234 NERVOUS SYSTEM. [SECT. Il6. 



of 0-0012"' to 0-003"', on an average 0-002'", in diameter, without 

 any intermixture of grey substance. These fibres, concerning the 

 special course of which but little is as yet known, never run in the 

 form of networks or bundles, but all parallel to each other, and 

 mostly in a straight direction. They undoubtedly proceed out- 

 wards from the corpus callosum and the ganglia of the cerebrum 

 to the superficial grey substance ; but it is uncertain whether or 

 not they divide in their progress. Besides the fibres now spoken 

 of and those of the anterior commissure, the fornix and the origin 

 of the optic nerves, the hemispheres contain other fibres, which 

 decussate with them at right angles. I found these last upon the 

 outer side of the corpus striatum and in the most superficial layers 

 of the white substance, not far from the grey covering of the con- 

 volutions, where they are met with in not inconsiderable numbers, 

 and in part taking an oblique course ; but nothing can be ascer- 

 tained with reference to their source. The grey substance of the 

 convolutions is, as regards its intimate structure, tolerably clear (see 

 my Micr. Anat., tab.iv. fig. 2). It may be most fitly divided into 

 three layers : an external, white ; a middle, pure grey ; and an in- 

 ternal, yellowish-red. The latter, which equals the two others in 

 thickness, usually has, on its outermost limit, a clearer, frequently 

 almost white streak, and occasionally, further inwards, a second 

 narrower and less white layer, so that three, four, or even six layers 

 are formed in the grey substance, viz., 1. yellowish-red layer, inner 

 part; 2. first white streak; 3. yellowish- red layer, outer part; 4. 

 second white streak; 5. grey layer; 6. superficial white layer. 

 The grey substance contains, throughout its whole thickness, both 

 nerve-cells and nerve-fibres, together with a large quantity of a 

 granular matrix, exactly like that of the cerebellum. The nerve- 

 cells, wdiich are most numerous in the middle grey layer, are rather 

 small, all having probably one to six processes ramifying in various 

 ways, and, at last, running out into extremely fine pale fibrils, of 

 about 0-0004'" in diameter. 



The nerve-tubes of the grey substance of the convolutions, as 

 can be readily demonstrated, come from the medullary substance 

 of the hemispheres, and penetrate in bundles, and parallel to one 

 another, into the yellowish-red layer, in a straight direction. Here 

 a number of the tubes separate from the bundles, and traverse the 

 yellowish-red layer in all directions, but especially parallel to the 

 surface, decussating accordingly with the main bundles. Where 

 these horizontal fibres are collected in larger numbers, the above- 

 described whiter or lighter streaks on this layer are produced, the 



