66 ATLAS OF NERVE CELLS 



THE SECOND LAYER, OR LAYER OF SMALL PYRAMIDAL CELLS 



Plate XXXVIII. shows the second layer of the cortex or the layer of small pyramidal 

 cells; and Plates XXXIX. and XL. demonstrate the characteristic features of the cells of 

 this layer. In Plate XXXVIII. the surface layer is seen, but its cells and fibres, excepting 

 a few deep tangential fibres, are not shown. Just beneath these deep tangential fibres a 

 number of small pyramid cells (d) are evident with their short apical processes, really a part 

 of the body of the cell, running up toward the superficial layer. The size of these cells is 

 from io/t to 12 p.. In one or two cases the apical process can be seen to bifurcate, and its 

 two branches can be seen to diverge and end in the superficial layer. Other apical processes 

 coming up from cells lying more deeply, and not shown, can also be seen to take the same 

 course. The small pyramid cells are seen to have a few dendrites coming off from their 

 basal corners and a true neuraxon coming from the base which can be followed for some 

 distance downward, and in some cases can be seen to give off collaterals. The majority of 

 the dark broad black lines in this plate are capillary blood-vessels. 



In the lower part of the plate the dense mass of nerve fibres interlacing and running 

 in all directions is noticeable. All through this layer it is evident that fine neuraxons as 

 well as apical projections of deeper cells are making their way toward the surface layer, many 

 of them giving off collaterals in their course. 



Plate XXXIX. shows this second layer under a higher power of magnification and brings 

 out the structure and appearance of a few of its cells and the enormous mass of its fibres. 

 Plate XL. shows the characteristics of its cells alone. 



The cells have a triangular shape with narrow base and long apex (e). The body gives 

 off a few branches which pass outward, usually downward, from the two basal corners of the 

 cell. These have the characteristic features of dendrites, tapering as they pass on, having 

 an irregular surface, giving off a few branches at an acute angle, which branches in turn 

 divide and give off secondary branches. The general direction of these dendrites is diagonal 

 to the vertical fibres of the layer. The neuraxon also comes off from the base of the cell, 

 but rarely from a corner, usually in the middle. It differs from the dendrite in being 

 smooth, fine, of uniform calibre, and giving off its finer collaterals at right angles to its 

 course. Cajal and Golgi believe that many of the collaterals turn in their course and ascend 

 through the second layer to the superficial layer. It is evident in the plate that this layer 



