NON-MEDULLATED FIBERS 67 



opaque white aspect of medullated nerves is due. The thickness of this 

 layer of nerve fiber varies considerably. It is a semifluid, fatty substance 

 of high refractive power. It possesses a fine reticulum (Stilling, Klein), in 

 the meshes of which is embedded the fatty material. It stains well with 

 osmic acid. 



The Axis-cylinder. The central thread of a medullated nerve fiber is 

 the axis-cylinder. It is the prolongation of a nerve cell and extends un- 

 interrupted for the full length of the fiber. It consists of a large number of 

 primitive fibrillcz, as shown in the cornea, where the axis-cylinders of nerves 

 break up into minute fibrils which form terminal networks. From various 

 considerations, such as its invariable presence and unbroken continuity in 

 all nerves, there can be no doubt that the axis-cylinder is the essential con- 



FIG. 87. Transverse Section of a Portion of the Sciatic Nerve of the Rabbit, Hardened 

 in Chromic Acid and Stained with Picro-carmine, to show medullated fibers in end view. 

 X 275. a, Perifascicular connective tissue; b, lamellar sheath; e, axis-cylinder. 



ducting part of the fiber, the other parts having the subsidiary function of 

 support and possibly of insulation. 



The size of the nerve fibers varies, figure 87. The largest fibers are 

 found within the trunks and branches of the spinal nerves, in which the 

 majority measure from 14/4 to igp in diameter. In the so-called visceral 

 or autonomic nerves of the brain and spinal cord medullated nerves are 

 found, the diameter of which varies from i . S/i to 3 . 6/1. In the hypoglossal 

 nerve they are intermediate in size, and generally measure 7 . 2fj. to 10. 8//. 



Non-medullated Fibers. The fibers of the second kind, figure 86, 

 which are also called fibers of Remak, constitute the principal part of the 

 trunk and branches of the sympathetic nerves, the whole of the olfactory 



