104 HISTOLOGY. 



B. Nerve Fibres. 



The continuation of the axone of a nerve cell forms the 

 axis cylinder of a nerve fibre. This is the only essential part 

 of the,nerve fibre. All the other parts may be wanting. 



In a cross-section of a nerve fibre possessing all the accessory 

 coverings we see in the centre the axis cylinder. Around this 

 in concentric arrangement we have, from within outward, the 

 medullary sheath, Schwann's sheath, and Henlds sheath (Figs. 

 74-79). ' 



The axis cylinder runs uninterruptedly from the nerve cell 

 to the nerve-ending. It is characterized by being highly 

 refractive, and possesses a fibrillar structure similar to that of 

 the cell of which it is a process, but contains no tigroid bodies. 



Fr. 74. 



Medullary sheath **^^ : .'. '' 



Space between two 



Schmidt-Lantermann 



segments 



Axis cylinder 



From a cross-section through a nerve treated with osniic acid. X 350. 



Some authors regard it as quite structureless. By special 

 methods it is seen to be made up of a large number of primi- 

 tive fibrils (neuro fibrils), between which there is a small amount 

 of soft neuroplasm (Fig. 80). It is almost generally acknowl- 

 edged that these neurofibrils are capable of carrying nerve 

 impulses, and that each one is a separate conduction path 

 (Apathy). 



The medullary sheath (Fig. 74) which surrounds the axis 

 cylinder consists of myelin, a fatty,, semifluid, homogeneous, 

 highly refractive substance. Medullated fibres have a double 

 contour, formed by the inner and outer margins of the medul- 

 lary sheath seen in optical section. 



Soon after death the so-called coagulation phenomena begin. 



