94 



HAXDBOOK OF PHYSIOLOGY. 



erably, at one time being very well developed, at another forming but a 

 very thin investment of the axis cylinder. It is a semi-fluid, fatty sub- 

 stance, and in the fibre possesses a double contour. It is said to be 

 made up of a fine reticulum (Stilling, Klein), in the meshes of which is 

 embedded the bright fatty material. It stains well with osmic acid. 



According to McCarthy this sheath is composed of small rods radiat- 

 ing from the axis-cylinder to the external sheath of Schwann. Some- 

 times the whole space is occupied by them, while at other times the 

 rods appear shortened and compressed laterally into bundles embedded 

 in some homogeneous substance. According to other ob- 

 servers the sheath is made up of segments which are 

 either cylindrical or funnel-shaped (sections of Lanter- 

 mann). It is not definitely decided that these divisions 

 exist naturally in the nerve-fibre. In nerves hardened in 



Fig. 97. Fig. 98. 



Fig. 97. A node of Ranvier in a medullated nerve-fibre, viewed from above. The medullary 

 sheath is interrupted, and the primitive sheath thickened. Copied from Axel Key and Ketzius. 

 X 750. (Klein and Noble Smith.) 



Fig. 98. Gray, pale, or gelatinous nerve-fibres. A. From a branch of the olfactory nerve of the 

 sheep; two dark-bordered or white fibres from the fifth pair are associated with the pale olfactory 

 fibres. B. From the sympathetic nerve. X 450. (Max Schultze.) 



alcohol, it is possible to demonstrate a very chromatic recticulum in the 

 medullary sheath, which is supposed to be of a horny nature, since it 

 offers much resistance both to chemical reagents and to digestive fluids 

 (horny reticulum or neuro-keratin network). 



The axis-cylinder consists of a large number of primitive fibriHa. 

 This is well shown in the cornea, where the axis-cylinders of nerves 

 break up into minute fibrils which form terminal networks, and also in 

 the spinal cord, where these fibrilla? form a large part of the gray matter. 

 From various considerations, such as its invariable presence and un- 

 broken continuity in all nerves, though the primitive sheath or the 

 medullary sheath may be absent, there can be little doubt that the axis- 

 cylinder is the essential part of the fibre, the other parts having the 

 subsidiary function of support and possibly of insulation. 



