OCR Text |
Show 6 6 MR. F. E. BEDDARD ON THE ENCEPHALIC [May 16, A noteworthy difference exists between the two specimens in relation to the course of the large left carotid. In the one brain this artery lies outside of the dura mater for a large part, of its course, and gives off at least one branch to the brain which perforated that membrane ; thus giving additional proof of the fact that the carotid itself lies outside of the dura mater. In the other brain I did not observe this state of affairs. It follows that the left carotid exhibits an aloofness from the brain which is remarkable, and that the branches therefrom do not run on the same plane with it. The arteries to the optic lobes arise from the basilar artery after its bifurcation, between this point and the entrance of the carotids, and further back still there is a smaller cerebellar artery. The posterior cerebral artery is small and arises just in front of entrance of the carotids. The next important artery is a cerebral, which arises in front of the inflow of the carotids. This artery is the middle cerebral or Sylvian of other animals, since it runs along the rudimentary Sylvian fissure. The posterior cerebral is also partly represented by several small branches of the artery to the optic lobe. In front of the middle cerebral artery is a smaller anterior cerebral artery. Anteriorly to this the circle of Willis is completed in the following way : the large left carotid bifurcates to form the two nearly equally stout ophthalmic arteries which of course accompany the optic nerves. Just before this bifurcation the slender right carotid effects a junction with the common trunk. Immediately in front of this a single trunk arises from the point of bifurcation of the left carotid, which at once divides into two. These vessels run closely side by side in the furrow which separates the two hemispheres and rejoin at the extreme anterior end of the brain, their course in fact recalling that of the callosal arteries in mammals. The arteries are by no means inconspicuous, as is shown in the annexed figure (text-fig. 19, p. 65). I now draw, of course quite in a preliminary and tentative way, a series of comparisons between the Ophidian and Lacertilian brain arteries, enumerating the characters of the former in the same order as already given (on p. 64) for the latter. § Cerebral Arteries in the Ophidia. (1) The entrance of the vertebral arteries into the anterior spinal marks the end of the medulla oblongata. These arteries seem to be stouter than in the Lacertilia. (2) There is no markedly large pair of cerebellar arteries ax-ising from the basilar ax-tery, but a nuxnber of more or less equisized arteries supplying the cerebelluxn and adjacent region. (3) The two branches produced by the bifurcation of the basilar are equisized. The anterior cerebellar arteries ax'ise froxxi the bifui-cated x'egion. (4) The point of entx'ance of the carotids appeal's to be x'atlxer |