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Show 460 MR. A. H. GARROD ON THE [May 6, Meckel, Nitzsch, and Owen stating that there is only a right carotid in Phoenicopterus. I have had the opportunity of examining two specimens of Phoenicopterus antiquorum and two of P. ruber ; and in all of them both carotids were present in the lower part of the neck, the right being much the larger and being joined by the left to form one trunk at the point in the neck where they first meet, as in Botaurus stellaris. Both vessels carried blood ; but the calibre of the left was extremely small, and that of the right was nearly the same as it would have been if it alone had been present. From the list at the end of this paper it is shown that of 300 genera in which the arrangement of the carotids has been observed, in 193 of them both are present, in 107 the left only ; in one only are both equal when they join in the neck ; in another they join, the left being the smaller; and in one other the right is the smaller under similar conditions; whilst perhaps one possesses the right only. So it may be generally stated that in birds either both carotids are present separate, or the left only exists. Several attempts have been made by different authors to account for these peculiarities. According to Bauer, the simplicity of the carotids (in other words, the presence of the left instead of two) is dependent ou the size of the individual, the smaller species having the single trunk. Undoubtedly the great majority conform to this rule; but there are too many exceptions, as shown by Meckel, to make the generalization of much value, Rhea, Podiceps, Cacatua, Talegalla, and Menura possessing only the left. Meckel originally thought that a correlation existed between the length of the neck and the simplicity of the carotids; but when he found two carotids in Struthio, Dromceus, Cygnus, and Ardea he acknowledged that such was not the case. Prof. Owen remarks*, " Birds as a rule are peculiar in sleeping with their long necks much bent or twisted; and this position might be expected to exercise some effect on the vessels subject thereto. Accordingly we find that the carotids are frequently of unequal size; in the Dabchickf 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 sleeping; 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 explanation would be, that they did so because the arrangement of their cervical vessels would not allow of their doing otherwise, and consequently an argument in a circle would be the only result. The ultimate cause is most probably as yet some way beyond our grasp ; but I would offer * Anatomy of Vertebrata, vol. ii. p. 190. t In the Grebes (Podiceps), according to m y observations, the right carotid is not found to be present at all.-A. H. G. |