6 4 



ATLAS OF NERVE CELLS 



THE FIRST, OR SUPERFICIAL LAYER 



Plates XXXIV. to XXXVII. show the superficial layer of the cortex. This is also known 

 as the molecular layer, from its punctate appearance under the carmine stain and from its 

 supposed lack of nerve cells. The modern methods of staining have demonstrated the existence 

 of a peculiar type of cell in this layer and also the existence of a fine interlacing mass of 

 fibres. These cells having been first described by Cajal are now known by his name ; but 

 Retzius l has studied them more thoroughly in the human brain, and to him we owe most of 

 our knowledge of these structures. There are several varieties of Cajal cells, all of which are 

 shown in the plates. 



Plate XXXIV. shows two of these cells, one fusiform, one triangular. The fusiform cell 

 has an oval body, whose long axis is parallel to the surface of the convolution ; and two 

 long protoplasmic processes, called stalks by Cajal, which pass out nearly straight in the hori- 

 zontal plane. These stalks, whose surface is smooth and not covered with gemmules, give off 

 at right angles numerous fine filamentous fibres. These appear to extend vertically toward the 

 surface, and at their ends little knob-like terminations are often seen. They sometimes divide 

 or give off fine ramifications, which in turn become horizontal in their course. These fibres 

 do not leave the surface layer of the convolution. The end of the stalk terminates in a fine 

 fibre, which is very long, and extends a great length horizontally through this layer. The 

 second cell in the plate is a rectangular, diamond-shaped cell. From this cell the stalks come 

 off at right angles to each other, two being horizontal, two being vertical. From both hori- 

 zontal stalks fine fibres arise, which take a vertical course, and the right-hand stalk finally 

 turns upward. From the lower pointed process of the cell a neuraxon comes off which gives 

 off a collateral, then divides and becomes a tangential, fibre. This is the course taken by the 

 neuraxons of all these Cajal cells, as has been clearly demonstrated by Retzius. They do not 

 leave the superficial layer, but become tangential fibres. The plate shows the large number 

 of tangential fibres. These appear somewhat thicker than in the subsequent plates, owing to 

 a difference in the photographic method adopted. 



Plate XXXV. shows a large triangular Cajal cell with two long stalks, both giving off 



numerous fine vertical branches which pass upward to the surface of the cortex. One of the 



branches becomes a horizontal fibre and can be traced for some distance. The neuraxon is 

 not shown. 



1 Retzius, Biologische Untersuchungen, Bd. III.; 1894. 



