OCR Text |
Show 114 MR. F. E. BEDDARD OX THE [Feb. 7, front of the sixth nerves. The arteries of the corpora bigemina are nearly symmetrical with each other; each divides into two branches just before reaching its corpus bigeminum. Of these branches the anterior is the smaller. There is nothing specially noteworthy to record concerning the cerebral arteries. Goura coronata.- In this bird the two vertebral arteries reach the brain just behind the medulla ; before passing forward they give off, as in the case of many mammals (as, for instance, Man), a very delicate anterior spinal artery, thereby contrasting with many birds, where the anterior spinal is of as great a calibre as the basilar artery. The vertebrals then run forwards separately, and unite to form the basilar artery well behind the origin of the cerebellar arteries. The latter arise much further forwards than in the Ostrich (described above, p. 103). Furthermore, the branch of this artery which runs backward along the lateral aspect of the medulla differs from that of the Ostrich in its relation to the adjacent nerve; moreover, the main continuation of the artery to the cerebellum has also a different relation to adjacent nerves. A final difference in the arterial system of this region of the brain is that, while in the Ostrich the main branch of the cerebellar artery passes over the flocculus and is distinctly the most important artery of the cerebellum, in Goura the artery which follows the same course is not derived from the posterior cerebellar, but from an anterior cerebellar arising from the basilar artery further forwards, and which is of the same calibre as the posterior cerebellar. This artery exists in the Ostrich, but is not nearly so large as the posterior cerebellar. The posterior cere-bellars, it should be observed, arise symmetrically from the basilar ; the left anterior cerebellar arises in advance of the right anterior cerebellar. As in the Ostrich, the left postexior communicating artery is much the stronger. The anteriorly situated cerebral arteries have a curiously asymmetrical and compensative arrangement. As in other birds, the carotids curve round and each ends in the ophthalmic artery. The main cerebral arteries are three in number. The first two of these are exactly as in Struthio, lying respectively between the cerebrum and the corpus bigeminum and along the Sylvian fissure. The third artery, however, consists of two branches, of which the more important approaches its fellow of the opposite side and runs mesially forward to the olfactory lobes; the branch running forwards and supplying the under surface of the hemisphere to the side of this is less important. In Struthio the precise reverse is the case. Moreover, this branch, lying to the side of the olfactory branch of the anterior cerebral artery, is only well developed on the right side ; it exists on the left side, but is functionally replaced on that side by a branch of the middle cerebral artery.‘ This branch is present on the right side but very small. Gymnorhina leuconota.- There are a number of features (see |