290 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



right and left ventricles. The internal structure of the heart is in most 

 particulars like that of the pigeon (p. 245), but the right auriculo- 

 ventricular valve is not muscular but made up of three membranous flaps 

 (giving the name tricuspid valve), connected by chord* tendinew with 

 papillary muscles in the right ventricle. These muscles are also found in 

 the left ventricle, and are more numerous than is the case in the pigeon. 

 The opening of the postcaval is guarded by a fold, the Eustachian valve, 

 and that of the left precaval by a similar Thtbesian valve. 



Arteries. The aorta takes origin from the left ventricle, and 

 its arch curves round to the left to form the dorsal aorta, giving 

 off vessels which supply head, neck, and fore-limbs. The dorsal 

 aorta runs back ventral to the backbone, supplying the trunk 

 with its contained viscera, and finally forks into two iliac arteries, 

 which supply adjacent parts and continuations of which run into 

 the hind-limbs. 



The aorta dilates at its origin into the 3 sinuses of Valsalva, situated 

 behind the pouches of the semilunar valves. Two coronary arteries, for 

 the supply of the heart-walls, are given off here. Near the beginning of the 

 arch a short innominate artery runs off, which at once gives off the left 

 carotid, and soon after divides into right subclavian and right carotid 

 arteries. The left subclavian arises from the left side of the arch. Each 

 carotid runs forwards on one side of the trachea, giving off branches to the 

 neck as it does so, and divides in front into internal carotid for the brain, 

 and external carotid for the outside of the head. The subclavian arteries 

 supply the fore-limbs, on entering which they become the brachial arteries. 

 A vertebral artery runs doraalwards from each subclavian near its com- 

 mencement, and enters the vertebrarterial canal to supply the brain and 

 spinal cord. 



The dorsal aorta gives off in the thorax small paired intercostal arteries 

 to the thoracic walls. In the abdomen a large unpaired cwliac artery 

 takes origin a little way behind the diaphragm, and quickly divides into 

 the hepatic artery for the liver, and lieno-gastric artery for the spleen and 

 stomach. Another unpaired artery, the anterior mesenteric, for the small 

 intestine, pancreas, caecum, and colon, arises a short distance posterior to 

 the coeliac. Paired renal and spermatic (or ovarian) arteries are next given 

 off to the kidneys and spermaries (or ovaries), and still further back a small 

 unpaired posterior mesenteric artery to the rectum. The dorsal aorta 

 bifurcates at the posterior end of the abdominal cavity into the iliac 

 arteries, each of which, after giving off an ilio-lumbar artery to the body- 

 wall, divides into an internal iliac artery for the pelvic region and an 

 external iliac artery, which, after giving off a branch that supplies the 

 bladder (and uterus in the female), enters the thigh as the femoral artery 

 for the hind-limb. 



Shortly before the aorta bifurcates, it gives off from its upper surface a 

 small median sacral artery, which runs back into the tail. 



The pulmonary artery arises from a forward prolongation of 

 the right ventricle on the ventral side, and, curving round the 



