6 Mr. W. A. Forbes on the late Professor Garrod's 



II. The Carotid Arteries. 



The variations in the position of the carotid arteries in 

 birds had been studied by Meckel, Bauer, Barkow, and 

 others ; but their opportunities of observation were limited, 

 for the most part, to European species. Prof. Garrod, in his 

 paper on the subject'^, has recorded their condition in 400 

 species of birds, of 300 different genera ; in his subsequent 

 papers, or MS. notes, many additional species are included. 



From a consideration of these, six different modifications 

 in the disposition of these vessels may be traced : — 



(1) The two carotids, each springing, as usual, from the 

 innominate artery of its side, after the latter has given off 

 the pectoral and subclavian branches, run up in a converging 

 manner into the neck, and then continue, closely parallel to 

 but quite free from one another, up along the under surface 

 of the neck, in a bony canal or passage formed by the hypa- 

 pophyses of the cervical vertebrae, to near the head, where 

 they again diverge and break up for the supply of that part. 

 This may be considered the most typical and least modified 

 form : it is present in a very large number of birds. 



(2) Where, instead of both carotids being developed, only 

 one, the left, is so, the right having entirely disappeared. 

 This is a condition constant in all Passeres, as well as in 

 sundry other birds. 



(3) Where the right artery is present in its normal posi- 

 tion in the hypapophysial canal ; but the left runs up the neck 

 superficially in company with the left jugular vein and vagits 

 (pneumogastric) nerve. This condition is present only in 

 certain Parrots. 



(4) Where the two arteries, instead of running parallel, 

 blend together at the lower part of the neck, running up then 

 as a single trunk in the normal position till its bifurcation 

 near the head. This is an exceptional condition. The two 

 trunks before blending may be equal in size {Botaurus stel- 

 laria), or either the right {Phcenicopterus) or the left {Cacatua 

 sulphurea, according to Meckel f), may be the bigger. 



* '' On the Carotid Arteries of Birds," P. Z. S. 1873, pp. 457-472. 

 t Prof. Garrod was unable to confirm this statement in the only speci- 

 men dissected by him (P. Z. S. 1874, p. 



