xm PHYLUM CHORDATA 435 



three semi-lunar 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 semi-lunar valves. The corresponding vein opens into 

 the terminal part of the left pre-caval. 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. 1028, a. ao.), 

 to run backwards through the thorax and abdomen, in close 

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

 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 right subclavian (s. da.) and the right (r. c. c.) and left 

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

 limb as the brachial 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 internal mammary, 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 and external 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 

 coeliac artery (cce.), w r hich 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 spermatic (spm.) or ovarian 

 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 posterior 

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

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

 abdominal cavity. Posteriorly the dorsal aorta divides to form 

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

 limb, a small median sacral (caudal) artery (ms. c.) passing back- 

 wards in the middle line to supply the caudal region. 



The system of caval veins which open into the right auricle 

 consists of the right and left prc-cavals and of the single post-cai-al. 

 The right pre-caval is formed by the union of the right jugulai- 

 (e.ju.) vein and right subclavian (scl. v.). The vena azygos (az. v.), 

 the right anterior intercostal (i. cs.), and the right internal mam- 

 mary also open into it. The left pre-caval receives a series of 



F F 2 



