346 



HISTOLOGY. 



before reaching the hemispheres and their course is prolonged by a new 

 set of nerve cells. 



The central portion of the hemispheres is a mass of white substance. 

 The peripheral zone of gray in which these fibers arise or terminate is called 

 the cortex; it is divided into four ill-defined layers, an outer molecular 

 or neuroglia layer, a layer of small pyramidal cells, a layer of large pyram- 

 idal cells, and next the white substance, a layer of polymorphous cells. 

 From the pyramidal cells the fibers of the descending tract arise. The 

 layers are shown in Figs. 403 and 404. 



The molecular layer which in ordinary sections appears finely punctate 

 or reticular contains, besides many neuroglia cells, a network of medullated 



fibers parallel with the surface, the tangen- 

 tial fibers. The Golgi method shows that 

 these fibers are partly neuroglia and partly 

 the dendrites of pyramidal cells. The 

 "cells of Retzius" found in this layer have 

 irregular bodies, and processes some of 

 which are parallel with the surface; their 

 branches, together with other processes from 

 the cell body, descend into the pyramidal 

 layer (Fig. 402). These cells are probably 

 neuroglia. 



The layer of small pyramidal cells is 

 characterized by large ganglion cells with 

 pyramidal bodies measuring 10-12 //. Since 

 they taper into a dendritic process their 

 length cannot be definitely determined. 

 The chief dendrite, after producing small 

 lateral branches, enters the molecular layer 



where it arborizes freely; its terminal branches often show small, 

 irregular projections. Lesser dendrites proceed from the sides and basal 

 surface of the pyramidal cell body. The neuraxon always arises from 

 the basal surface, and. after producing branched collaterals it generally 

 enters the white substance where it may divide in two (Fig. 402, 3). 

 Sometimes the neuraxon turns toward the molecular layer, joining the 

 tangential fibers; infrequently an inverted pyramidal cell is found. The 

 neuraxons and collaterals are medullated. 



The layer of large pyramidal cells contains those with bodies 20-30 // 

 long (the "giant pyramidal cells" of the anterior central convolution 

 measure even 80 /<). The very large neuraxon always goes to the white 

 substance, after sending out several collaterals in the gray. 



a.t 



d.t 



FIG. 401. TRANSVERSE SECTION OF 

 THE BRAIN. About i natural size. 



The gray substance is stippled; the 

 white is blank, a. t., Ascending 

 tract, including the fillet; c. c., 

 corpus callosum; d. t., descending 

 tract, entering the hemisphere 

 from the cerebral peduncle; n. 1., 

 nucleus lentiformis; th., thalamus; 

 v., third ventricle. 



