THE CAROTID ARTERIES OF BIRDS. 165 
carotids are frequently of unequal size; in the Dabchick* the left is 
the largest; in an Emu I found it the smallest.” I may here remark 
that on several occasions I have watched the Flamingos sleepmg; and 
they do so, some with the neck bent one way and some the other, in a 
manner quite independent of the constant peculiarity in the arteries of 
their necks. 
All these explanations, therefore, fail to show why birds should 
have two or only one carotid artery ; and it is the last of them only 
that takes into consideration which carotid would be absent when 
there is any deficiency. If it were proved that all birds with a left 
carotid slept with their necks bent in one direction, the only explana- 
tion would be, that they did so because the arrangement of their 
cervical vessels would not allow of their doing otherwise, and con- 
sequently an argument in a cirele would be the only result. The ulti- 
mate cause is most probably as yet some way beyond our grasp; but I 
would offer the following as a step towards it. In birds possessing Page 461. 
two carotids those vessels, after they have once met, run close to- 
gether in the hypapophysial canal, but do not blend or anastomose in 
any way. 
In Botaurus stellaris, Cacatua sulphurea (according to Meckel, as 
shown in the diagram, Fig. 6 [p. 164]), and the genus Phenicopterus, 
- 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 disap- 
pear, leaving the left solely to maintain the cerebral and cervical cir- 
culation; 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. 163] (which shows the distribution of the arteries at the base of the 
neck as they would appear immediately after the fusion of the caro- 
tids), 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, 
connects the left carotid with the aorta distad of the point at which 
* In the Grebes (Podiceps), according to my observations, the right carotid is 
not found to be present at all—A. H. G. 
