78 ATLAS OF NERVE CELLS 



In the description of the cortex given above it has been described as consisting of four 

 layers. It is easily possible to distinguish everywhere in the cortex the four types of cells 

 described in these layers; viz. Cajal cells, small pyramidal cells, large pyramidal cells, polygonal 

 cells. But the relative number of these cells varies greatly in different regions of the brain, 

 and their arrangement in layers also varies. Thus few authors have agreed as to the num- 

 ber of layers of cells to be described, and it is erroneous to suppose that any single scheme 

 corresponds to all parts of the brain. Bevan Lewis has shown some of the differences exist- 

 ing in the cortical structure in different regions (see Quain's "Anatomy," loth edition). But the 

 most careful .study was made by the late Dr. Carl Hammarberg of Upsala, in " Studien iiber 

 Klinik und Pathologie der Idiotic nebst Untersuchungen iiber die Normale Anatomic der 

 Hirnrinde," and his plates are reproduced in Plate LI II. 



Hammarberg's drawings of sections through* various parts of the cortex made to a scale 

 have proven that great variations in number and arrangement of the cells exist, that the frontal, 

 parietal, central, occipital, temporal, and median portions of the brain as well as the gyrus 

 fornicatus and the hippocampus have distinctive characteristics, and that no description of one 

 portion holds good for all. It is beyond the scope of this work to describe these variations, 

 which are rather in the mutual relation of such cells as have been shown than in any 

 peculiarities of cell structure. It has seemed necessary, however, to observe that the four- 

 layer type of structure is by no means universal, and should not be taken as typical of cor- 

 tical structure in man, however universal in lower animals. 



Plate LI 1 1. shows sections from eight different regions of the adult human cortex, drawn 

 to a scale, and demonstrates the varying thickness of the cortex and the varying arrangement 

 of the cells in different parts. The first four sections are from the anterior regions of the 

 brain, and present the appearances supposed to characterize the motor cortex. They are 

 from the superior frontal (Fig. i); anterior central (Fig. 2); third frontal, posterior to the 

 ascending limb of the fissure of Sylvius (Fig. 3) ; third frontal, anterior to the ascending 

 limb of the fissure of Sylvius (Fig. 4). The second four sections are from the posterior regions of 

 the brain, and show the arrangement of cells prevailing in the sensory areas of the cortex. They 

 are from the posterior central (Fig. i); superior temporal (Fig. 2); superior parietal (Fig. 3); 

 and superior occipital (Fig. 4). It will be noticed that the larger pyramidal cells are found more 

 constantly in the motor region, and that the contrast between layers of large and small cells is 

 more apparent in the sensory region. It is as yet impossible to assign any separate functions to 

 the various kinds of cells. Hammarberg's studies, however, demonstrate conclusively that the 

 degree of mental power depends directly upon the number and perfection of development of these 

 cortical cells, for in the brains of idiots the cells are few in number and imperfectly developed. 



