CHAPTER V. 



Elements of Nervous Tissue. 



THE analysis of nervous tissue reduces its anato- 

 mical elements to two forms, ^iz. the fibre, and the 

 cell. 



nee t flSe. of The fibres of nerve tissue present variations in the 

 details of their structure; sometimes consisting of a 

 tube of a certain diameter, with envelope and con- 

 tents perfectly distinct ; whilst, at others, on the con- 

 trary, the envelope and its contents so blend together 

 as to constitute a simple homogeneous fibre. 



The envelope of the nerve tube is entirely struc- 

 tureless, and possessed of a certain amount of elas- 

 ticity. Immediately in contact with its internal sur- 

 face we find a soft amorphous substance of an albumi- 

 nous and fatty nature this is the nervous marrow, 

 or medullary sheath. In fresh nerve fibres this me- 

 dulla is homogeneous, and constitutes a regular tube ; 

 but soon after death it becomes disintegrated, and 

 separates itself into lumpy masses, between and upon 

 which the envelope contracts so as to give the fibre a 

 varicose appearance (PL XII. fig. I.). Finally, the 

 central axis of the tube is occupied by a cylinder of 

 amorphous material, of an albuminous nature, more 

 compact and more tenacious than the medulla, which 

 is known by the name of axis-cylinder (PL XII. fig. 

 I. 7). It is exceedingly difficult to make out the axis- 



