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Show 1904.] OF THE BRAIXS OF MAMMALS. 193 anterior cerebral arteries diverge. This was also the case in a brain of Mustela subpalmata. On the other hand, in Rodents it at least sometimes is the case that the ophthalnxics arise farther back along the circle of Willis ; and when, as in the Porcupine (Hystrix cristata), a carotid is present and x-eaches the circle of Willis, it joins the circle at a point considerably anterior to the emergence of the ophthalmic artery. § Anterior Cerebral Artery. Tandler, in the memoir referred to, does not pursue the course of the anterior cerebral arteries far beyond their origin from the circle of Willis. That is indeed not his object. I may therefore direct attention to a few facts relating to the distribution of the anterior cerebral ax-tery and its branches. In M a n tlxe two antex-ior cerebrals give off comparatively inconspicuous branches to the anterior lobes of the brain, and themselves form entix-ely the two arteries which run over the corpus callosum and supply the hemisphex-es right and left. The opposite extreme appears to m e to be shown in the brain of Myopotamus and some othex- types. In Myopotamus (see text-fig. 22, A, p. 195) the anterior cerebx-al of each side divides early into two branches, of which the inner soon meet and fuse and are continued forward as the at first single but soon double callosal artery. The circle of Willis is thereby completed. The outer of the two arteries preseixtly gives off a branch, on each side of which, as will be seen from the drawing just referred to (text-fig. 22, A,f p. 195), I traced only one-that on the left side-from beginning to end. This artery joixxs the callosal after its division into the two forwardly running arteries. The callosal arteries are thus formed by two bx-axxches ax-ising from the anterior cerebx-al in Myopotamus, and by only one branch in Man. It is possible that the anterior communicating artery of M a n represents in a rudimentary way the second and stronger* tributary of tlxe callosal artery in the Rodexxt. Both of these two types show modifications in other mammals. In the brain of the diprotodont Marsupial Beltongia penicillata the circle of Willis is quite incomplete anteriorly; there is no communication between its two sides-no anterior communicating artery as it is termed in human anatomy. The middle cerebral (the artery running along the Sylvian fissure) joins at an acute angle an artery running forwards, which is evidently the anterior-cerebral . This vessel runs on each side close to the interhemispheral edge of the anteriox- lobe; the two arteries are thus nearly in contact when tlxe brain is undisturbed. These arteries run right forward to the anterior end of the bx-ain, and each gives off at any rate two branches of importance. One of these runs towards the outside of the brain in the furrow below the olfactory bulb. The other branch, arising a little further on and at right angles to the main trunk, passes over the commissural region and supplies P R O C . Z O O L . Soc-1904, V O L . I. No. XIII. 13 |