788 COMPARATIVE ANATOMY 



arise from the mandibular arteries. There is, however, an external and 

 internal carotid artery supplying the head and brain coming from each 

 half of this first arch (Fig. 448). As various changes take place, how- 

 ever, the relation of the carotids makes them appear as though they 

 arose from the first functional arch. 



In the cyclostomes and fishes the various arches do not undergo 

 much, if any, modification. Whatever modification does occur depends 

 upon changes in the gills. 



In all land vertebrates, and many of the fishes, the first arch on both 

 sides disappears at the point where the external carotid artery begins. 



When the spiracular gill is reduced, the second pair of arches is 

 partially or completely lost in the adult. 



The third pair persists. The blood for the internal carotids flows 

 through these. In fishes, gymnophiona, and a few urodeles, the blood 

 for the radices aortae also flows through the third pair. In all four- 

 footed animals the radix disappears between the third and fourth arches 

 so that it leaves the third arch purely carotid. In such a case that part 

 of the ventral aorta between the third and fourth arches carries the 

 carotid blood along and hence is known as a common carotid artery (Fig. 

 448, I, B), which usually then divides into a right and left branch. 



The fourth pair are the systemic trunks, in all four-footed animals, 

 and carry blood from the ventral to the dorsal aortae. 



The fifth has become smaller, disappearing in nearly all animals 

 except lizards and urodeles. In reptiles the left side of the fourth arch 

 becomes separated from the rest of the ventral aorta, having its own 

 trunk connected with the right side of the partially-divided ventricle, 

 thus carrying a mixture of arterial and venous blood. The blood from 

 the left fourth arch on- the dorsal side is largely distributed to the diges- 

 tive tract. The right side of the arch and the carotids are connected 

 with the left side of the heart and are consequently purely arterial, the 

 arch itself forming the main trunk connecting the heart with the dorsal 

 aorta. In birds the radix on the left side of the adult disappears caudad 

 to where the subclavian artery begins, so that this arch supplies blood 

 only to the left arm. The right arch is purely aortic in character. In 

 the mammals this is entirely reversed, the right arch being subclavian, 

 the left supplying the dorsal aorta and the subclavian of that side. 



The bird in its embryonic growth (as exemplified by the chick em- 

 bryo) turns upon its left side in about 80 per cent of all cases, and the 

 right arch persists, while in mammals, the embryo usually turns upon its 

 right side and the left arch persists. 



When lungs are developed, whether that be in the lung fishes or any 

 of the higher forms of animals, a pair of pulmonary arteries develop 

 from the sixth pair of arches on the ventral side of the pharynx. These 

 arteries grow caudad into the lungs. That part of the arch dorsal to 

 these newly formed pulmonary arteries becomes reduced to a small 



