has been divid­ed into three portions—the ascending aorta, the arch of the aorta and the descending aorta.

The aorta begins as ascending aorta and then curves dorsally and to the left to form the arch or aorta. From the level of the second and third thoracic vertebrae it is known as descending aorta. It runs caudally parallel to the vertebral column and bifurcates at the lumbar region of the abdomen forming iliac vessels.

Branches from the different portions of the aortic arch:

A. Ascending aorta:

The ascending aorta gives rise to two or three coronary arteries. The artery that goes to supply the right and dorsal surface of the heart is known as right coronary artery while that which supplies the left side is known as left coronary artery.

B. The arch of the aorta:

From the cranioventral aspect of the arch of aorta arise two large arteries—the brachiocephalic trunk and left subclavian artery (Fig. 10.24). These arte­ries supply blood to the neck, head and left forelimb. The arch of the aorta is bound to the pulmonary trunk by a fibrous band, called ligamentum arteriosum, immediately after the origin of the left subclavian artery.

The brachiocephalic artery is rather large and stout. It courses cranially. At the level of the first rib it gives rise to the left common carotid. The brachiocephalic artery continues further cranially and then bifurcates into right common carotid and right subclavian artery.

The two common carotid arteries lie late­ral and parallel to the trachea. From both arise similar types of arteries. The different bran­ches from left and right common carotids have been shown in Fig. 10.25. Both the carotids at the level of the cricoid cartilage of the larynx bifurcate to form the external and internal carotids.

The internal carotids are smaller and they supply blood to the deep structures of the head and the brain while the external carotids supply the respective sides of the head. Of the two subclavian arteries the left one is larger than the right and both course cranio laterally towards the left and right, respective­ly.

The subclavian arteries give the following branches:

(a) Costocervical trunk:

It arises from the craniodorsal aspect of the branch and courses dorsally. It divides soon after its origin into three branches (cranial) descending scapular, deep cervical, and supreme (caudal) inter­costal.

(b) Vertebral artery:

It arises from the craniodorsal border or from the costocervical trunk. It courses cranially for some distance and then curves to take a dorsal position.

(c) Internal thoracic artery:

It originates from the caudoventral border of the subcla­vian and lies opposite to the costocervical trunk. It courses caudally. The right internal thoracic artery gives rise to percardionephric, bronchial mediastinal and phrenic arteries of both sides, while from the left internal thoracic artery arises only one branch on the left side to the mediastinium.

(d) Superficial cervical trunk:

The super­ficial cervical trunk is the distal branch of the subclavians. It is a long artery and lies between shoulder and neck.

C. Descending aorta:

The part of the left aortic arch from the level of the second and third thoracic vertebrae and the rest is known as descending aorta. It runs caudally parallel to the vertebral column and bifurcates in the lumbar region to form the left and right iliac arteries. For the sake of description it is discussed under two heads—Thoracic subdivi­sion and Abdominal subdivision.

(a) Thoracic subdivision:

The part of the descending aorta lying between second or third thoracic vertebrae and second to fourth lumbar vertebrae is known as thoracic subdi­vision or thoracic aorta. It runs dorso caudally along the left of the vertebral column. Eight pairs of dorsal intercostal arteries arise from the dorsal border of the aorta and these extend laterally.

Each dorsal intercostal artery sends a dorsal branch and a spinal branch. The dorsal branch supplies the dorsal and lateral verte­bral muscles and the overlying skin while the spinal branch supplies the spinal canal. A number of short twigs called cranial phrenic arteries arise as separate vessels from the aorta near its point of entry into the diaphragm. They supply the dorsolateral peripheral border of the diaphragm.

(b) Abdominal subdivision:

The last part of the descending aorta from the level of the diaphragm is known as abdominal subdivi­sion. The branches from the abdominal subdi­vision or aorta are divided into two categories- paired and unpaired visceral arteries and the paired parietal or lumbar arteries.

A. Unpaired visceral arteries:

I. Coeliac trunk:

The coeliac trunk is the largest artery aris­ing from the abdominal aorta. It is a stout artery. There exists confusion regarding the nomenclature and variability of the different branches that emerge from it.

It gives the fol­lowing branches:

(i) Gastropancreaticosplenic artery:

It is the first branch of the coeliac artery. Near its origin it gives the left gastric artery which courses cranially and supplies blood to the ventral and dorsal surfaces of the lesser curva­ture of the stomach. The gastropancreatico­splenic artery continues laterally on the craniodorsal side and from it arises the splenic artery. Splenic artery sends many branches (pancreat­ic branch) to the pancreas.

(ii) Accessory middle coeliac artery:

It arises from a point just distal to the gastropan­creaticosplenic artery. It is short and supplies blood to the transverse and proximal distal colon.

(iii) Hepatic artery:

The branch located caudal to accessory middle coeliac artery is the hepatic artery. From it arises cystic artery that supplies the gall bladder.

(iv) Cranial mesenteric artery:

It is the last and largest branch from the coeliac artery. It supplies most of the intestine and mesenteries.

II. Caudal mesenteric artery:

It is a small branch and arises from the ven­tral surface of the aorta from a point just cau­dal to the kidneys. It courses ventrocaudally and supplies blood to the descending colon.

B. Paired visceral branches:

(i) Renal arteries:

There are two pairs of renal arteries—cranial and caudal (Fig. 10.26). They arise from the level of the 2nd and 3rd lumbar vertebrae and supply the cranial and caudal parts of the kidney, respectively.

(ii) Testicular artery:

In males a pair of testicular arteries is present. They course ventro laterally and supply blood to the male gonads.

In females these arteries are called ovarian arteries.

(iii) Parietal or Lumbar arteries:

There are six pairs of lumbar arteries that arise from the aorta extending between testicular or ovarian artery and the point of bifurcation of the aorta. They supply blood to the lumbar vertebrae and the body muscles adjoining these verte­brae.

(iv) Common iliac artery:

The abdominal aorta at the level of last lumbar vertebra bifur­cates into two and gives rise to common iliac arteries. It diverges caudo-laterally from the midline and then divides into two forming internal and external iliac arteries.

From the internal iliac artery arises the prostatic artery in males and vaginal artery in the females. Other branches arising from it supply blood to the reproductive and excretory organs. The external iliac artery enter the thigh part of the hind limb and supplies blood to it.

Venous system:

Deoxygenated blood returns to the heart through veins (Fig. 10.27). Veins follow the same general course as the arteries. Often they bear the same name as the arteries.

The different veins of the body may be grouped in the following way:

(a) Pulmonary group,

(b) Cardiac group,

(c) Systemic group and

(d) Portal group.

a. Pulmonary group:

The pulmonary veins lie ventrocaudal to the pulmonary arteries and dorsal to caudal vena cava. Three groups of pulmonary veins unite to form a single trunk that opens into the left auricle.

The three groups of the pulmonary veins are:

(i) Left cra­nial group:

It drains blood from the cranial lobe of the left lung,

(ii) Right cranial group:

It drains blood from the cranial lobe of the right lung,

(iii) Common caudal group:

It drains blood from the middle and caudal lobes of the left lung and from the middle and acces­sory lobe of the right lung.

b. Cardiac group:

A group of cardiac veins drain blood from the different regions of the heart. They generally follow the coronary arteries and open into the right auricle by coronary sinus or directly by tiny vessels.

c. Systemic group:

Deoxygenated blood is collected from the anterior and posterior parts of the body and is returned to the right auricle by two large veins. The one that collects blood from the anterior part is known as the cranial vena cava (in old nomenclature it was ante­rior vena cava or precaval). Blood from the posterior region is brought to the heart by cau­dal vena cava (posterior vena cava or postcav­al in old nomenclature).

Blood from the brain is collected by inter­nal jugular veins (left and right). It comes out of the cranial cavity by the jugular foramen and runs ventrocaudally lateral to the trachea.

Each one receives a good number of tribu­taries from the respective sides and joins with the external jugular vein of the same side at a point where the external jugular vein receives the axillary vein (Fig. 10.28). In some cases it has been seen that the internal jugular vein is opening directly into the proximal part of the brachiocephalic vein.

The external jugular veins (left and right) originate by the fusion of linguofacial and maxillary veins. It courses downwards and towards the medial border of the first thoracic rib.

Here it meets with the axillary vein on the lateral side and internal jugular vein on the craniomedial border and forms the brachio­cephalic veins. During its downward journey it receives many veins like superficial cervical, prescapular and veins coming from cephalic regions.

There are two brachiocephalic veins—left and right. Each of them has been formed by the union of external and internal jugular veins together with the axillary vein of the corre­sponding sides. The two brachiocephalics are short and converge at the level of the first rib.

Both the branches receive vertebral and costo­cervical veins. Left supreme intercostal vein opens to the left brachiocephalic and right intercostal to the right brachiocephalic.

The two brachiocephalic veins unite and give rise to the large cranial vena cava. It courses downward and enters the craniodorsal aspect of the right auricle after piercing the pericardium. It receives the common trunk of the internal thoracic vein on the left and the right azygos vein on the right.

It may be pointed out that the arrangement of azygos vein in guinea-pig is rather unusual. In most cases right and left azygos veins are present. But the left azygos vein opens in the left brachio­cephalic vein.

The caudal vena cava is formed in the posterior part of the abdominal cavity. The left and right common iliac trunk unite at the level of sixth lumbar vertebra. The common iliac drains the hind limb and the pelvic organs. Each common iliac is formed by the union of internal and external iliacs coming from the hind limb. The internal iliac is short and receives a number of small veins.

The impor­tant ones among them are—prostatic vein (in case of males) and vaginal vein (in case of females), visceral and parietal veins, penal vein, etc. The external iliac vein extends between its point of meeting with the femoral vein within the hind limb and its point of meet­ing with the internal-iliac vein, it receives a number of small veins.

The important ones amongst them are—caudal abdominal vein, deep femoral vein, iliolumbar vein, etc. The caudal vena cava runs towards the heart and lies right to the abdominal aorta.

On its journey towards the heart it receives the following paired veins in a poster anterior sequence:

(i) Six pairs of lumbar veins,

(ii) Testicular vein (in males) or ovarian vein (in females). In some cases these veins open into the renal vein instead of opening into the caudal vena cava,

(iii) Renal vein from the kidneys,

(iv) Cranial abdominal vein,

(v) Suprarenal vein,

(vi) Hepatic vein and

(vii) Cranial phrenic vein.

d. Portal group:

Only hepatic portal vein is present. The hepatic portal vein is formed by the union of a number of small veins coming from the intestine and mesenteries. The impor­tant veins that take part in the formation of hepatic portal vein are—lineogastric returning blood from the stomach and spleen, duodenal vein, cranial and caudal mesenteric veins, etc.

The hepatic portal vein enters the liver and breaks into capillaries. Blood is collected inside the liver by hepatic veins which pour their contents in the caudal vena cava.

Scheme of blood circulation:

The mechanism of blood circulation through the body of guinea-pig is shown in Fig. 10.29.

Lymphatic system:

Lymph is a watery liquid containing few leucocytes. It is circulated in the body through lymph vessels. There are a pair of large lymph vessels running one on either side of the dor­sal aorta.

Small lymph vessels coming from different parts of the body open into these large vessels. The large vessels in their turn open into the subclavian vein of the corresponding side. Contractile lymph hearts, located on the ves­sels, help in the propagation of the fluid.

Cavia: Type # 4. Nervous System:

The entire nervous system may be divi­ded into three groups:

(A) Central nervous system,

(B) Peripheral nervous system and

(C) Autonomic nervous system.

Central nervous system:

It includes the (1) Brain and (2) Spinal cord.

Brain:

In spite of general similarity in the pattern of brain with other vertebrates, there is considerable amount of specialisation of this organ in mammals. This specialisation is responsible for their advancement over other vertebrates. The brain consists of five divi­sions, telencephalon, diencephalon, mesen­cephalon, metencephalon and myelencephalon.

Following are the special features of the brain of Cavia:

(i) Meninges or coverings of the brain are three-layered. In between outer Dura mater and inner Pia mater, there is a distinct middle third layer, called arachnoid layer.

(ii) Size of the brain is large and the cere­bral hemispheres and cerebellum are much convoluted to increase the area.

(iii) Olfactory lobes are small and club- shaped.

(iv) Cerebral hemispheres are much enlarged and cover the diencephalon and mesencephalon. Each hemisphere is subdivi­ded into four lobes—frontal, parietal, temporal and occipital, by grooves, called sylvian fissure (Fig. 10.30A). The growth of cerebral hemisphere is due to growth of its roof, called neopallium.

(v) Transverse bands of nerve fibres, called corpus callosum, connect the two cerebral hemispheres.

(vi) The ventral side of diencephalon is well-developed and known as hypothalamus. It bears optic chiasma, pituitary body (Fig. 10.30B) and a pair of small round mass, called mammillary bodies. The dorsal side of dien­cephalon carries pineal body or epiphysis and a vascularized fold, called anterior choroid plexus.

(vii) Mesencephalon or midbrain is thick and contains four optic lobes, called corpora quadrigemina.

(viii) Cerebellum is enlarged, folded and divided into a median vermis and two lateral lobes. Each lateral lobe is with a short floccu­lus. A broad band, called pons varolii, is pre­sent on the ventral side of the cerebellum.

(ix) Medulla oblongata is prominent and carries a vascularized posterior choroid plexus on its non-nervous roof.

Spinal cord:

The spinal cord runs up to the posterior most end through the neural canal of vertebral column. Posteriorly, the spinal cord forms a narrow, triangular cone, called conus terminalis, from which a bunch of nerves arises. These are called filum terminale. In the brachial and lumbar regions, the spinal cord is slightly swollen.

Peripheral nervous system:

It includes nerves which are given out from brain and spinal cord. The nerves from the brain are called cranial nerves and those from the spinal cord are called spinal nerves. Twelve pairs of cranial nerves are present in guinea-pig besides the terminal nerve. The origin and distribution of the cranial nerves in Cavia are basically similar to the cranial nerves of Bufo and Calotes already described.

The tenth or vagus nerve after forming the vagus ganglion sends a branch, called cardiac depressor to the heart, and a branch anterior laryngeal to the larynx. The main trunk runs posteriorly through the neck region (Fig. 10.31).

Near thorax, the main trunk sends a branch, called recurrent laryngeal, which in the left side curves around aorta and around subclavian in the right side, and finally turns anteriorly to supply the larynx.

After entering the thoracic cavity, vagus sends usual bran­ches to lungs, heart and other visceral organs. Figs. 10.32, 10.33, 10.34 and 10.35 show the origin and distribution of the fifth and seventh cranial nerves of Cavia. Figs. 10.36 and 10.37 show the distribution of ninth and tenth cra­nial nerves of Cavia.

The eleventh cranial nerve or spinal accessory originates from the lateral side of medulla oblongata and innervates the muscles of neck region. A branch of it called ramus internus, supplies nerves to the muscles of pharynx and larynx. This is a motor nerve.

The twelfth cranial nerve or hypoglossal begins from the mid-ventral region of the medulla oblongata and innervates the tongue muscles through a number of branches. This is a motor nerve.

Thirty-two pairs of spinal nerves are pre­sent. They are built up in the same plan as that of toad. On each side, the fourth and fifth spinal nerves of the cervical region unite as phrenic nerve to supply the muscles of the diaphragm.

The brachial plexus is formed by the participation of the first four nerves in the cervical region and the first thoracic nerve. It innervates the forelimbs. Hind limb is innervated by sciatic plexus, which is formed by last two lumbar nerves and sacral nerves.

Autonomic nervous system:

It is built up in the same amphibian plan and consists of a pair of sympathetic nerve cords, one on each side of the aorta. Each cord bears one anterior and one posterior cervical ganglia.

Cavia: Type # 5. Endocrine System:

The chemical co-ordination within the body is done by several ductless glands, called endocrine glands. These glands pour their contents directly in the blood.

Following endocrine glands are present:

Thyroid gland:

It is placed beneath the thyroid cartilage of the larynx. It is a soft, two- lobed structure. It secretes an iodine-contain­ing compound, called thyroxine. It is responsi­ble for the growth of the animal.

Parathyroid glands:

Two pairs of round glands are present in close association with the thyroid. The secretion is responsible for calcium and phosphorus metabolism and thus helps in bone formation.

Thymus gland:

It is a soft, pink coloured gland present near the base of the aorta. In the young, it is large and the size gradually dimi­nishes with age. Its secretion retards untimely sexual maturity.

Pituitary gland:

Most important of all endocrine glands, it is present on the hypophysis of the diencephalon. Through its secre­tion it not only controls important vital activi­ties but also controls the functioning of other vital glands.

Adrenal gland:

Adrenal or suprarenal is an endocrine gland situated above each kidney. A sectional view reveals the presence of an outer cortex and an inner medulla. The cortex is encapsulated by a connective tissue capsule.

The cortex is subdivided into three zones, viz.:

(i) Zona glomerulosa,

(ii) Zona fasciculata and

(iii) Zona reticularis (Fig. 10.42).

The cortex produces a hormone, called the cortin. The medulla is composed of chromaffin cells which secrete a hormone, named adrenalin. Adrenalin influences sympathetic activity.

Islets of Langerhans:

These are present as minute patches within the pancreas. It pro­duces insulin. Insulin controls the level of blood sugar.

Gonads:

The activity of gonads affects the working of reproductive organs and specially the development of secondary sexual.

Cavia: Type # 6. Excretory System:

Removal of nitrogenous metabolic wastes and excess of water is done by a pair of kid­neys. The kidneys, together with several other structures, constitute the excretory system. A pair of metanephric kidneys is located on the dorsal posterior side of the abdominal cavity.

The kidneys are retroperitoneal in posi­tion. The two kidneys are not located on the same line. The right kidney is situated little up from the level of the left kidney. The bean- shaped kidneys are notched on their inner sur­faces.

The notched region is called hilus. Renal artery and renal vein enter into the kid­ney through hilus. Internally each kidney pre­sents two regions. The cortex is outer and medulla is inner in position.

Each kidney is made up of a number of nephrons. Each nephron is made up of a Bowman’s capsule having a tuft of capillaries, called glomerulus, inside and a long convoluted tube differentiat­ed into proximal tubule, loop of Henle and distal tubule (Fig. 10.43).

The cortex houses the Bowman’s capsules and medulla houses the tubules. All the tubules converge and open into collecting tubules. The collecting tubules in their turn unite at the point of hilus and form the ureter which comes out through the hilus. The ureter from each kidney runs posteriorly and opens into the urinary bladder.

The uri­nary bladder is a small, muscular and elonga­ted sac situated at the posterior most region of the abdominal cavity. The bladder continues posteriorly as urethra.

The urethra in males also receives the male reproductive ducts and serves as a common passage for urinary and reproductive substances. It opens to the out­side through an opening located at the tip of penis. In females the urethra is short. It is inde­pendent of the genital ducts and opens to the outside through an aperture, called urinary aperture.

Cavia: Type # 7. Reproductive System:

Sexes are separate but the secondary sex­ual characteristics are not much elaborated. In both the sexes the reproductive system includes reproductive organs, ducts and asso­ciated accessory glands.

Male reproductive organs:

The male reproductive organs are Testes, Epididymis, Deferent ducts, Spermatic cords, Urethra, Penis, Accessory genital glands, Perineal glandular modifications and Caudal glands (Fig. 10.44 and 10.45).

Testis:

In the young males the testes remain inside the abdomen. In adults, the testes descend down and remain lodged in a special fold of skin, called scrotum. The scro­tal cavity and the abdominal cavity remain in communication with each other through a canal, called inguinal canal.

The paired testes lie in the scrotal pouch and produce sperms. Each testis is oval in outline and its long axis remain, oriented craniocaudally. The testis remains suspended within the pouch by mesorchium (a fold of peritoneum) dorsally and caudally by a fibrous ligament, called gubernaculum. Each testis is divided inter­nally into lobules by many thin septa. Each lobule contains tiny seminiferous tubules.

Epididymis:

Each testis is associated with an epididymis. The epididymis is a highly con­voluted tube consisting of a head, a body and a tail. The head is the largest and most cranial portion of it.

The body is narrow and runs along the medial border of the testis. The cau­dal part opens in the ductus deferentus. The function of the epididymis is to store sperms for ejaculation. Some workers are of the opin­ion that the epididymis helps in the maturation of sperms.

Ductus deferens:

The paired deferent ducts are 40 to 60 mm long and are divisible into a coiled epididymal portion and an uncoiled portion. They open separately in the urethra. The openings are located ventral to seminal vesicles, dorsal to the neck of the bladder and medial to the ducts of the coagu­lating and prostate glands.

The two ducts then open into a median slit within the urethra. Each deferent duct is supported by a mesoductus deferens which extends through the spermatic cord as a separate fold of peri­toneum.

Spermatic cord:

The two spermatic cords (one each for the deferent ducts) contain the deferent ducts and their mesenteries. Each spermatic cord begins at the inguinal ring and ends within the scrotal pouch.

Accessory glands:

The accessory genital glands consist of the following—seminal vesi­cles, the coagulating glands, the ventral and dorsal lobes of the prostate gland and bul­bourethral gland. A common sheath envelops the coagulating glands, the prostate and the urethral end of the deferent ducts and seminal vesicle.

Seminal vesicle:

Paired seminal vesicles are the largest amongst the accessory glands. Each seminal vesicle is a cylindrical and elon­gated structure, the free end of which may be bifid. They converge medially and enter the urethra by a pair of ducts that lie dorso-caudal to the deferent ducts. The lumen of the semi­nal vesicle remains filled up with a milky white fluid and its walls are granular.

Coagulation glands:

These are paired and pyramid-shaped bodies. They lie latero-dorsal and in close proximity to the seminal vesicle. Each gland has a single duct that opens in the urethra cranio-lateral to the opening of deferent ducts and seminal vesi­cles. The secretion of these glands coagulates the secretion of the seminal vesicle and produces vaginal plug.

Prostate gland:

It consists of two pairs of lobes—large dorsal and small ventral and the lobes are joined by isthmus. It lies caudomedial to the coagulation gland. A single pair of ducts comes from the ventral lobe and many pairs of ducts come from the dorsal lobe. These ducts open into the urethra and the openings are located cranio-lateral to the open­ing of the coagulating gland.

Bulbourethral glands:

These paired glands are small, lobulated and lie ventrolateral to the rectum. From each gland a duct arises and the ducts open into the urethra on the dorsal sur­face. Besides these glands a peculiar structure, called uterus masculine’s, is present in male guinea-pig.

It is considered homologous to the uterus of the female. This is a flat bilobed, hollow organ that lies between the mesentery connecting the deferent ducts and that of the two seminal vesicles and its unpaired central body opens to the urethra.

Urethra:

The male urethra differs func­tionally from the female urethra because it transports seminal fluids as well as urine. The urethra is divided into two portions—pelvic portion and spongy portion.

The pelvic portion extends from the neck of the urinary bladder to the penis and receives most of the ducts of accessory glands. The spongy portion remains embedded within the corpus spongio­sum of the penis. It opens externally through the urethral orifice located at the tip of the penis.

Penis:

Penis is the male copulatory organ. It is eversible but during sexual inactivity lies retracted within a sheath of skin, called preputial sheath. The penis is made up of two parts—body and glans.

The body is composed of two layers of corpora cavernosa on the dor­sal surface and its ventral midline is made up of corpus spongiosum which houses the spongy portion of the urethra. The penis is enveloped by tunica albugenia. The glans is shorter than the body. It is cylindrical and ends in a roun­ded tip. The tip bears the urethral orifice. The os penis or baculum is present on the dorsal surface of the glans.

Female reproductive system:

The female reproductive system consists of a pair of ovaries which produce ova or eggs; two oviducts which carry the ova to the uterus; a uterus which supports the developing guinea- cranial tubal and caudal uterine extremity.

The pigs, and a vagina which is a passage between uterus and the external genitalia or the vulva (Fig. 10.46). The mammary glands may be Oviducts. The oviducts or Fallopian tubes considered under the system as they are functionally associated with these organs.

Ovaries:

There are two ovaries. They are intra-peritoneal, oval and dorsoventrally flattened. The right ovary is caudolateral to the right kidney and the left ovary is craniolateral to the left kidney. The long axis of the ovary lies parallel to the axis of the animal. Each ovary has a medial and a lateral surface and a straight medial border and the tubal extremity is invested by mesovarium.

Oviduct:

The oviducts or Fallopian tubes are paired and lie along the lateral regions of the ovary. Each oviduct consists of three parts— the infundibulum, the ostium and the tubal part. The infundibulum is the most cra­nial part of the oviduct.

It is triangular, flared and its border is provided with many papillary elevation, called fimbriae. The ostium is the small opening into the oviduct at the cranial apex of the infundibu­lum. The tubal portion is a thin coiled tube. The terminal part of it joins with the uterine horn.

Uterus:

The uterus is bicornuate and hap­pens to be the largest organ of the female reproductive tract. It is a ‘Y’-shaped structure extending between the oviducts cranially and vagina caudally. It has two horns, a body and a cervix. The cervix opens into the vagina.

Vagina:

It is a pink-red canal extending between cervix and vulva. The vagina is dorsoventrally flattened and bears on its inter­nal side a number of longitudinal ridges.

Vulva:

It is the external female genitalia and includes clitoris, the vaginal orifice and labia. The clitoris is homologous to the male penis. It consists of paired roots, body and glans. The vaginal orifice is ‘U’-shaped and usually remains closed by a vaginal closure membrane. The labia are a pair of lateral thick folds of skin bordering the vaginal orifice.

Mammary glands:

A single pair of mam­mary glands is located on either side of the midline of the caudal abdomen near inguinal region. During pregnancy the glands become large and start producing milk following partu­rition. The glands are apocrine in nature. In males they remain rudimentary.

Breeding and life history:

Fertilization is internal. During coitus the males insert numerous sperms within the female genital tract. A few minutes after copulation during oestrous period, a vaginal copulation plug forms to prevent the outflow of sperms after ejaculation by the male. The plug is formed out of the secretions from the coagulating and prostate glands.

The plug is expelled a few days after copulation. Mature eggs liberated from the Graafian follicles enter the oviduct through oviducal funnel and wait in the Fallopian tube for some days. One egg is ferti­lized by one sperm. The fertilized ovum travels along the posterior part of the oviduct and enters the uterus.

It then implants itself on the uterine wall. Gradually a placenta grows between the uterine wall and the developing embryo. The placenta is a joint structure formed by the contribution of both uterine wall of the mother and the embryonic tissue.

Development within the uterus continues for ten weeks. This period is called gestation. A female usually produces three to five litters per year and each litter may contain one to six individuals. At the end of this period partu­rition or expulsion of the embryo to the outside occurs.

Youngs are born alive, well- developed, fully hairy and with teeth and so guinea-pigs are viviparous. Immature youngs suck the milk secreted by the mammary glands of the mother and remain with her twelve to fourteen days after birth. The animal attains sexual maturity early. The males become sexually mature in 70-75 days and the females become mature in 40-60 days.