1030 HISTOLOGY OF CORTEX. [BOOK in. 



diminishing in 'size from below upwards, and the layer has been 

 called the "layer of large pyramidal cells," though in certain 

 regions the largest pyramidal cells, and notably the giant cells are 

 found in the preceding, fourth, layer. The cells are on the whole 

 scattered somewhat sparsely, though frequently gathered into 

 small groups, and among them occur small ' nuclear ' and other 

 cells. The bundles of vertical fibres spread out rapidly in this 

 layer so that the columnar arrangement becomes lost, and many 

 of the fibres undoubtedly become axis cylinder processes of the 

 pyramidal cells. Though the layer varies in thickness (1 mm. 

 to *4 mm.) and in some of its features in different regions, the 

 characteristic pyramidal cells are present over the whole surface 

 of the hemisphere. In the lower part of the layer a second 

 horizontal streak of closely interwoven horizontal fibres frequently 

 makes its appearance. 



The second layer, generally a thin one, though varying from 

 '25 mm. to '75 mm. in thickness, is also formed by pyramidal cells 

 but is distinguished from the layer below by the absence of large 

 and medium sized cells and by the presence of numerous small 

 cells closely packed together; it has been 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 neuroglial in nature than in 

 the other layers, and, as we said above, its extreme surface 

 appears to be furnished by neuroglia alone. The layer is gener- 

 ally spoken of as the ' peripheral ' or ' superficial layer,' or some- 

 times 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 somewhat fine medullated fibres 

 also traverse it in a horizontal direction. 



651. The general arrangement just described varies as we 

 have said in different regions of the cerebral surface. We must 

 content ourselves here with pointing out the characteristics of 

 two or three important regions. 



The region which we have ( 632) 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 pyramidal cells, of the kind 

 which are referred to above, 649, as being frequently 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 



