in the Fwtus of Vertehrated Animals. 275 



these remain to form the gills, the anterior being partly obliter* 

 ated, gives rise to the roots of the carotid or head artery. 3. la 

 batrachia, we have seen, there is a gradual transition from the 

 structure of fishes to that of the higher reptiles. 'JJhe gills in 

 the batrachia are, during some period of their existence, de- 

 veloped along the course, or from particular parts of the bran- 

 chial arches, in which, as in fishes, minutely subdivided bran- 

 chial arteries and veins are formed ; but these last gradually dis- 

 appear, and more or fewer of the primitive branchial vessels re- 

 main, a. In the batrachia with permanent tails, the aorta is 

 formed, as in the foetus, by the union of the whole four bran- 

 chial arches on each side, the pulmonary artery arising from the 

 posterior arch ; b. while in the batrachia without tails, as in 

 the frog, only one branchial vessel remains on each side, so as 

 to form the right and left roots of the aorta; and the pulmonary 

 artery, which in the foetus was given off from the posterior 

 branchial arch, appears to spring from the aortic root itself, in 

 consequence of the obliteration of the posterior part of the arch 

 communicating with the descending aorta. 4. We have seen 

 that two branchial arches also remain entire in the saurian and 

 chelonian reptiles ; but in these, as well as in all the other 

 animals in which the ventricular part of the heart is more or less 

 divided in the progress of development, the pulmonary arteries 

 — formed, as in batrachia, by the posterior branchial arch-^are 

 separated from the aorta and its branches ; each of these sets of 

 vessels communicating directly with its proper ventricular cavity. 

 5. In birds, the second pair of arches, and the fifth arch of 

 the right side, are wholly obliterated without giving rise to any 

 branches. The first and third form the arteriai innominatae or 

 carotid and subclavian arteries on both sides, the communicat- 

 ing branches between these arches and the roots of the aorta, 

 being obliterated at an early period. The fourth arch on the 

 right side alone remains entirely pervious during the whole of 

 life, and forms the proper trunk of the aorta from which the 

 innominatae spring. The fourth arch on the left, and the fifth 

 on the right side> united in a common root, give rise to the pul- 

 monary arteries. These arches remain pervious till birth, 

 forming the ductus botalli or arterial ducts leading from the 

 right ventricle into the aorta. 6. In mammalia, nearly the same 



