OCR Text |
Show 1879.] MR. W. OTTLEY ON THE GROUND-HORNBILL. 467 singular fact that in this bird, and in this bird alone, so far as is at present known, such a remarkable event should occur as the complete obliteration of the principal vessels for the supply of the head. The presence of two quite distinct vestiges of the missing arteries, in the shape of the two fine cords, which have been already described, would seem to indicate that this obliteration must have occurred after the arteries in question had been fully formed and for some Plan of the internal carotid and its branches, with the arteries in the orbit (eidarged). Right side. time in use. The manner in which the obliterated cord joins the vertebral artery above suggests that the vessel from which the internal carotid springs was originally a direct continuation of the common carotid trunk. And the alteration that would be necessitated by the obliteration of the main artery would be simple. The vessel which extends from 10 to the vertebral canal would then have been the occipital, which normally should anastomose with the vertebral in this position, while the part from 10 to 2 would be the external carotid, giving the usual branches. It seems probable therefore that at some time in the history of this bird the distribution of its vessels differed but little from that usually met with ; but this fact perhaps adds to the difficulty of accounting for the change that has taken place. The theories hitherto proposed to account for such obliterations of the vessels of the neck in birds have only dealt with a change affecting one side of the body. This is the only instance which I a m acquainted with of the symmetrical closure of two such important arteries at a period which, if I a m correct in m y supposition, was subsequent to their full development and functional activity. 30* |