THE CEREBRUM 263 



marginal convolutions beyond the calloso-marginal fissure from 

 the preceding and between it and the edge of the longitudinal 

 fissure; the continuation of the parieto-occipital fissure running 

 downward and forward to meet the calcarine fissure, between 

 which is the cuneus; the internal aspect of the temporal lobe, 

 the uncinate gyrus. 



Structure. The cerebral hemispheres are composed of white 

 and gray matter, but here the gray matter is situated externally. 

 To increase its amount, with economy of space, the gray matter 

 is thrown into many convolutions, to some of which reference has 

 been made. The sulci separating these convolutions have a 

 depth in the average human brain of about one inch. The thick- 

 ness of the gray matter of the cortex varies from -^ to J in., 

 being thinnest in the occipital and thickest in the front parietal 

 region. 



The cells found in the superficial and deep portions of the 

 gray matter are not uniform in size or shape. In a general way 

 it may be said that they increase in size as the surface is left, 

 but in addition to the comparatively large cells in the deep parts 

 there are also numbers of small ones. Passing in the same direc- 

 tion there are found in succession small pyramidal, larger pyram- 

 idal, and irregular branching cells. 



Fibers from the Cerebrum. Fibers pass from each cerebral 

 hemisphere to (a) the spinal cord, (b) the cerebellum, (c) the 

 opposite cerebral hemisphere, and (d) different parts of the same 

 hemisphere. 



(a) Fibers converge from the anterior and middle (particularly 

 the latter) parts of the cortex to pass by the corona radiata to 

 the corpora striata, from which fibers are continued to the crusta, 

 pons, pyramids of the medulla and pyramidal tracts of the cord; 

 most of these pass down through the internal capsule to reach 

 the corpora striata. From the same regions also some fibers 

 pass directly through the internal capsule, without connection 

 with the corpora striata, to be actually continuous themselves 



