1904.] OF THE BRAINS OF MAMMALS. 193 



anterior cerebral arteries diverge. This was also the case in a 

 brain of Mitstela suhpahnata. On the other hand, in Rodents it 

 at least sometimes is the case that the ophthahiiics arise farther 

 back along the circle of Willis ; and when, as in the Porcupine 

 {Hystrix cristata), a carotid is present and reaches the circle of 

 WilHs, 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 oiigin 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 artery and its branches. In Man the two 

 anterior cerebrals give off comjjaratively inconspicuous branches 

 to the anterior lobes of the brain, and themselves form entirely 

 the two arteries which run over the corpus callosum and supply 

 the hemispheres right and left. 



The opposite extreme appears to me to be shown in the brain 

 of Myopotamus and some other types. In Myopotamus (see text- 

 fig. 22, A, p. 195) the anterior cerebral 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 outei' 

 of the two arteries presently gives ofi" a branch, on each side of 

 which, as will be seen from the drawing just referred to (text- 

 fig. 22, A,/, p. 195), I traced only one — that on the left side — from 

 beginning to end. This artery joins the callosal after its division 

 into the two forwardly running ai-leries. The callosal arteries ai-e 

 thus formed by two branches arising fi-om the anterior cerebral in 

 Myopotamus, and by only one bi'anch in Man. It is 230ssible that 

 the anterior communicating arteiy of Man represents in a rudi- 

 mentary way the second and sti-onger tiibutary of the callosal 

 artery in the Rodent. 



Both of these two types show modifications in other mammals. 

 In the brain of the diprotodont Marsupial Beitongia pifinicillata 

 the circle of Willis is quite incomplete antei-ioi-ly ; 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 anteiior 

 cerebral. This vessel runs on each side close to the interhemispheral 

 edge of the anteiior lobe ; the two arteries are thus neaid}^ in 

 contact when the brain is undisturbed. These artei-ies run right 

 forward to the anterior end of the brain, and each gives ofl'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 

 Proc. Zool. Soc— 1904, Yoi,. I. No. XIII. " 13 



