4(54 ZOOLOGY sect, xiij 



more strongly developed than in the right. At the basal (anterior) 

 end of the left ventricle is the opening of the aorta, guarded by 

 three semikmar valves similar to those at the entrance of the 

 pulmonary artery. The coronary arteries, which supply the mus- 

 cular substance of the heart, are given off from the aorta just 

 beyond the semilunar valves. The corresponding vein opens into 

 the terminal part of the left precaval. The pulmonary artery 

 divides into two, a right and a left, each going to the corresponding 

 lung. 



The aorta gives origin to a system of arterial trunks by which 

 the arterial blood is conveyed throughout the body. It first runs 

 forwards from the base of the left ventricle, then bends round the 

 left bronchus, forming the arch of the aorta (Fig. 1091), 

 to run backwards through the thorax and abdomen, in close 

 contact with the spinal column, as the dorsal aorta (d. au.). 

 From the arch of the aorta are given off two large arteries, the 

 innominate (in.) and the left subclavian. The innominate divides 

 to form the i-ight suhclavian (s. cl.a.) and the right (r. c. c.) and left 

 (I. c. c.) common carotid arteries. The right subclavian passes to the 

 fore-limb as the hrachial artery, giving origin first to the vertebral 

 artery, which, after passing up through the vertebrarterial canal, 

 enters the cranial cavity, having first supplied branches to the spinal 

 cord ; and then to. the anterior epigastric or interned mammary (et.cjjg), 

 which supplies the side of the chest behind the root of the fore- 

 limb. The right carotid divides opposite the angle of the jaw 

 into internal (i.e.) and external (e.c.) carotids. The left carotid and left 

 subclavian correspond in their distribution and branching to the 

 right carotid and right subclavian respectively. The aorta, in 

 passing through the thorax, gives off a series of small paired inter- 

 costal arteries (i. cs.). In the abdomen its first large branch is the 

 caliac artery (coj.), which supplies the liver, stomach, and spleen. 

 Behind this it gives origin to the anterior mesenteric (a. m.), which 

 supplies the intestine and the pancreas. Opposite the kidneys it 

 gives off the two renal arteries (r.) for the supply of these organs, 

 and a good deal further back the si^ermatic (sjwi.) or ovetrian 

 arteries for the testes or ovaries as the case may be. Just in 

 front of the origin of the spermatic arteries is given off a }wsterior 

 mesenteric (p.m.), which supplies the hinder part of the rectum. 

 A series of small lumbar arteries supply the side-walls of the 

 abdominal cavit3^ Posteriorly the dorsal aorta divides to form 

 the two common iliac arteries (c. il. a.) which supply the pelvic 

 cavity and hind-limbs, a small median sacral (caudal) artery (m.sc.) 

 passing backwards in the middle line to supply the caudal region. 



The system of caval veins which open into the right am-icle 

 consists of the right and left preeavals and of the single piostcaval. 

 The right precaval is formed by the union of the right jugular 

 (e.ju.) vein and right subclavian (s.cl. v.). The azygos vein {az. v.), 



