CHAP. ii.J THE BRAIN. 1031 



at the summit of the postcentral convolution, and in the para- 

 central lobule, acquiring their greatest size at the top of the 

 precentral convolution. 



The occipital region is characterized by the prominence of the 

 ' granule ' or ' nuclear ' cells. These not only form a distinct 

 division of the fourth layer, but are also conspicuous in other 

 layers, their arrangement being such that some authors have 

 been led to divide the cortex of this region into seven or even 

 eight layers. In the present state of our knowledge we may 

 be content with insisting that the great mark of this occipital 

 region is the abundance of these small ' nuclear ' cells together 

 with other small ' angular ' cells, whereby the pyramidal cells seem 

 to be made less conspicuous. It is worthy of notice however 

 that in the third, but more especially in the fourth layer, a 

 few cells of very large size are met with, which by their large 

 branched cell substance and conspicuous axis cylinder process 

 resemble the large cells in the motor region; but it should be 

 noted that while these large cells occur, (at least in man and in 

 the monkey, though not in some of the lower animals as the 

 rabbit), in very definite clusters in the motor region, they occur 

 singly in the occipital region. In this occipital region the layer 

 of horizontal fibres in the fourth layer is very conspicuous, and 

 owing to the number of ordinary medullated fibres present forms 

 a white streak visible even to the naked eye. 



In the frontal region, in front of the motor region, the arrange- 

 ment is more in accordance with what we have described as the 

 general plan. The two pyramidal layers are well marked as is also 

 the fourth layer ; but the layer of large pyramidal cells is much 

 thinner than in the motor region, as is also, though to a less 

 extent, the fourth layer, while the fifth layer, that of fusiform cells, 

 is thicker than elsewhere. Small ' nuclear ' cells are perhaps 

 more abundant in this region throughout all layers than in the 

 motor region, but are far less conspicuous than in the occipital 

 region. 



We may here remark that the transition in structure from one 

 region to another is very gradual, not sharp and distinct, and is 

 perhaps especially gradual in passing from the motor region 

 backwards to the occipital region. It is not possible to recognize 

 histologically the limit, for instance, of the motor region as 

 determined experimentally. 



In special regions of the brain, for instance in the olfactory 

 bulb of which we shall speak later on, very great modifications 

 of the general plan may be observed in the cortex. We cannot 

 enter upon these but may just refer to the cornu ammonis or 

 hippocampus. At the ventral end of the temporal lobe, the gyrus 

 hippocampi, the structure of whose cortex follows the general plan, 

 is thrust inward so as to project into the cavity of the descending 

 horn of the lateral ventricle, forming the ridge-like prominence 



