66 



CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



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

 primitive fibrillce, 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 little 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 y. to 19 //. 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 1.8 [j. to 3.6 /*. In the hypoglossal nerve 

 they are intermediate in size, and generally measure 7.2 fj. 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 

 nerve, and are mingled in various proportions in the cerebro-spinal nerves. 

 They differ from the preceding chiefly in not possessing the outer layer of 

 medullary substance; their contents being composed exclusively of the axis- 

 cylinder. 



The non-medullated nerves are only about one-third to one-half as large 

 as the medullated nerves, they do not exhibit the double contour, and they 



