1873. J CAROTID ARTERIES OF BIRDS. 461 



the following as a step towards it. In birds possessing two carotids 

 those vessels, after they have once met, run close together in the 

 hypapophysial canal, but do not blend or anastomose in any way. 



In Botaurus stellaris, Cacatna sulphurea (according to Meckel, 

 as shown in the diagram, fig. 6, p. 45!)), and the genus Phceni- 

 copterus, the carotids join to become one vessel at the spot where, 

 in others, they come into contact, each proximal portion persisting. 

 What I desire to show is, that on simple mechanical principles it 

 is much more likely, when the two vessels do so blend, that the 

 right should disappear, leaving the left solely to maintain the cere- 

 bral and cervical circulation ; in other words, the assumption that 

 there is a blending of the left with the right carotid in early 

 life is sufficient to explain the absence of the right in birds thus 

 affected. The diagram fig. 4, p. 459 (which shows the distribution 

 of the arteries at the base of the neck as they would appear imme- 

 diately after the fusion of the carotids), will help to explain my 

 meaning. The blood-current, almost immediately it has passed the 

 aortic valve, divides into two, one going along the left innominate, 

 and the other following the course of the aorta until it very shortly 

 further divides into that traversing the right innominate, and that 

 which continues on to the abdomen and posterior extremities. Such 

 being the case, and the two carotids being of equal calibre, it is 

 evident that, just as in Wheatstone's Bridge the electric current is 

 less intense in the bridge itself than in the branches, the current 

 in the right carotid, which, in the case under consideration, con- 

 nects the left carotid with the aorta distad of the point at which 

 the left innominate springs, is less than in the vessels it connects ; 

 consequently the current there tends to stagnate ; but a tendency 

 to stagnate in blood is a tendency to coagulation, as is seen in the 

 proximal end of a ligatured arterial trunk ; and the tendency to 

 coagulation is a tendency to obliteration of the vessel in which the 

 coagulation occurs ; consequently the right carotid must tend to 

 disappear, which it does in nearly every case. Since this explana- 

 tion occurred to me, I have not had the opportunity of examining 

 any of the birds in which the right artery persists after it has fused 

 with the left, to see if there is any peculiarity in their vascular 

 arrangement which will account for its persistence. When the 

 carotids do not blend there is evidently no reason why either should 

 disappear ; and when they do join, the presence of a large pectoral 

 and subclavian branch from each innominate does not alter the 

 problem ; it only indicates that the obliteration must occur distad of 

 it, as is the case. 



The following list includes all those species of birds in which I 

 have had the opportunity of observing the disposition of the carotid 

 arteries. They are arranged nearly according to the classification 

 adopted in Mr. Sclater's revised List of the Vertebrated Animals in 

 the Gardens of this Society. 



