LESSON 87.] NERVOUS SYSTEM IN MAMMALIA AND MAN. 



283 



substance (Fig. 398), obtained by the same agency; nothing can exceed 

 the transparency, combined with brilliancy, of these two last prepara- 

 tions. It will be seen that the white substance consists of nucleated 

 cells. 



1270. Another axis-cylinder, containing some corpuscles of the 

 white substance, copied from the same preparation, is given (Fig. 



FIG. 897. 



FIG. 398. Fm. 399. 



White substance of Schawn ; from the 

 median nerve, human. 



Axis-cylinder contain- 

 ing white substance, 

 median nerve. 



Axis-cylinder. 



399) ; it will readily be perceived that they differ considerably in 

 size. A common occurrence is, to find the axis-cylinder with the 

 white substance covering the termination of it (Fig. 400). 



1271. The appearance of the tubes of the median nerve is shown 

 in Fig. 401, the granular contents indicating the white substance. 



1272. The nerve tubes of the sympathetic system differ from 



FIG. 400. FIG. 401. FIG. 402. 



Median nerve. 



Sympathetic nerve, human. 



"White substance covering ter- 

 mination of axis cylinder. 



those just described ; firstly, they are so much smaller that they do 

 not exceed one-third the diameter of the former ; and, secondly, they 

 do not present the two distinct substances within the tubular invest- 

 ment, which has been shown to exist in the ordinary nervous tubuli. 

 1273. A figure of the tubes of a sympathetic nerve is given at 

 Fig. 402 ; the contents of these tubes appear to be perfectly homo- 



