THE AETJEEIES 329 



iv. The posterior mesenteric vein lies in the meso- 

 rectum or hindmost part of the mesentery, and 

 returns blood from the terminal portion of the 

 rectum. 



3. The pulmonary system consists of the pulmonary veins, 

 which have already been described (p. 324). 



C. The Arteries. 



The arteries form two distinct systems, of which one, the 

 aortic system, conveys arterial blood from the left ventricle to 

 all parts of the body ; while the other, or pulmonary system, 

 carries venous blood from the right ventricle to the lungs. 

 The arteries are thicker-walled, and hence paler in colour than 

 the veins. 



The aortic system should be dissected first. Clean the 

 aorta along its whole length, and follow the arteries arising 

 from it to their distribution. 



1. The aorta arises from the base of the left ventricle, 



and, turning towards the left side to form the arch 

 of the aorta, runs backwards through the thorax 

 and abdomen. It lies on the ventral surface of the 

 vertebral column, and divides about an inch in front 

 of the level of the pubic symphysis into the two 

 common iliac arteries. 



2. Arteries arising from the aorta in the thorax. 



a. The innominate artery arises from the commence- 

 ment of the arch of the aorta : it gives off, close 

 to its origin, the left carotid artery, and then runs 

 forwards a short distance and divides into the 

 right subclavian and right carotid arteries. 



i. The right subclavian artery runs outwards just 

 in front of the first rib : it gives branches to 

 the shoulder, and runs on as the brachial 

 artery into the right fore-limb. Its priacipal 

 branches are the following. 



