194 NINETEENTH LECTURE. 



ous bodies, as well as lecithin and cerebrin. This investment 

 originally conceals the axis cylinder. 



As soon as we isolate broad nerve tubes, we encounter the 

 cadaveric form of the medullary sheath (Fig. 168). "They 

 are now coagulated," is a customary expression of the histolo- 

 gists. We meet with the most varying stages of coagulation, 

 often close to each other, and even in the course of one and 

 the same primitive tube. 



As a commencing stage, we discover on both sides a 

 double contour, a sharp but dark external, and a closely 

 applied finer border (Fig. 167, a, b, 168, b, above). 



Later, the double contours no longer run parallel with each 

 other, and the inner one appears frequently interrupted (Fig. 

 168, b, below). The latter becomes constantly more and 

 more irregular, and in the previously homogeneous axis por- 

 tion, dark bordered, lumpy substances are formed (a, b). 

 The process of coagulation may, it is true, be arrested at an 

 earlier stage. The cortex then forms to a certain extent, a 

 protective mantle around the axis portion. In other cases, 

 the latter also does not escape its final destiny ; together 

 with the cortex it is completely disintegrated into clots (c). 



It was a long time before the just described structure of 

 the nerve tubes could be agreed upon. The existence of the 

 axis cylinder, especially, gave rise to heated debates. It is 

 to-day a child's play to recognize the latter in any transverse 

 section of a hardened peripheral nerve or — which amounts to 

 the same — each primitive tube in a white column of the spinal 

 cord (Fig. 170). 



The nerve tubes of medium size have a 

 similar constitution. 



A similar structure— envelope, axis cylin- 

 der and medullary sheath — is also perceived 

 in the fine filaments of the nerve trunks. 



Fig. 170. — Trans- 



verseiy divided nerve The medullary sheath ( Fig. 167, c,d) remains 



fibres from the postern ir ' 



column of the human clear, and simply demarcated, even with 



spinal cord. l J 



advanced post-mortem changes. Osmic 

 acid, which rapidly blackens the medulla of the broad nerve 



