THE CEREBRAL HEMISPHERES. 603 



brain weighs 11-65 ounces (327'8 grammes), for the male, and 10 ounces 

 (283-5 grammes), for the female. In both sexes, " the weight of the brain 

 generally increases rapidly up to the seventh year, then more slowly to be- 

 tween sixteen and twenty, and again more slowly to between thirty-one and 

 forty, at which time it reaches its maximum point. Beyond that period, 

 there appears a slow but progressive diminution in weight of about one 

 ounce (28-3 grammes) during each subsequent decennial period ; thus con- 

 firming the opinion, that the brain diminishes in advanced life." 



The comparative weights of the several parts of the encephalon, calcu- 

 lated by Reid from observations upon the brains of fifty-three males and 

 thirty-four females between the ages of twenty-five and fifty-five, are as fol- 

 lows : 



The proportionate weight of the cerebellum to that of the cerebrum, in 

 the male, is as 1 to 8f , and in the female, as 1 to 8J (Quain). 



The specific gravity of the whole encephalon is about 1036, that of the 

 gray matter being 1034, and of the white, 1040 (Quain). 



THE CEREBRAL HEMISPHERES. 



'Cortical Substance. The surface of the cerebral hemispheres is marked 

 by fissures, sulci, and convolutions, which serve to increase the extent of the 

 gray substance. The sulci between the convolutions vary in depth in dif- 

 ferent parts, the average being about an inch (25-4 mm.). The grcy mat- 

 ter, which is external and follows the convolutions, is ^ to -J of an inch 

 (2-1 to 3-2 mm.) in thickness. Anatomists have described this substance as 

 existing in several layers, but this division is mainly artificial. In certain 

 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 super- 

 ficial cells are small and present a net-work of delicate, anastomosing fibres. 

 The deepest cells are much larger. Between these two extremes, in the 

 intermediate layers, there is a gradual transition in the size of the cells. 

 Fig. 220 shows the layers of cells in a vertical section of a cerebral convolu- 

 tion. The most superficial layer is very thin. It contains much neuroglia 

 and a fine net-work of fibrils, with a few small nerve-cells. The second layer 

 presents a large number of small, so-called pyramidal cells. The third layer 



