94 



LECTURE IV. 



plainly prove tliat they exercise different functions, ttough we 

 have not yet succeeded in determining them. 



On turning our attention first to the constituent elementary 

 forms, we find in the brain of man and animals two groups of 

 substances, a grey substance, more or less blackish or yellowish, 

 appearing to the naked eye as a uniform mass, and a white 

 substance apparently distinguished from the other by being 

 composed of fibres which run in various directions. The grey 

 substance consists of cells containing nuclei and finely granu- 

 lated matter, from which issue processes subdivided into very 

 deUcate threads, which either form delicate networks or are lost 

 in the fibres of the white substance. These nerve-cells (see 

 fig. 28) exhibit a variety of forms which are probably connected 

 with their respective functions, an hypothesis rendered more 

 probable by the fact, that the grey substance no doubt origi- 



Fig. 28. Multipolar Cells, witli Processes in the Human Brain. 



1. CeU, the process of which, a, becomes the cylinder-axis of the sheathed 

 primitive fibre, 6. 2. Two cells, o and h, connected by processes. 3. Three 

 cells, a, connected by commissures, 6, sending forth nerve fibres, c, 4. Mul- 

 tipolar cells, with much more black pigment. 



