ON SOME HISTOLOGICAL FEATURES OF THE BRAIN. 659 



called " the layer of small pyramidal cells." As we have said, these smaller 

 pyramidal cells differ somewhat from the larger cells ; and the cells in this 

 layer are sometimes described as " angular." 



The first and most superficial layer is characterized by the predominance 

 of the molecular ground substance, the cells being few. far between, small, 

 and irregular. The ground substance itself seems to be more largely neur- 

 oglial iii nature than in the other layers, and, as we said above, its extreme 

 surface appears to be furnished by neuroglia alone. The layer is generally 

 spoken of as the " peripheral " or " superficial layer," or sometimes as the 

 " molecular " layer. The tapering vertical processes of the pyramidal cells 

 may be traced into this layer, which indeed varies in thickness according to 

 the abundance of pyramidal cells in the subjacent layers ; numerous some- 

 what fine medullated fibres also traverse it in a horizontal direction. 



564. The general arrangement just described varies, as we have 

 said, in different regions of the cerebral surface. We must content our- 

 selves here with pointing out the characteristics of two or three important 

 regions, 



The region which we have ( 545) called the " motor area " or " region " 

 is characterized on the one hand by the great thickness (1 mm.) of the third 

 layer, that of large pyramidal cells, as well as by the number and size of 

 the cells contained in it, and on the other hand, and especially, by the 

 prominence in the fourth layer of remarkable clusters of very large pyram- 

 idal cells, of the kind which are referred to above ( 562) as being fre- 

 quently called " ganglionic ; " it is in this region that " giant cells " are 

 found in the fourth layer, namely, in the upper part of the precentral and 

 at the summit of the postcentral convolution, and in the paracentral 

 lobule, acquiring their greatest size at the top of the precentral con- 

 volution. 



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 arrangements 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 con- 

 tent with insisting that the great mark of this occipital region is the abun- 

 dance 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 arrangement 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 



