THE CEREBRAL GANGLIA. 



323 



perior longitudinal fissure, including a tract on the inner 

 aspect of the hemisphere, called the paracentral lobule, is the 

 centre for movements of both extremities, especially the lower. 

 On account of the anatomical variability of the convolutions 

 in different brains, these centres must be allowed some lati- 

 tude, and should not be made so small and exactly located 

 as they are by some authors. Their location has been pretty 

 definitely determined, however, by experimentation on animals, 

 and lesions in man, such as trauma- 

 tisms, neoplasms, abscesses, hemor- 

 rhages, atrophy following amputa- 

 tions, retarded development, etc. 



Possessing such important proper- 

 ties we should naturally expect the 

 cerebral cortex to be a very complex 

 structure, and so it is. 



Minute structure of the cortex. 

 In order to get a satisfactory view of 

 the elements of the cortex, great care 

 has to be exercised in making sections. 

 It is not enough to make a section 

 exactly perpendicular to the cortex. 

 The plane of the section must exactly 

 coincide with the direction of the fibres 

 of the corona radiata as they enter the 

 convolution. This can be rather easily 

 accomplished by paying close atten- 

 tion to the arrangement of the white 

 and gray matter in the piece from which the sections are to 

 be made. Cuts with any obliquity will give erroneous impres- 

 sions as to the exact shape and structure, especially of the 

 cellular elements of the cortex. The cortex cerebri is generally 

 divided into five layers, but it is easily divisible into three only. 



The outer layer, lying immediately under the pia mater, is 

 more transparent than the rest, and is composed of a fine net- 

 work of neuroglia containing many quite large openings, giving 

 it a spongy appea,rance. It also contains a few large, round 

 nuclei, and a small number of triangular nerve-cells. 



The second layer, thicker than the first, consists of a gray 

 basis-substance, dense and granular, holding an immense num- 

 ber of small, triangular and conical cells, their apex being di- 



FIG. 150. Diagram showing the 

 elements and relation of parts in the 

 cerebral cortex. (See text.) 



