68 ATLAS OF NERVE CELLS 



THE THIRD LAYER, OR LAYER OF LARGE PYRAMIDAL CELLS 



Plate XL I. gives a general view of the second and third layers of the cortex, or the 

 layers of pyramidal cells. These two layers are only to be distinguished from one another by 

 the size of their cells, the deeper cells being much larger and reaching from 15 //, to 40 /x in 

 diameter. There is no actual boundary between the two layers, and Cajal prefers to consider 

 the two as one. In fact, the cells are small, intermediate, and large, as they lie deeper. 

 Their apical processes or stalks reach up to the superficial layer. The cells are seen to lie 

 at various levels, yet they are so nearly together as to make quite a distinct stratification of 

 the cortex. The characteristics of the cells in the deep or third layer are the great size of 

 the cells, the length of their apical process, and the greater number of the vertical fibres 

 which give this layer a striated appMfSfee. The numerous dendrites of the cells, running out 

 from their bases, and also the '^n^lraxoiW with collaterals, can be seen in the plate. The 



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special features of this layer are *eesHto better advantage in the plates which follow. 



Plate XLII. shows very clearly a group of intermediate-sized pyramidal cells in a human 

 embryo of eight months. 



The cells of the second layer did not stain, hence the course of the apical process can 

 be traced to the superficial layer. 



The characteristic triangular appearance of these cells is to be noticed ; their dendrites 

 coming off at the corners and subdividing; their long apical projection running far up into 

 the second layer, giving off branches as it ascends, and finally splitting up into two parts, 

 which diverge and enter the superficial layer of tangential fibres ; and lastly, their axis cylin- 

 der process, coming out of the base, passing downward and giving off one or two collaterals 

 in its course. 



It is quite evident that the apex projection has a rough surface covered by gemmules or 

 little spike-like or thorn-like excrescences. These appear to be better marked on this process 

 than on the other protoplasmic processes, though they can be seen on them as well. On 

 several of the apical processes, extending through the plate, varicosities are seen. Andriezen 1 

 is inclined to attach a pathological significance to these, but Berkley 2 considers them as nor- 

 mal. Their significance is not known. In this plate and in Plates XL 1 1 1. and XL VI. a few 



1 W. L. Andriezen, On Some of the Newer Aspects of the Pathology of Insanity. Brain, Vol. XVII., p. 548; 1894. 



2 H. ]. Berkley, A Theory of the Causation of Permanent Dementia. The Medical News; Nov. 9, 1895. 



