238 STRUCTUKE OF THE VERTEBRATES 



larger auriculo-ventricular valves making a slightly deeper note. 

 Any weakness or hardening of the valves may cause them to 

 close imperfectly and permit a backflow, the regm'gitating blood 

 causing a murmur as it rushes past the valves. 



5. The Aorta and Its Branches. In their fundamental plan 

 the branches of the aorta remain remarkably constant in the 

 classes of vertebrates. The coronary arteries supply the heart. In 

 the dogfish they arise from the efferent loops around the third 

 gill pouch. In the amniotes they arise directly from the proximal 

 portion of the ventral aorta. The origin of the carotid arteries, 

 which are next in order from anterior to posterior, was discussed 

 with the development of the branchial arteries. 



The subclavian arteries supply the pectoral appendages. The 

 vessel receives other names as it passes distally, the changes 

 being based upon human terminology. It is the axillary artery 

 as it passes the arm-pit; the brachial in the upper arm; and when 

 it divides at the elbow the two main branches are the radial 

 and ulnar. In the classes which have both radices of the aorta 

 present, the subclavians are given off as branches from each 

 side. The condition in the mammal and bird has been dis- 

 cussed. 



Posterior to the subclavians the blood vessels can be divided 

 into two groups: (1) the parietal and (2) the visceral. The 

 parietal arteries are primitively metameric, although complete 

 metamerism may be lost in the higher animals. In the thoracic 

 region the parietal arteries pass to the muscles of the back and 

 ribs, the latter being the costal arteries. Posterior to these are 

 the lumbar and sacral arteries. The large iliac arteries, which 

 supply the pelvis and pelvic limbs, are branches from the aorta. 

 In most vertebrates the dorsal aorta is continued into the tail 

 as the caudal artery. 



The visceral vessels are: (1) the coeliac axis which divides 

 into several large vessels supplying the liver, pancreas, stomach 

 and upper parts of the intestine; (2) the anterior mesenteric 

 which is more widely distributed in the mammal than in the dog- 

 fish, passing to most of the small intestine in the former class; 

 (3) the posterior rnesenteric which supplies the posterior part of 

 the digestive tract, including the colon and rectum. 



The amniotes have several visceral arteries arising directly 



