Anatomy of the Brain. 569 



They are those which compose the corpus callosum, and anastomosing 



and connecting at their extremities with the converging fibres, they cross 



under the f alx cerebri from one hemisphere to the other, and tie the two 



together. The layer of gray matter on the outside is called the cortex. 



This is from JL to -| of an inch in thickness. By the great number of 



its convolutions its surface is greatly increased. The average surface 



of an adult cerebrum, counting what is gained by these foldings, is said 



to be about 670 square inches. There is a certain general plan in these 



FIG. 278. Section of Cortex of Cerebrum. 



1 .Superficial layer with scattered corpuscles. 



2. Layer of small pyramidal corpuscles. 



3. Broader layer of pyramidal corpuscles rendered 

 partly into columns separated by nerve fibres. 



4. Narrow layer of small, irregular corpuscles. 



6. Layer of fusiform ( spindle-shaped ) and irregular 

 corpuscles in medullary center. 



( Quain, after Meynert.) 



convolutions due to race inheritance, and there 

 are certain of the principal ones that are prac- 

 tically alike in human brains, and some are 

 common to man and other mammals, but in 

 the small details they are not alike in any two. 

 Nor are the two hemispheres in any one brain 

 precise counterparts of each other, either in 

 respect to these convolutions or in respect to 

 weight. The gray matter is more abundant 

 in the anterior lobes than in the posterior, and 

 its quantity varies at different ages and in dif- 

 ferent races. It is greater in races of large 

 stature. It is darker -in the negro than in the 

 white man. The composition of the gray 

 matter consists chiefly of cells, with their 

 nerve fibres and connective tissue, and the cap- 

 illaries for the circulation of the blood, which, 

 originating in the pia mater, penetrate down- 

 ward into the spaces between the cells of the 

 cortex. From these capillaries, and around 

 them and attached to them, is the connective 

 tissue, which, spreading around, fills all the 

 FIG. 278. areolar space, enclosing and holding all the 



cells with their innumerable ciliary fibres and branching nerve fibres. 

 This is called the neuroglia, and all through it the blood capillaries per- 

 meate, distributing the vital fluid to the cells and fibres. Around each 

 of the capillaries is a sheath of areolar tissue, through which the nour- 

 ishment is strained to the nerve fibres and cells. There is a consider- 

 able difference in the size of the cells. Those in the bottom layers are 



