THE CEREBRUM. 321 



The Cerebrum. 



The anatomical description which we have just given of 

 the encephalon will answer for most of the points of physio- 

 logical interest connected with the cerebrum. As we have 

 seen, the cerebrum constitutes more than four-fifths of the 

 encephalic mass. Its gray matter, which is external and 

 follows the convolutions, is from -^ to -J- of an inch in thick- 

 ness. 1 Writers have described this substance as existing in 

 several layers, but this division is mainly artificial. In cer- 

 tain parts, however, particularly in the posterior portion of 

 the cerebrum, the gray substance is quite distinctly divided 

 into two layers, by a very delicate intermediate layer of a 

 whitish color. 



There is a marked difference in the appearance of the 

 cells in the most superficial and in the deepest portions of 

 the gray substance. The superficial cells are small, and 

 present a net-work of delicate, anastomosing fibres, re- 

 sembling the cells of the posterior cornua of the gray 

 substance of the cord ; while the deepest cells are large, and 

 resemble the so-called motor cells of the cord. Between 

 these two extremes, in the intermediate layers, there is a 

 gradual transition in the size of the cells.' This anatomical 

 fact points to the possibility of distinct functions of the cells 

 belonging to the superficial and the deep layers ; viz., that 

 the larger cells are for the generation of the motor stimulus, 

 while the smaller are for the reception of sensory impres- 

 sions. This, however, is mere supposition, incapable, as 

 yet, of positive demonstration. 



1 LUYS, Systeme nerveux, Paris, 1865, p. 161. 



2 The above general description of the peculiarities of the nerve-cells of the 

 cerebral convolutions is the one given by most anatomists. Lately, Lockhart 

 Clark has described the structure of the convolutions very minutely, dividing 

 the gray substance into seven distinct layers. This description is interesting, 

 but chiefly so from an anatomical point of view. (LOCKHART CLARK, The 

 Structure of the Cerebral Convolutions. Quarterly Journal of Psychological 

 Medicine, Xew York, 1869, vol. in., p. 517.) 



