310 



LECTURE XIII. 



FIG. 93. 



A circumstance worthy of particular observation here 

 is that in the petromyzon, in the whole substance of the 

 spinal marrow, no medullary matter exists in an isolated 

 form, as is the case in man ; we only find simple, pale 

 fibres, which Stannius has without 

 hesitation pronounced to be naked 

 axis-cylinders. But without taking 

 into account the fact that some of 

 them have an enormous diameter, we 

 find upon more accurate examination, 

 as in the case of the gelatinous grey 

 fibres in man, a membrane very 

 clearly seen in transverse sections, 

 especially after it has been coloured 

 with carmine and in the centre a 

 finely granular matter, so that they 

 seem rather to correspond to entire 

 nerve-fibres. 



Hitherto, gentlemen, in considering the nervous sys- 

 tem, I have only spoken of the really nervous parts of it. 

 But if we would study the nervous system in its real rela- 

 tions in the body, it is extremely important to have a 

 knowledge of that substance also which lies between the 

 proper nervous parts, holds them together and gives the 

 whole its form in a greater or less degree. 



It is by no means very long ago since the existence of 

 such interstitial masses of tissue was really only conceded 

 in the case of peripheral nerves, and since the neurilemma 

 was only traced back as far as the membranes of the spi- 

 nal cord and brain, such an enveloping tissue being at 

 most allowed to exist within the ganglia and in the sym- 

 pathetic. In the nervous centres properly so called, and 



Fig. 93. Pale fibres from the spinal marrow of Petromyzon fluviatilis. A. Broad, 

 narrow, and extremely fine fibres. B. Transverse sections of broad fibres with a dis- 

 tinct membrane and granular centre. 300 diameters. 



