XXXI.] CEREBRUM. 353 



method. In it may be seen sections of blood-vessels passing from 

 the pia. 



(ii.) The second layer, or layer of small pyramidal cells, is usually 

 narrow, with several rows of small pyramidal cells, the peripheral 

 processes of the latter pointing towards the surface of the convolu- 

 tion. It passes gradually into 



(iii.) The third layer, or layer of large pyramidal cells, which is 

 much thicker than the others, and contains large pyramidal cells, 

 each with a peripheral process, which can sometimes be traced 

 outwards for a considerable distance. The axis-cylinder process 

 gives off several collaterals, and can be traced into a medullated 

 fibre of the white matter. The cells, however, may be cut obliquely, 

 and then they appear triangular. Usually the cells are larger in 

 the deeper layers, and become smaller in the outer layers. 



(iv.) Tk& fourth layer, or layer of irregular or polymorphous cells. 

 This is a narrow layer of small, usually angular but irregularly- 

 shaped cells. They lie between the nerve-fibres which pass into 

 the cortex. In the motor areas of the frontal and parietal convolu- 

 tions large pyramids arranged in groups or nests may be found 

 between these cells. 



(v.) T\\Q, fifth layer, or layer of fusiform cells, also a narrow layer, 

 with a few fusiform cells with nerve-fibres between them. This 

 layer abuts on the white fibres of the medulla, and may in some 

 regions be fused with the previous layer. In the grey matter 

 superficial to the island of Reil, it exists as a separate layer consti- 

 tuting the clanstrum. 



Not unfrequently in bichromate and chromic acid preparations, 

 the place of the pyramidal cells is partly represented by clear 

 spaces, produced by vacuolation of the cells. It is by no means 

 easy to distinguish a nerve-cell from a neuroglia-cell in the cere- 

 brum. Usually the glia-cell nuclei are smaller. Between the rows 

 of cells may be seen fine longitudinal striation, indicating the exist- 

 ence of nerve-fibres in the grey matter. 



The structure of the cortex is not identical, although similar, 

 throughout. In the motor areas especially, i.e., in the ascending 

 frontal, ascending parietal, and part of the marginal convolutions, 

 the pyramidal cells are usually larger than in those areas which are 

 described as sensory e.g., the occipital, temporo-sphenoidal, or even 

 in the anterior part of the frontal. 



It is advisable, therefore, to provide the student with sections 

 from these different areas. There is no abrupt transition between 

 oiK 1 , type of cortical structure and another. 



B. Course of White Fibres in the Cortex Cerebri. This cannot 

 be made out in sections prepared as above. All that can be seen is 

 that fine strands radii of medullated fibres pass from the white 



Z 



