536 PROCEEDINGS OF THE NATIONAL MUSEUM vol. io4 



changes in the arrangement and orientation of the other associated 

 vessels. 



In the 5K-day chick a vessel arises de novo from the dorsal carotid 

 at a point about halfway between the third arch and posterior border 

 of the eye. At a later stage this vessel comes to communicate with the 

 ventral carotid, thus forming the fork of the external carotid. 



Mackay (1887) maintains that the ventral carotid does not contrib- 

 ute to the formation of the external carotid, but Twining (1906) and 

 Hughes (1934) have shown that Macka3^'s conclusions were in- 

 correct. 



With elongation of the carotid arch, the ventral carotid comes to 

 assume a somewhat more dorsal position, and in the GK-day chick 

 embryo the secondary subclavian artery forms an anastomosis with 

 the third arch somewhat ventral to the ventral carotid. Consequently, 

 Mackay's contention that the definitive subclavian and the ventral 

 carotid join in a common stalk is not substantiated by Twining's study. 



The dorsal carotid and anterior branches of the ventral carotid 

 undergo an anastomosis between the sixth and seventh days of incuba- 

 tion. This connection results in a dual blood supply to the upper and 

 lower jaws. The portion of the dorsal carotid anterior to the anas- 

 tomosing branch is referred to by Twining as the internal carotid. 



In the chick embryo of 7 to 8 days, the ventral carotid is reported 

 to lose its anterior connection. The carotid arch elongates anteriorly, 

 and with this there is a dorsal and anterior migration of the thjrroid 

 gland. 



Twining states that the vertebral is generally a branch of the 

 common carotid. Glenny, in a long series of systematic studies, has 

 shown that the vertebrals may vary considerably in the point of origin 

 (dorsal radix anterior to the thhd arch, the common carotid, or as a 

 branch of the superficial cervical or ventral carotid). 



With the interruption of the ventral carotid at a point about midway 

 between the basal portion of the third arch and the cephalic end of the 

 external carotid (Hughes, 1934; Twining, 1906) the entire blood 

 supply to the head (other than that carried by the vertebrals) traverses 

 the dorsal carotids. The earlier communicating vessel, which con- 

 nects the dorsal and ventral carotids, then comes to supply the vessels 

 which were previously connected with the ventral carotid. Both 

 Twining and Hughes have demonstrated that the anterior or cephalic 

 portion of the ventral carotids function as descending oesophageal 

 arteries. This corresponds with Glenny's (1944d) findings on the 

 Canada goose. Thus the shunt which develops between the dorsal 

 and ventral carotids during the sixth and seventh days of incubation 

 in the chick embryo serves to carry the cephalic blood supply previ- 

 ously carried by the ventral carotid. 



Wliile Twining considered the proximal portion of the ventral 

 carotid to degenerate or atrophy, Hughes, in his studies on the 9-day 



