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Show 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, Cacatua sulphurea (according to Meckel, as shown in the diagram, fig. 6, p. 459), and the genus Phoenicopterus, 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 cerebral and cervical circulation; in other words, tbe 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 immediately after the fusion of the carotids), will help to explain m y 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 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 explanation 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 w h y 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 M r . Sclater's revised List of the Vertebrated Animals in the Gardens of this Society. |