790 COMPARATIVE ANATOMY 



ventricle into two portions, the conus arteriosus and the ventral aorta 

 are divided so that those portions which are derived from the sixth arch 

 are connected with the right side of the heart, while the rest of the 

 ventral aorta, with the exception already noticed in the reptiles, receives 

 its blood from the left side of the heart. 



It is necessary to study the figures very carefully in order to see 

 how, already in the vertebrates such as the elasmobranchs, there is a 

 differentiation of the fifth and sixth arches from the rest of the series. 

 It will be remembered that the fifth arch is almost completely obliterated 

 in vertebrates possessing lungs, and that the sixth is completely sepa- 

 rated from the rest. 



The dorsal aorta comes into existence by the fusion of two primitive 

 vessels running caudad. These lie dorsal to the mesentery and run al- 

 most parallel to the notochord to the very end of the body. This fusion 

 varies; it may extend as far forward as the aortic arch. It will be re- 

 membered that the portions which would normally be called the dorsal 

 aortae, when these segment, or when there is a division between the 

 various arches, are called radices aortae. Sometimes the dorsal aorta 

 extends still farther forward than the last aortic arch and involves the 

 whole of the radices, so that the dorsal aorta in this case extends to the 

 first arch. 



As many students studying comparative anatomy are preparing for 

 medicine, dentistry, and other professions, it is necessary to call atten- 

 tion here to the fact that the names in human anatomy are somewhat 

 different from those adopted in books on Comparative Anatomy. In 

 the study of human anatomy that part of the ventral aorta which per- 

 sists is called the ascending aorta; that part of the fourth arch which 

 continues in existence is known as the arch of the aorta; and the rest 

 of the dorsal aorta running downward toward the feet is called the 

 descending aorta. This, in turn, is divided into one portion (passing 

 into the thorax) known as the thoracic aorta, while from the diaphragm 

 downward (as it passes into the abdominal cavity) it is known as the 

 abdominal aorta. The last two names are thus only convenient terms 

 to show the location of the descending aorta. 



ARTERIES OF THE DORSAL AORTA 



These are known as visceral (splanchnic), and somatic. As these 

 terms are already familiar to the student it is merely necessary here to 

 state that the visceral arteries run through the mesenteries where the 

 double layers of serosa are found, and furnish the blood supply of the 

 digestive tract. Many of the blood vessels are in a primitive condition 

 though they are not metameric. Usually, especially in vertebrates, these 

 smaller vessels become united into larger trunks. The principal ones are 

 as follows : 



