In this article we will discuss about the dissection of guinea pig. Also learn about:- 1. The Venous System 2. The Arterial System.
The rat and Guinea Pig (Fig. 20.1) belong to the same order Rodentia, class Mammalia. Still, structural differences in the circulatory system (heart, venous and arterial system), to a certain extent are present in the two species. The renal portal system is absent.
Put the specimen on its back on a dissecting tray. Fix it with pins passing through the limbs. Lift the skin of the abdomen with a pair of forceps and make a small cut at about the middle of the abdomen.
Starting from the cut give an incision extending up to the snout anteriorly and the genital opening posteriorly. Give transverse incisions along the length of the limbs. Separate the skin from the underlying muscles and pin it down.
Give a transverse incision on the abdominal wall in the posterior region. Proceed laterally and anteriorly on both sides cutting the ribs up to the anterior border of the thoracic cavity. Cut the clavicles and the muscles there. Lift the sternum and cut the attachment of the diaphragm to the abdominal wall.
The ventral wall of the thorax and abdomen with the sternum is separated and the organs in the thoracic and abdominal cavities are exposed (Fig. 19.2). The structures in the neck region have already been exposed with the removal of the skin.
The Venous System:
The cranial vena cava draining blood from anterior region and the caudal vena cava from the middle and posterior region of the body open respectively into the cranio- caudal and craniodorsal region of the right atrium. Only hepatic portal system is present (Fig. 20.2).
1. Cranial vena cava:
The cranial vena cava is formed by the joining of right and left brachiocephalic veins. A brachiocephalic vein is formed by the joining of external and internal jugulars and subclavian.
A. External jugular formed by the union of:
a. Linguofacial from tongue and adjacent region.
b. Facial from facial region.
c. Maxillary from maxillary region.
d. Deep cervical from pectoral muscles.
e. Supra and subscapular from scapular region.
B. Internal jugular formed by the union of:
a. Pharyngeal from pharynx.
b. Oesophageal and trachal from oesophagus and trachea.
The two jugulars unite and open into the brachiocephalic vein of the side.
C. Right subclavian:
Formed by the union of:
a. Brachial from the extremity of the fore limb and continues inward as axillary vein receiving costocervical from the thoracic wall, neck muscles and phrenic from the diaphragm.
b. The axillary continues as subclavian and receives:
i. Vertebral from vertebral column
ii. First intercostal from anterior intercostal space
iii. Supreme intercostal from first two intercostal spaces
D. Left subclavian:
Formed by veins similar to those of right side:
1. Cranial vena cava:
It is a short and stout vessel, directly receives:
i. Vertebral from vertebral column
ii. Internal thoracic from fifth and sixth intercostal spaces
iii. Azygos from the thoracic wall and diaphragm (phrenic)
2. Caudal vena cava:
It is formed by the union of veins bringing blood from the middle and posterior region of the body.
a. Median sacral from the sacral and coccygeal region
b. External iliac by the joining of:
i. Femoral from hind-limb
ii. Pseudo gastric from abdominal muscles and urinogenital organs
iii. Internal iliac from urinary bladder and pelvic region
iv. Common iliac is formed by the joining of two iliacs
The caudal vena cava in its forward course receives:
v. Caudal abdominal from the posterior abdomen and rectum and joins the common iliac
vi. Lumbars from lumbar region
vii. Genitals from testis/ovaries
viii. Renals from kidneys receiving adrenals
ix. Caudal phrenic from diaphragm
x. Cranial abdominal from abdomen
xi. Hepatic a single vein from the liver.
The caudal vena cava pierces the diaphragm following a middle course and forms a shallow arch to the right and opens in the right atrium.
3. Hepatic portal system:
Hepatic portal vein is formed by joining of following veins from the visceral organs.
i. Cystic from gall bladder
ii. Cranial mesenteric from intestine
iii. Caudal mesenteric from rectum
iv. Gastro-duodenal from stomach, pancreas and anterior part of the intestine.
The hepatic portal vein branches in the lobes of the liver.
4. Pulmonary veins:
A single vein from the right and two veins joining to form one from the left lung, join to form a common pulmonary vein opening in the left atrium.
The Arterial System:
I. The left aorta (right is absent) arises from the base of the left ventricle, turus left, forms an arch round the left bronchus and runs posterodorsally as the dorsal aorta, through an aperture in the diaphragm. The aortic arch has three portions—the ascending aorta, aortic arch and dorsal aorta.
1. Coronary artery:
Two or three small vessels arise close to the base of ascending aorta and supply blood to the heart.
2. Brachiocephalic artery:
First branch of aortic arch. It divides into a lateral branch, the right subclavian artery and a medial short, stout branch forming the base of the right and left common carotids.
A. Right subclavian:
Arising from the brachiocephalic artery it runs outward between the first rib and clavicle and sends following branches.
i. Vertebral to cranial cavity, paravertebral musculatures and spinal cord.
ii. Caudal thyroid runs forward obliquely to the left, divides into right and left caudal thyroid, proceed anteriorly by the sides of the trachea, supply the thyroid gland.
iii. Superficial cranial supply the muscles of neck region.
iv. Internal thoracic supply the sternal region.
v. Costocervical supply the neck, upper part of thoracic wall and diaphragm.
The portion of the artery beyond the origin of costocervical up to the brachial artery.
It sends the following branches:
i. External thoracic supplies pectoral and trapezius muscles and axillary lymph nodes.
ii. Lateral thoracic supplies muscles of pectoral region.
iii. Subscapular supplies upper arm, scapula and thorax.
4. Brachial continues to the extremity of the forelimb and divides into a number of branches.
5. Common carotid:
Left and right common carotid arise from a common base.
a. Right Common carotid runs to the head lying parallel and right to the trachea. Sending a branch to the lymph node the artery divides into two—external and internal and supplies the right side in the anterior region.
i. Internal carotid enters the skull and supplies to the brain and deep structures of the head.
ii. External carotid branches to supply head, tongue, jaw, parotid gland, pharynx, oesophagus and thyroid gland.
b. Left common carotid branches similar to right common carotid.
B. Left subclavian:
Arises directly from the aortic arch at a short distance from the brachiocephalic artery. Branches are similar to those of the right subclavian, except lacking caudal thyroid.
6. Dorsal aorta:
Runs posteromedially and sends following branches to the middle and posterior part of the body:
A few pairs of small arteries supply thoracic region.
The dorsal aorta pierces the diaphragm, enters the abdominal cavity.
The branches in the abdominal region are:
A large unpaired vessel, send branches to oesophagus, stomach, spleen, pancreas, liver, major part of intestine, caecum, ascending transverse and descending colon, and other associated structures.
Two pairs. The anterior pair supply kidneys, and send branches to adrenal and diaphragm as caudal phrenic arteries. The posterior pair ends in Kidneys.
Paired, supply gonads and their ducts.
A few paired arteries posterior to genitals, supply dorsomedian muscles of the lumbar region.
vi. Median sacral:
Arises at the junction of two common iliacs and supplies sacral and coccygeal region.
vii. Caudal mesenteric:
Unpaired, arises from the dorsal aorta in between second and third pairs of lumbars and sends branches to descending colon, rectum and anal region.
viii. Common iliac:
Paired, the right and left common iliacs are formed by the bifurcation of the dorsal aorta. Each divides into two major branches, external and internal.
a. External iliac:
Supplies abdominal wall and urinogenital organs through pseudoepigastric branch and continues as femoral artery in the hind-limb.
Two. Arise from external iliacs and supply the structures in the posterior abdomen.
b. Internal iliac:
Runs backward and supplies the pelvic region and urinary bladder.
II. Pulmonary trunk:
Originate from the right ventricle, courses towards the dorsal side and bifurcates into right and left. The right pulmonary artery passes dorsal to the aortic arch, divides into four branches to supply four lobes of the right lung. The left pulmonary artery passes ventral to the aortic arch, divides into three branches to supply three lobes of left lung.