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301 



tomes and fishes they undergo considerable modification, and in the 

 fishes they are frequently more or less reduced in correlation with the 

 reduction of the gills (p. 254). The modifications may be outlined 

 as they occur in the successive pairs of arches. 



In many fishes and all tetrapoda the first arch on either side dis- 

 appears beyond the point where the external carotid arises (fig. 

 319, 5 to Z)) while, correlated with the reduction of the spiracular gill, 

 the second pair of arches is partially or completely lost in the adult. 

 The third pair is always persistent and through them flows the blood 

 for the internal carotids and, in the fishes, gymnophiona and a few 

 urodeles (fig. 319, C) and reptiles (£), blood for the radices aortae as 

 well. In all other tetrapoda the radix disappears between the third 

 and fourth arches (fig. 319, D) and consequently here the third 

 arch i? purely carotid in character. When this occurs the por- 



FiG. 321. — Aortic arches of amniotes, after Hochstetter. A, Varanus; B, snake; C, 

 alligator; D, bird; E, mammal, b, basilar artery; cc, common carotid; ei, ce, internal 

 and external carotids; da, dorsal aorta; p, pulmonary; s, subclavian. 



tion of the ventral aorta between the third and fourth arches car- 

 ries blood for the carotids alone and hence forms a common carotid 

 trunk, usually divided into right and left common carotid arteries. 



The fourth pair of arches are the systemic trunks in all tetrapoda, 

 carrying blood from the ventral to the dorsal aortae, while the fifth, 

 reduced in size, perform a similar function in a few lizards and uro- 

 deles (fig. 319, C), but elsewhere they entirely disappear. The 

 fourth arches show a differentiation between the two sides in many 

 reptiles. That on the left side becomes separated from the rest of 

 the ventral aorta (fig. 319, E, F) and has its own trunk connecting 

 with the right side of the partially divided ventricle, and, as will be 

 understood from the relations of the heart (p. 297), it may carry a 

 mixture of arterial and venous blood. From the dorsal side, this 

 blood of the left fourth arch is largely distributed to the digestive 



