TRANSACTIONS OF SECTION H. 779 



convoliitionary pattern it was very early found that the simple cerebral suvfuce of 

 the ape's brain in many cases afforded the key to tlie solution of the problem. 

 More recently the close 'study of the manner in which the convohitions assume 

 shape during" their frrowth and development has yielded evidence of a still mori' 

 valuable Idnd. We now know that the primate cerebrum is_ not only disl iuguished 

 from that of all lower mammals by the possession of a distinct occipital lobe, but 

 also by havinq- Imprinted on its surface a convolutionary design, which in all but 

 a few "fimdamental details is diilerent from that of any other order of mammals. 



There are few matters of more interest to those antln-opologists wiio make a 

 study of the htuuan skull than the relationship which exists between the cranium 

 and the brain during the period of ac^ive growth of both. Up to the time imme- 

 diately prior to the pushing out of the occipital lobe, or, in other words, the period 

 in cerebral development which is marked by the transition from the quadrupedal 

 type to the primate type of cerebrum, the cranial wall fits lilre a tight glove on the 

 surface of the enclosed cerebrum. At this stage there would appear to be a, growth 

 antagonism between the brain and the cranial envelope which surrounds it. The 

 cranium, it would seem, refuses to expand with a speed sufficient to meet the 

 demands made upon it for the accommodation of the growing brain. In making 

 [his statement it is riii'ht to add that Ilochstetter, in a carefully reasoned memoir, 

 has recently cast doubt upon the reality of the appearances which have led to this 

 conclusion,' and at the recent meeting of the Anatomische Gesellschaft, in Bonn, 

 Professor Gustaf Retzius,' one of the numerous observers responsible for the 

 description of the early cerebrum upon which the conclusion is based, showed 

 some inclination to waver in his allegiance to the old doctrine. This is not the 

 time nor the place to enter upon a discussion of so technical a kind, but I may 

 be allowed to say that whilst I fully recognise the necessity for further and more 

 extensive investigation into this matter I do not think that Ilochstetter has satis- 

 factorily accounted for all the circumstances of the case. 



When the occipital lobe assumes shape the relationship of the cranial wall to 

 the enclosed cerebrum undergoes a complete change. The cranium expands so 

 rapidly that very soon a wide interval is left between the surface of the cerebrum 

 and the deep aspect of the cranial envelope within which it lies. This space is 

 occupied by a soft, sodden, spongy meshwork, termed the subarachnoid tissue, and 

 it is into the yielding and pliable bed thus prepared that the convolutions grow. 

 At first the surface of the cerebral hemisphere is smooth, but soon particular areas 

 of the cortex begin to bulge out and foreshadow the future convolutions. These 

 suffer no growth restriction, and they assume the form of round or elongated 

 elevations or eminences which rise above the general surface level of the cerebral 

 hemisphere and break up its uniform contour lines in the same manner that moun- 

 tain chains protrude from the surface of the globe. 



As growth goes on, and as the brain gradually assumes a bulk more nearly in 

 accord with the cavity of the cranium, the space for surface protrusions of this 

 kind becomes more limited. Tlie gyral elevations are now pressed together : they 

 become flattened along their summits, and in course of time they acquire the 

 ordinary convolutionary shapes. While this is going on the valleys or intervals 

 between the primitive surface elevations become narrowed, and ultimately assume 

 the linear slit-like form characteristic of the fissures. These changes occur shortly 

 before birth, but are not fully completed until after the first few months of infancy. 

 The final result of this process is that the convolutions come into intimate relation 

 with the deep aspect of the cranial wall and stamp their imprint upon it. 



It is obvious that certain of the later changes which I have endeavoured to 

 portray might be ascribed to a growth antagonism between the brain and the 

 enclosing cranium at this period. In reality, however, it is merely a process by 

 which the one is brought intp closer adaptation to the other — a using up, as it were, 

 of superfluous space and a closer packing together of the convolutions— after the 

 period of active cortical growth is past. Nevertheless the convolutionary pattern 

 is profoundly aflected by it, and it seems likely that in this process we find the 



' Anatomische Gesellschaft, Bonn, May 28, 1891. Gustaf Retzius, TransUorische- 

 Furchcn des (rrnxshirns. 



