310 AN INTRODUCTION TO ZOOLOGY 



off the same branches as the right with the exception of the 

 intercostal. 



The aorta then runs on giving off a short series of paired 

 intercostal arteries to the posterior intercostal spaces, and then some 

 distance behind the diaphragm gives off a large median cceliac 

 artery which, after a short course in the mesentery, divides into a 

 hepatic trunk going to the liver and a lieno-gastric branch supplying 

 the stomach and spleen. A second large artery, the anterior 

 mesenteric, arises just posterior to the foregoing and divides up to 

 go to the duodenum, pancreas, small intestine, ccecum and colon. 

 This is followed by a pair of renal arteries which, after giving off 

 a branch to the musculature of the dorsal body wall, proceeds to 

 the kidney. The right vessel arises a short distance ahead of the 

 left. Some distance further back arise a similar asymmetrical 

 pair of genital arteries differently named in the two sexes. In 

 the female the ovarian arteries pass out laterally to the ovaries, but 

 in the male, owing to the migration of the testes, the spermatic 

 arteries run outwards for a short distance and then back and into 

 the scrotal sacs. The posterior mesenteric artery is a small median 

 vessel originating just before or just behind the genitals and going 

 to the hinder part of the rectum. Posterior to this the median 

 sacral artery is given off from the dorsal side of the aorta and it 

 runs parallel with this to the tail : it is probably to be regarded as 

 being actually the continuation of the aorta itself. Shortly after 

 this the main vessel the so-called aorta bifurcates to form the 

 common iliac arteries, which pass out at an angle laterally towards 

 the legs. A short way along each vessel gives off on its anterior 

 side an ilio-lumbar artery to the dorsal body wall. This is followed 

 by a pair of arteries, the internal iliac and the vesicular, on the 

 posterior wall. The former spreads over the dorsal wall of the 

 pelvic cavity and the latter supplies the bladder. Another small 

 artery, the posterior epigastric, is given off from the anterior wall of 

 the main trunk which may now be termed the external iliac. It 

 passes up the ventral abdominal wall. Finally the external iliac 

 passes on into the leg as the femoral artery. 



Venous System. 



The veins of the body can be considered as falling into 

 three more or less independent, but nevertheless related groups of 

 vessels and dealt with under the headings : the pulmonary system, 

 the hepatic portal system and the system of the caval veins. 



The pulmonary veins bring the blood back from the lungs. 

 Each one is formed by the union of two main trunks coming from those 

 bodies quite close to the heart and they open into the right atrium. 



