NEEVE-TISSUE. 295 



(a) Medullated Nerves. These compose the white substance 

 of the brain and of the spinal cord, and all the nerves springing 

 from the brain and the spinal cord, with the exception of the 

 olfactory and auditory nerves. They invariably run their whole 

 course without branching. The constituent parts of medullated 

 nerves are best studied in specimens where, owing to a wavy 

 course of the nerves, the razor in cutting produces alternately 

 longitudinal and transverse sections. (See Fig. 124.) 



We see that each nerve-bundle is made up of a varying number 

 of fibers. The bundle is surrounded by a somewhat broader layer 

 of fibrous connective tissue, the external perineurium^ from which 

 arise delicate membranes of connective tissue, ensheathing each 

 single fiber j this formation has received the name of the internal 

 perineurium. Both the external and internal perineurium are 

 provided with a large number of blood-vessels. Next, there is a 

 delicate hyaline layer with distinct nuclei, the myeline sheath, or 

 sheath of Schwann. This sheath incloses a layer of a semi-fluid 

 fatty substance, the myeline or nerve-fat, or the white substance 

 of Schwann. At comparatively regular intervals the myeline 

 investment is traversed, according to L. Ranvier, by transverse 

 septa, which he found to be in connection with the axis-cylinder 

 sheath (Mauthuer), and closely related to the development of 

 the medullated nerves. The myeline investment is separated 

 from the central axis-cylinder by a delicate hyaline sheath, the 

 axis-cylinder sheath (Mauthner). This incloses the central axis- 

 cylinder, which may be either round or oblong, and is the con- 

 ducting part of the nerve-fiber. In transverse sections this is 

 easily recognized from its bright, almost homogeneous, appear- 

 ance, while in longitudinal sections it is invisible if the myeline 

 be present, and only faintly visible if the myeline be absent. 

 Not infrequently we find two axis-cylinders in one nerve-fiber. 

 Together with the large medullated nerve-fibers in the same 

 bundles there are sometimes found fibers without and also fibers 

 with a very delicate myeline investment. The latter formations 

 probably belong to the sympathetic system. 



The myeline is a fatty substance, diifering from ordinary fat, 

 however, in both its optical and chemical characteristics. From 

 the fact that a delicate reticulum is still preserved in the space 

 between the sheaths of the myeline and that of the axis-cylinder 

 (see Fig. 123), and from the fact that fat is a product of living 

 matter, we may conclude that the myeline is, perhaps, produced 

 from only a part of the living matter, while the other part 



