542 PROCEEDINGS OF THE NATIONAL MUSEUM vol. i04 



the space occupied by the carotids within the hypapophysial canal. 

 Further reduction in size of the hypapophysial canal, by the en- 

 croachment of the aforementioned cervical muscles, may account, 

 in part, for the fusion of the two carotid arteries and the resulting 

 formation of the unicarotid arrangement. 



While I have noted this orientation of left and right carotids within 

 the hypapophysial canal many times, Bhaduri and Biswas (1954) 

 have made particular mention of the condition. 



Commonest of the modifications which occur because of the posi- 

 tion of the dorsal radices (dorsal carotids) is that of fusion of these 

 vessels between the third arch and the base of the head. As a result 

 of this fusion of the two primary dorsal carotids, a single vessel trav- 

 erses the length of the neck. In some orders of birds the basal por- 

 tion of both vessels are present, while in other orders or families only 

 the basal portion of one of the conjugate vessels is present. In still 

 other instances, a vestige of the atrophied vessel remains as evidence 

 of its earlier embryonic relationship in the system. When both basal 

 portions of the conjugate vessel are present they may be equal or 

 one side may be reduced in diameter. At the cephalic end of the 

 conjugate carotid both left and right carotids are given off before they 

 further divide into the several internal and external branches. These 

 branches, as Hughes (1934) has pointed out, are not the same for 

 birds as for mammals. 



Reduction in the lumen of the basal portion of the dorsal carotids 

 may occm- on either side, and still further alteration in this portion 

 of the carotid may occur in the form of atrophy, with retention of 

 either a complete or an incomplete ligament. Insofar as I can de- 

 termine, this ligament has never been described in any of the literature 

 heretofore, and no name has as yet been assigned to it. Ottley 

 (1879) described the presence of two white imperforate cords lying 

 within the hypapophysial canal of Bucorvus abyssinicus. These he 

 believed to be the remnants of the dorsal carotids. In recent studies 

 I have had the opportunity of observing the same or similar structiu"es 

 which are definitely the ligamentous vestiges of the dorsal carotids. 

 Since these structures were originally noted by Ottley, it would be 

 well to refer to them as the ligamenti ottleyi. In forms which present 

 ligaments on both sides (ligamenti ottleyi), the blood supply to the 

 head is carried by enlarged vertebral and superficial cervical arteries. 



When the paired dorsal radices aortae (anterior) do not enter the 

 hypapophysial canal, the dorsal carotids may become fm-ther modified 

 and may be reduced in size. In both Zanclostomus javanicus javanicus 

 and Phaenicophaeus pyrrhocephalus the dorsal carotids were found to 

 be superficial vessels, much reduced, and functionally modified as 

 oesophageal arteries in addition to the normal function of cephalic 

 blood supply. In Rhainphococcyx curmrostris erythrognathus the left 

 dorsal carotid was superficial and modified to form an oesophageal 



