422 NERVE-FIBRES. [BOOK I. 



der, and (3) between the neurilemma and the axis cylinder a white, 

 highly refracting substance, known as the medullary sheath or white 

 siibstance of Schwann. At intervals, the white substance is inter- 

 rupted (nodes of Ranvier). 



2. Non-medullated nerve-fibres. These differ from the medul- 

 lated variety in the absence of the white substance of Schwann. 

 They consist of an axis cylinder sheathed in a nucleated neurilemma. 



The neurilemma may be absent from both medullated and non- 

 medullated nerve-fibres. 



The neuri- The very transparent and thin membrane which 



forms the wall of the nerve-fibre appears to possess 

 characters which closely resemble if they are not identical with those 

 of the sarcolemma. When a medullated nerve-fibre enters a muscular 

 fibre, the neurilemma loses itself upon, and becomes continuous with, 

 the sarcolemma (Kiihne). By prolonged boiling both neurilemma and 

 sarcolemma yield gelatin. 



The axis This structure, which under a high power of the 



cylinder. microscope presents the appearance of a cylindrical 



band, exhibiting marks of fibrillation, is certainly of solid consistence 

 during life, and is composed of a mixture of proteid with complex 

 fat-like bodies. It is partly soluble in a weak aqueous solution 

 of hydrochloric acid (1 to 1000), and in a 10 per cent, solution 

 of common salt. It is not collagenous. It reduces gold solutions 

 very readily in the presence of light ; it is stained by ammoniacal 

 solutions of carmine, which leave the white substance of Schwann 

 unstained ; this action of carmine is probably dependent upon changes 

 which occur in the axis cylinder at death. 



Chromic acid, potassium bichromate, ammonium monochromate 

 and certain other reagents, harden the axis cylinder and render it 

 more easily seen. Perosmic acid, though hardening it, does not stain 

 it black. 



The white That the m-edullated nerve-fibre is not homogeneous 



substance of while it is in a physiological condition, i.e. that a distinc- 

 tion between the axis cylinder and medullary sheath 

 exists, may be proved by various considerations, for which the reader 

 is referred to works on histology. The white substance of Schwann 

 appears during life to have a semi-liquid consistence ; from optical 

 considerations it would seem to contain suspended solid bodies. At 

 death it undergoes a kind of coagulation. The white substance of 

 Schwann instantly reduces solutions of perosmic acid and becomes 

 black from the presence of metallic osmium. When fresh, the white 

 substance can be squeezed out of the nerve-fibres, .and is found to be 

 insoluble in water in which it swells; it is partially soluble in alcohol. 

 The white substance of Schwann is doubtless specially rich in the 

 complex phosphorized fats, in the cerebrin group of bodies, and in the 

 cholesterin, which will be described as the chief constituents of the 

 nervous matter. 



