452 HANDBOOK OF PHYSIOLOGY. 



Klein), in the meshes of which is imbedded the bright fatty material. 

 It stains well with osmic acid. 



According to McCarthy, the medullary sheath is composed of small 

 rods radiating from the axis-cylinder to the sheath of Schwann. Some- 

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

 rods appear shortened, and compressed laterally into bundles imbedded 

 in some homogeneous substance. 



(3. ) The axis-cylinder consists of a large number of primitive fibrillce. 

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

 break up into minute fibrils which go to form terminal networks, and 

 also in the spinal cord, where these fibrillae form a large part of the gray 

 matter. From various considerations, such as its invariable presence 

 and unbroken 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. 



At regular intervals in most medullated nerves, the nucleated sheath 

 of Schwann possesses annular constrictions (nodes of Ranvier). At these 

 points (Figs. 310, 311), the continuity of the medullary white substance 

 is interrupted, and the primitive sheath comes into immediate contact 

 with the axis-cylinder. 



Size. The size of the nerve-fibres varies; it is said that the same 

 fibres may not preserve the same diameter through their whole length. 

 The largest fibres are found within the trunks and branches of the 

 spinal nerves, in which the majority measure from 14.4/j 1 to 19/* in 

 diameter. In the so-called visceral nerves of the brain and spinal cord 

 medullated nerves are found, the diameter of which varies from 1.8/f to 

 3.6/*. In the hypoglossal nerve they are intermediate in size, and gen- 

 erally measure 7.2yU to 10.8/*. 



(B.) Non-medullated Fibres. The fibres of the second kind (Fig. 

 312), which constitute the whole of the branches of the olfactory and 

 auditory nerves, the principal part of the trunk and branches of the 

 sympathetic nerves, and are mingled in various proportions in the cere- 

 bro-spinal nerves, differ from the preceding, chiefly in their fineness, 

 being only about -J- to as large in their course within the trunks and 

 branches of the nerves; in the absence of the double contour; in their 

 contents being apparently uniform; and in their having, when in bun- 

 dles, a yellowish-gray hue instead of the whiteness of the cerebro-spinal 

 nerves. These peculiarities depend on their not possessing the outer layer 

 of medullary substance; their contents being composed exclusively of 

 the axis-cylinder. Yet, since many nerve- fibres may be found which 

 appear intermediate in character between these two kinds, and since the 



1 UL = .001 mm. 



