448 THE INTERNAL STRUCTURE 



branched connective tissue cells. The second layer has abont the 

 Bame depth as the first ; to the naked eye it is api>arent as a reddish- 

 grey band abruptly marked oiF from the pale layer beneath it. On 

 microscopic examination it is found to consist " of a series of 

 closely aggregated pyramidal and oval cells of small size, whose 

 apical processes are arranged radially to the surface of the cortex. 

 [Numerous other processes arise from the basal angles, and radiate 

 outwards and downwards from the cell, including an extensive area 

 in their distribution." Each of these cells contains a large nucleus 

 of round or pyramidal form. The third layer is about three times 

 as broad as the last and contains nerve elements of precisely the 

 same kind except that they are larger and not nearly so closely 

 packed. The cells seem uniformly to increase in size from above 

 downwards, and in the lower part of this stratum they are two or 

 three times as large as those of the second layer. This statement 

 is, however, subject to the qualification that some smaller cells exist 

 throughout, interspered amongst those of larger size. The fourth 

 layer is not radically different from the last. It has only about 

 one-third of its depth, and differs, moreover, by reason of the great 

 increase in the size of its cells — otherwise similar in type. In 

 consequence of their considerably superior size these cells appear 

 to be more closely packed. They are on an average about three 

 times as long and broad as those of the third layer. Interspersed 

 between them are a number of small angular cells: and in certain 

 portions of this 'frontal convolution' the small cells alone exist as 

 representatives of the fourth layer — the above described large, or 

 so-called ' giant cells,' being, in these parts, wholly absent. The 

 fifth layer is again much broader than the fourth. It contains 

 irregularly fusiform or spindle-shaped cells of a smaller and pretty 

 uniform size, often arranged in irregular columns owing to the 

 interposition of the bundles of medullary fibres which ascend from 

 the subjacent white matter. 



More recent observations still* have shown, (1) that in many 

 other portions of the Cerebral Hemispheres a six- rather than a 

 five-laminated Cortex is found — the additional stratum in the 

 six-laminated regions being produced by the interposition, between 

 the above described ' third' and ' fourth ' layers, of one containing 

 "small pyramidal and angular cells": (2) that the five-laminated 



* See Bevan Lewis, " On the Comparative Structure of the 

 Cortex Cerebri," Proceed, of Royal Soc, Jane, 1879, p. 234. 



