OCR Text |
Show 1905.] ARTERIAL SYSTEM IN SAUROPSIDA. basilar artery are the posterior cerebellar, and these arise a little behind the middle of the medulla. The two arteries are perfectly symmetrical with each other as to their point of origin. They are, however, different in their branching. The light artery gives off, shortly after its origin from the basilar, a strong artery running backwards along the side of the spinal cord. This branch exists and pursues the same course on the left side; but on that side of the brain it arises separately from the basilar artery. Between the origin of the posterior cerebellar arteries and the bifurcation of the basilar anteriorly are three pairs of small arteries supplying adjacent regions of the medulla. A slightly larger artery, which is the anterior cerebellar, arises from the fork of the basilar. This fork is U-shaped in the Teguexin (text-fig- 18, p. 65), and not V-shaped as in the other Lizards described here. The U -shape is due to the fact that the two carotids run parallel to and almost in contact with each other for some distance before they join the circle of Willis. The carotids, moreover, lie within the area bounded by the third nerves very close to and about on a level with those nerves. The artery formed by the junction of the basilar and carotid on each side, often spoken of merely as the carotid, passes outwards and slightly backwards at first, when it is practically at right angles with the basilar. In this region the artery shows different relations on the two sides of the body. On the left side it runs in front of the third nerve; on the right side it lies behind that nerve. The first branch arising after the carotid is at the bend of the artery, where it turns forward ; this very stout artery supplies the cerebellum and optic lobe; immediately in front of this is the artery of the optic lobe. This state of affairs occurred on the left side of the body; on the right side the two arteries arose by a common trunk. On both sides the artery of the corpus bigeminum gives off an artery to the cerebral hemisphere which buries itself in the furrow between the hemisphere and the optic lobe. From the inner side of the circle of Willis, just opposite to the bigeminal artery on the left side and to the conjoined arteries just mentioned on the right side, arises an artery which runs to the optic chiasma. This artery is precisely like that of other Lacertilia. The next artery to be given off' is the posterior cerebral, which plunges at once into the furrow lying between the optic lobe and the cerebral hemisphere. The middle cerebral artery, which is the largest of the cerebral arteries, runs in the usual way along the Sylvian depression, and just in front of the point of origin of this the circle of Willis practically ends in the strong ophthalmic arteries which follow the optic nerves. There are therefore no differences of importance between the arterial system of the brain of Tupinambis and of the other genera of Lizards reported upon in the present communication. § Cerebral Arteries in the Lacertilia. We may deduce from the facts just described the chief |