| OCR Text |
Show 1905.] ARTERIAL SYSTEM IN SAUROPSIDA. olfactory branch arises beneath the olfactory lobe at the junction of the latter with the cerebral hemisphere ; before this point of junction a branch is given off to the olfactory lobe. The circle of Willis is completed anteriorly; it also ends in two strong branches which run along the under surface of the brain, anteriorly, giving off numerous branches at the junction of the hemispheres with the olfactory lobes. One or more of these bend downwards (as the brain is viewed from beneath) and pass through the gap between the hemispheres running to the dorsal side of the brain in a way which suggests the callosal artery of the mammals. I could not detect anything more than a very small branch arising where the ophthalmic arteries arise in the Lacertilia. I cannot think that this artery is absent, but it is clearly not so conspicuous as in the Lizards. It is evident that the encephalic arterial system differs quite as much from that of either Lizards or Snakes as do the encephalic arterial systems in the two last mentioned groups. It is thus plainly possible to distinguish between several types of distribution of the cerebral arteries among the different divisions of the Sauropsida, and there is, as is well known, another type characteristic of mammals. It will be interesting to ascertain how far these several types confirm views as to the relative positions of the groups of Sauropsida under consideration. It will not be held by anyone, I presume, that the class Aves represents a primitive Sauropsidan type ; and in agreement with this presumption we find clear evidence of modification in the encephalic arteries *, in the abortion of one or other of the normal two branches of the basilar. On the other hand, the arteries in question of birds are, as I think, undoubtedly primitive in that there is no completion of the circle of Willis anteriorly. A completed circle of Willis appears to me to be a secondary modification mainly for the reason that in mammals, where it occurs universally, it is there brought about in more than one way, and is moreover associated with strong arteries in the anterior region of the brain in close communication, or rather in close apposition, and there is apt to be confluence between closely apposed spaces and vessels. If this view be correct, wre can set aside the brain of the Python and that of Testudo as showing primitive characters by virtue of the fact that they have a closed circle of Willis. And in addition to this, it may be pointed out that the asymmetry of the arterial system in the Snake indicated by the carotids, and the changes in the disposition of the vessels due to the prevalence of the left carotid, can be fairly regarded as being secondary. This conclusion is obviously in accord with the current views of the relations of the Ophidia to other reptiles. There only remains the Lacertilia. It is, in my opinion, probable that in this group (and in Hatteria) the whole question lies of the antiquity of existing * P. Z .S . 1905. vol. i. p. 102. |
