72 Type of Cell Lamination [CHAP. 



Layer of Medium-sized Pyramidal Cells. 



These cells closely resemble those of the " precentral " cortex, but the lamina they form 

 is appreciably shallower, apparently owing to the better development of the next layer. 



External Layer of Large Pyramidal Cells. 



The appearance presented by this layer constitutes an important point of distinction between 

 the " postcentral " and " precentral " types of lamination. In the former the cells are much 

 larger and more numerous, and they form a well-marked, almost prominent lamina, '40 mm. 

 in depth, showing several superimposed rows of elements. Here, the cells average 22 by 43 //, 

 in diameter, and have a nucleus measuring 10 x 7 /A. In shape they are elongated and pyriform, 

 and not so plump as the corresponding cells in the precentral cortex; three or more distinct 

 processes spring from the lower angles and sides of the body, and the apical process, which 

 becomes attenuated very gradually as it leaves the body, is of such length that, even in 

 Nissl specimens, it can be traced for a distance of 200 //,. In the substance of the cell 

 chromophilic elements can be recognised, but they are not sufficiently numerous to give much 

 depth of staining to the body. On enumeration 35 to 40 of these cells are found per transverse 

 millimetre of cortex, but while this number is maintained throughout the gyrus, they suffer 

 a reduction in size as the convolution is descended. While the cells just described constitute 

 the predominant element in this lamina, others of a different character have to be mentioned. 

 These cells are similar in shape but of larger size, a feature which proclaims itself clearly 

 when they are examined, and especially when they are drawn, under a high magnification. 

 Along with the increase in size a greater prominence of the nucleus and the nucleolus is 

 noticeable. Further, the substance of the cell is now packed with large chromophilic elements, 

 so that altogether it comes to resemble an elongated but miniature Betz cell. Their average 

 measurement is 30 by 75 /u.. Such cells exist in greatest abundance on the paracentral portion 

 of the gyrus, where one is found for every five other large pyramidal cells of the same layer; 

 on the convexity they are much less numerous, 1 to 20, and they gradually disappear as 

 the convolution is descended. 



Layer of Stellate Cells. 



The remarkably good development of this layer enables one to distinguish postcentral 

 from precentral cortex at a glance. Sharply defined, intercalated between the two laminae 

 of large pyramidal cells, '23 mm. in depth, and packed with small deeply-stained elements, 

 the layer stands out very distinctly; and in the paracentral region, where there is no fissure 

 of Rolando to act as a divisional guide between the two areas, the advent of this lamina 

 immediately proclaims a change in type. Just in the same way, when the Rolandic cortex 

 is inspected, this lamina is always plainly demonstrable, descending the posterior wall in band 

 form, but ceasing the moment the floor is touched. 



Internal Layer of Large Pyramidal Cells. 



It has been stated by S. Ramdn y Cajal that this layer is equivalent in general repre- 

 sentation to the external layer of corresponding cells, but this is an error. To satisfy myself 

 on this point I have made drawings of sections taken at intervals of 1 cm. all the way 



