OCR Text |
Show 1905.] ARTERIES OF THE BRAIN IN BIRDS. 105 which the anterior bends inwards and gives off blanches which anastomose with those of the anterior cerebral. The latter artery is about equal in size to either the middle or the posterior cerebral, and arises along the circle of Willis some way in front of the middle cerebral. The two anterior cerebrals anastomose anteriorly in the middle ventral line of the brain. Dromons ?wvce-hollandice.-A beautifully injected brain of this species shows some differences from that of the Ostrich. The space enclosed by the anterior bifurcation of the spinal artery and the reunion of the vessels to form the basilar artery is somewhat more extensive than in Struthio. There is an asymmetry in the relations of the cerebellar arteries to the trunk of which they are branches ; but the asymmetry is different. The spinal artery arises from or joins the right cerebellar, and both of the cerebellar arteries lie in front of the sixth pair of nerves, instead of one in front and one behind as in Struthio. The bifurcation of the basilar artery anteriorly is rather peculiar in the specimen before me. There is the usual asymmetry, but it is unusual in its character. Just behind the optic chiasma the basilar artery bends to the right side of the brain and becomes continuous with the carotid in the usual way. About halfway between the point where the basilar artery becomes deflected to the right and its bifurcation posteriorly to form the cerebellar arteries, an artery of one-half of the diameter of the basilar arises from it on the left, and after giving off branches to the medulla runs forward and becomes connected with the left carotid. Anteriorly the carotids give off the usual arteries ; but their main stem is the middle cerebral artery, which passes along the Sylvian fissure. The ophthalmic artery arises at the root of this, and immediately afterwards, apparently almost by a common stem with the ophthalmic, the anterior cerebral. This artery divides on each side into two branches, fairly equisized, of which the inner supplies the olfactory bulbs, which are here large. It is as well developed as in Goura (described below), and much more conspicuous than the minute corresponding artery of Struthio. The outer branch of the anterior cerebral again divides into two equisized branches, as is the case with Struthio. Ara hyacinthina (text-fig. 16, p. 106).-The anterior spinal artery is single throughout and slender, but shows no such great disproportion' in calibre to the basilar artery such as is apparent, for example, in the Penguin, Spheniscus demersus, described below. It is, however, rather more slender than the basilar artery. The two arteries do not pass directly into each other; for the anterior spinal opens into the left cerebellar artery, quite close, however, to its point of origin from the basilar artery. The cerebellar arteries, with the slight exception just mentioned, are symmetrical and arise behind the origin of the sixth pair of nerves. It is important to note that in this bird the basilar artery is |