FUNCTIONS OF THE CEREBRO -SPIN AL NERVOUS SYSTEM. 555 



FIG. 188. 



of Ilemak, or the non-medullatod fibres. The nuclei lie between the mem- 

 brane of Schwauu and the white substance of Schwanu, in which last 

 they are included. They are surrounded by a layer of albuminoid sub- 

 stance, which does not stain with perosmic acid. In the ganglion-cells con- 

 tained in the brain and spinal cord, and in the ultimate peripheral rami- 

 fic-itioMs of the nerves, both the neurilemma and the white substance of 

 Schwann become indistinct or wholly disappear, 

 whilst the axis-cylinder has been observed to break 

 up into fibrils of extreme tenuity, termed primitive 

 nerve-fibrils ;' these are represented in Fig. 189, a. 

 The oleaginous composition of the white or medul- 

 lary substance is shown by its ready solubility in 

 ether. It possesses great refrangibility, is extremely 

 extensible, but inelastic, and of a peculiar viscid na- 

 ture, so that when its continuity is interrupted it 

 has little or no tendency to return to its original 



FIG. 189. 



FIG. 188. a, Axis-cylinder, showing its fibrillar structure; at the 

 upper part, t, x, it is seen to arise from a ganglion-cell, a, only par- 

 tially rrpn .s'-nted, and to become incrustecl by a medullary sheath at 

 a' ; b, naked axis-cylinder, from the dorsal region of the spinal cord 

 of the ox. The medullary sheath has been removed. 



FIG. 189. Primitive nerve-fibrils: a, from the nervous fibre-layer 

 of the retina ; 6, from the external granular layer of the retina, show- 

 ing at z a larger varicosity, resulting from imbibition; c, from the 

 olfactory nerve of the pike, showing a thick nerve inclosed in a 

 sheath breaking up into fibrillse. 



position. 2 The peculiar appearances it presents in polarized light have led 

 Klebs 3 to maintain that it consists of, or contains, doubly-refracting bodies 



1 See Max Schultze, Art. on The Histology of the Nervous System, in the first volume 

 of Strieker's Huniiin and Comparative Histology, Syd. Soc. Trans., 1870, p. 147. 



2 See Locktmrt Cl irke's Observations on the Structure of Nerve-fibre, in Journ. of 

 Mecl. Sci., vol. viii, p. 1. 



3 Quoted in Kiihne's Phys. Chemie, 1868, p. 339. 



