330 HISTORY OF THE HUMAN BODY 



both sides form complete arches and unite to form the dorsal 

 aorta, and VI becomes the pulmonary (here the pulmo-cutane- 

 ous). Of the two aortic arches IV is the principal one and V 

 is a subordinate, and is of such slight functional importance 

 that in the higher Classes it is destined to disappear altogether. 

 These arches are usually continuous,, and are not as a rule 

 interrupted in the midst by the interposition of respiratory 

 capillaries as in fishes; in larval urodeles, however, and in a 

 few adult forms, the perennibranchs, a branch from the ven- 

 tral side of the arch supplies the external gill-bushes with 

 capillaries, from which a collecting branch returns the blood 

 to the arch at its dorsal end. When such a gill-bush is of much 

 functional importance these lateral branches are large, .and in 

 extreme cases it is possible that practically all the blood of a 

 given arch may pass through these indirect channels. In most 

 cases the external gills, and with them their supplying 

 branches, disappear at the expiration of larval life, and the 

 arches form continuous vessels, as in higher forms. The sixth 

 arch is in the larva a complete one, and joins the dorsal aorta, 

 as do the two preceding ; with the development of the lungs 

 and the integumental respiration a small branch, which arises 

 from this arch near the middle, becomes engaged in supplying 

 the lungs and skin, and increases so much in size that it 

 ultimately transmits all of the blood that enters the arch, 

 leaving the distal half of the arch without employment. This 

 part then closes its lumen and is retained as a connecting 

 band, the ligamentum arteriosum [Botalli], extending along 

 its old path between the pulmonary artery and the dorsal 

 aorta. A similar ligament, or in many cases a small pervious 

 artery, is also retained between the carotid arch and the main 

 aortic arch (III and IV). 



In reptiles (Fig. 91, d) the metamorphosis of the arterial 

 system is pushed back into embryonic life, and, from this point 

 on, no longer appears after birth. In other words, the transi- 

 tion from water to land, an historic scene actually enacted 

 during the post-natal existence of amphibians in the form of 

 the metamorphosis, -with all the changes involved, not only in 



