n6 VERTEBRATE ZOOLOGY 



carotid, the larger of the two, passes dorsally and then forward 

 and supplies the brain, the orbit, and the mucous membrane of the 

 roof of the mouth. The external carotid passes directly forward 

 and supplies the tongue and the muscles of the lower jaw. 



The posterior aortic arch, the pulmocutaneous, takes blood to 

 the lungs and skin to be oxygenated. On each side just back of the 

 carotid gland it divides into two arteries, the pulmonary and the 

 great cutaneous. The former passes back a short distance and 

 then divides into three arteries which traverse the walls of the 

 lung. The latter passes first forward and dorsally, and then back- 

 ward along the inner surface of the skin ; its branches supply the 

 head and the muscles of the body wall. 



Exercise 22. Draw a semidiagrammatic sketch showing the distribu- 

 tion of these arteries so far as observed. 



The middle arch, the systemic, supplies the greater part of the 

 body with blood. The two sides of the arch pass dorsally, one 

 on each side, around the oesophagus to the upper side of the body 

 cavity, where they meet and form the dorsal aorta, or aorta de- 

 scendens. This vessel runs just beneath the spinal column to the 

 hinder part of the body cavity, where it divides into two arteries, 

 the iliacs, which go to the legs. 



Lift up the stomach and find the aorta descendens; follow it 

 forward to the meeting point of the two sides of the systemic arch, 

 and then follow each side of the arch to the heart. Each side of 

 the arch gives off the subclavian, occipito-vertebral, oesophageal, 

 and laryngeal arteries, of which the subclavian is foremost in 

 position. 



The last three of these arteries leave the arch near together. 

 The subclavian is the largest and supplies the shoulder and foreleg. 

 Follow it and its branches. The oesophageal and occipito-vertebral 

 are close together and go to the head ; the vertebral, a branch of 

 the occipito-vertebral, passes back along the dorsal surface of the 

 spinal column. The laryngeal arises in front of the others and 

 goes to the head. 



Just behind the point of union of the two sides of the systemic 

 arch the dorsal aorta gives off the coeliaco-mesenteric artery. Ob- 



