AORTIC ARCHES OF BIRDS — GLENNY 537 



chick and subsequent stages, points out that this portion of the ventral 

 carotid becomes functionally modified to form the ascending oeso- 

 phageal artery. This view is likewise shared by Glenny (1944d). It 

 should be pointed out that in several orders of birds this functionally 

 modified vessel may be short and greatly reduced, with the result that 

 during its development it may be readily overlooked and even in 

 the adult bird may be detected only after the most careful examination 

 or upon injection with colored materials. Bhaduri and Biswas 

 (1945, 1947, 1954) have shown that it may be continuous, and retain 

 its natm^al connection. I have observed the superficial cervical artery 

 to be continuous and uninterrupted for the entire length of the neck 

 in several orders of birds. This is probably the basic or ancestral 

 arrangement, whereas the discontinuity of these ventral carotids is 

 probably a modification rather than the usual condition. 



At about the eighth to ninth day of incubation the innominate artery 

 may be recognized as originating from the basal portion of the third 

 aortic arch. 



Therefore, it may be seen that (1) the innominate arteries are 

 derived from the basal portion of the third arch; (2) the vessel from 

 the point of junction with the subclavian to the region of the thyroid 

 gland represents, for the most part, the dorsal portion of the third 

 arch; and (3) the vessel lying beyond this point up to the base of the 

 head represents the dorsal radix aortae, anterior to the third arch. 



The origm and development of the cephalic branches of both the 

 internal and external carotid arteries are extremely well treated by 

 Bauer (1825), Hughes (1934), Ottley (1879), Twming (1906), and 

 others in both general and specific studies on the vascular system of 

 birds. Hughes has pointed out that there are several important 

 differences in the connection of the cephalic branches of the external 

 and internal carotids between birds and mammals and, as a result, 

 there cannot be a direct transfer of information from one group to the 

 other. An exposition of these differences is of no great significance 

 in this study. 



The ventral radices aortae (ventral carotids), as has been noted, 

 may become functionally modified to form the ascending oesophageal 

 artery from the posterior portion of the ventral carotids and the de- 

 scending oesophageal artery from the anterior portion of this same 

 vessel after disjunction. The external carotid, as a result, receives 

 blood by way of the dorsal carotid artery subsequent to the disjunc- 

 tion. It is possible that extensive reduction of the proximal portion 

 of the ventral carotid may result in a very short and much reduced 

 ascending oesophageal artery in many families of birds, while in stiU 

 others it is a prominent structure. 



Hafferl (1933) points out that the subclavian artery in birds is not 

 the primary blood vessel which is formed at first in the embryo but 

 that it arises from the ventral part of the third aortic arch, so that in 



