OCR Text |
Show 1891.] BRAIN OF THE MALE THYLACINE. 143 coupled with the divergence of the two hemispheres posteriorly, leaves the corpora quadrigemina partially exposed. The degree to which the corpora quadrigemina are exposed is about the same as in the Kangaroo, but considerably less than in the Wallaby or Opossum, or for the matter of that in the Rodent Dolichotis, the brain of which I propose to describe later. The cerebral hemispheres are not greatly convoluted. Judging from Owen's figures of the Opossum and the Dasyurus ursinus, there is a progressive complication of the folds in passing from the smaller to the larger forms, such as is often seen among mammals ; the Thylacine, which is the largest animal, has the greatest development of furrows of the three. Nevertheless the brain of this Marsupial is much smoother than that of a Kangaroo of about the same size. Sir Richard Owen's figure of the Dasyure's brain 1 is a little indistinct; I am not therefore able to compare it with the Thylacine very accurately. As compared with the Kangaroo2, the sulci are less numerous and often shallower. In the Kangaroo's brain the Sylvian fissure is deep and the convolutions are arranged in a series of arches round and above this fissure, as in the Carnivora; the " arched " arrangement is perhaps not so plain as in the Carnivora, and there are only two arches. In the Thylacine such an arrangement of the gyri could not be made out, the principal furrows passing obliquely so as to divide the brain into three unequal segments. The furrow separating the hemispheres proper from the olfactory portion (the hippocampal gyrus) is well marked, and as usual is bent upwards at about the middle of its course, but the angle formed is not so acute as in Macropus. From the highest point of this bend arises the Sylvian fissure (S, fig. 2, p. 141), which passes nearly vertically upwards and is about half an inch in length. On one side of the brain the Sylvian fissure could be followed as a very shallow groove into the posterior of the two principal sulci. On a dorsal view the hemispheres are seen to be divided into three unequally sized areas by two furrows running obliquely and approximately parallel to each other. The posterior fissure reaches tbe middle line of the brain 29 m m . in front of its posterior boundary, i.e. 19 m m . behind anterior boundary of hemispheres. On the left side of the brain this fissure runs parallel with the rhinal fissure; the commencement only is shown in Gervais's figure: near to its posterior termination it gives off a short descending fissure which does not reach the rhinal fissure, but stops short about a 1 Loc. cit. pi. v. fig. 5. 2 I follow Owen's figure (Phil. Trans. 1837, pi. v. fig. 4, and pi. vi. fig. 1), which, except for some slight differences, probably individual, agrees with a brain in m y possession. Gervais's figure of the brain of a " Kangurou geant " (loc. cit. pi. 13. fig. 1), which I take to be the same species, is that of a larger individual (?) and is more convoluted, and the convolutions are a little different; but the cast which he figures is like the brain before me. Sir W . Turner's figure (" The Convolution of the Prain; A Study in Compai'ative Anatomy," Journ. Anat. Phys., Oct. 1890, p. 118. fig. 11) of Macropus major is nearly identical with the brain I have examined. |