1887.] 



The Brachial Arterial Arches in Birds. 



431 



of its having been overlooked by previous observers. In chicks at 

 the close of the sixth day and in older forms it is pointed out that 

 the artery may be followed, by dissection under water. The innomi- 

 nate artery has been observed to be formed, not by the gradual 

 fusion of the subclavian and carotid arteries at their bases, but by 

 the splitting up of the truncus arteriosus into canals continuous 

 with the three permanent arches, the innominate artery belonging to 

 the third, and the basal portions of the aorta and pulmonary artery 

 to the fourth and fifth respectively. 



The author has also examined the relations of the subclavian 

 artery in the different groups of vertebrate animals, and finds that 

 instead of there being but one artery prolonged into the limb, as 

 Hathke held, there are in reality two such vessels. One is represented 

 by the mammalian subclavian, and also in lizards and amphibians 

 is found arising from the aortic root, and passing outwards to 

 the limb dorsal to the pneumogastric nerve and jugular vein. The 

 other, present in birds and in crocodilian and chelonian reptiles, 

 arises from the ventral end of the third arch, and crosses outwards 

 ventral to vein and nerve. In most of the lower forms representa- 

 tives of both vessels are present, and one or other is specially 

 -enlarged and supplies the greater part of the limb, but it is 

 pointed out that in the forms where the two vessels co-exist they 

 •anastomose with one another in the body-wall at the base of the limb. 

 This anastomosis may be dissected out in lizards, where the dorsal 

 vessel is specially enlarged, and in crocodilian reptiles, where the 

 ventral artery plays the important part. 



In the higher forms (birds and mammals) one of the arteries alone 

 is present, but the cetacean group of mammals forms an exception to 

 this rule. In this group both arteries are to be found, and it is the 

 ventral, not the dorsal as in mammals generally, which specially 

 supplies the limb. 



With reference to the development of the carotid artery in birds, 

 Hathke believed that the external carotid was the prolongation of the 

 ventral trunk from the extremity of the third arch towards the head, 

 and regarded the branches of the external carotid as derivatives of 

 this ventral vessel. The internal carotid he looked upon as represent- 

 ing the third arch and its dorsal continuation towards the .head, while 

 the common carotid was believed by him to be the portion of the 

 ventral vessel between the third and fourth arches. 



It is pointed out that if the observations already explained as to the 

 origin of the subclavian artery be accepted, and that' vessel be held to 

 arise from the ventral extremity of the third arch,*then, if Rathke's 

 theory of the external carotid be true, the subclavian should be found 

 in the adult as a branch of the external carotid ; but this is not the 

 case. The common carotid artery, which Rathke regarded as a ventral 



VOL- XLII. 2 I 



