132 SPECIAL HISTOLOGY. 



fibres of the white substance become gradually attenuated, 

 most of them to a diameter of 0001 2'", and in this state enter 

 the external grey layer. This layer, although to outward 

 appearance everywhere perfectly homogeneous, consists of two, 

 not well defined laminae, the inner of which contains nerve- 

 fibres and very well marked, large nerve-cells, whilst the outer 

 presents nothing but a finely granular, pale, light yellowish 

 substance, which is distributed generally throughout this 

 grey layer, and contains no nerve-cells. The granular sub- 

 stance agrees chemically, morphologically, and physically in 

 all respects with the already described contents of the nerve- 

 cell ; it is tenacious, elastic, rendered more opaque by acetic 

 acid, and more transparent in caustic soda, in which it is, for the 

 most part, dissolved, and exists in the purest form in the outer 

 half of the grey layer, that is to say next to the pia mater. 

 The small nerve-cells, speaking generally, are very few in 

 number and indistinct. They occur scattered throughout the 

 grey layer, having a diameter of 0-004 — 0-008'", more frequent 

 towards the ferrugineous layer than more externally, and 

 when successfully prepared, particularly by means of chromic 

 acid, most of them exhibit delicate processes, which, however, 

 can never be traced to any distance, and are frequently torn 

 off close to the cells. Besides these cells, there also occur here 

 and there, but on the whole rarely, nuclei of 0-002 — 0-0048"', 

 which, to all appearance, are free, as they are met with even 

 in the most carefully made preparations. Entirely different 

 from these smaller elements, and very peculiar, are the large 

 cells of the grey layer (fig. 118) discovered by Purkinje. 

 These cells, measuring 001 G — 0-03'", and of a round, pyriform, 

 or oval figure, with finely granular, colourless contents, occur only 

 in the innermost portions of the grey, close to the ferrugineous 

 layer, and they are, not unfrequently, at least some of them, 

 partly imbedded in its nuclei, in single or multiple layers, and 

 presenting 2 — 3, rarely 1 — 1, long and much branched 

 processes, directed particularly towards the outer surface 

 of the convolutions, which are, almost without exception, 

 at all events the strongest of them, given off from the sides 

 of the cells which look from the ferrugineous layer. At 

 their origin these processes are even 0*007, or as much as 

 0008'", thick, and extremely finely granular or very delicately 



