TO BIRD-ANATOMY AND CLASSIFICATION. 193 



(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 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 position in the 

 hypapophysial canal; but the left runs up the neck superficially in company 

 with the left jugular vein and vagus (pneumogastric) nerve. This con- 

 dition 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 stellaris), or either the right (Phoenicopterus) or the left 

 (Cacatua sulphur ea, according to Meckel*), may be the bigger. 



To these four conditions, duly noticed in Prof. Garrod's paper above Ibis, 1881, 

 quoted, may be added two more : P' 



(5) When the right carotid only is present, as is the case t, as dis- 

 covered by him, in the Bustards of the genus Eupodotis. 



(6) In Bucorvus abyssinicus, as discovered by my friend Mr. W. Ottley, 

 who, at Prof. Garrod's request, undertook a reexamination of the question, 

 the two carotid arteries are reduced to fibrous imperforate cords, and 

 their place is taken (functionally) by two vessels which are apparently 

 enormously enlarged equivalents of the comes nervi vagi of other birds J. 



From these facts it is evident that, taken per se, the disposition of the 

 carotid arteries has not much significance amongst birds, there being 

 many families in which, whilst the majority of the species have two, 

 some have only one carotid. This, for instance, is the case with Toccus 

 and Buceros amongst the Bucerotidae, Plotiis and PTialacrocorax amongst 



* Prof. Garrod was unable to confirm this statement in the only specimen dissected 

 by him (P. Z. S. 1874, p. 588). 



t P. Z. S. 1874, p. 473. 



J " On a Peculiarity in the Carotid Arteries and other Points in the Anatomy of 

 the Ground-Hornbill," P. Z. S. 1876, pp. 60, 61. Also " A Description of the Vessels 

 of the Neck and Head in the Ground-Hornbill (Bucorvus abyssinicus), by W. Ottley, 

 F.R.C.S.," P. Z.S. 1879, pp. 461-467. 



o 



