468 



THE NERVOUS SYSTEM. 



the tissues in which they are distributed, they gradually 

 become smaller. In the grey or vesicular substance of the 

 brain or spinal cord, they generally do not measure more 

 than from -^ to TT ^ nr of an inch. 



The fibres of the second kind (fig. 134), which constitute 

 the whole of the branches of the olfactory nerves, the prin- 

 cipal part of the trunk and branches of the sympathetic 

 nerves, and are mingled in various proportions in the 

 cerebro-spinal nerves, differ from the preceding, chiefly in 

 their fineness, being only about 1 or J as large in their 

 course within the trunks and branches of the nerves ; in 



Fiy. I34-* 



the absence of the double contour ; in their contents being 

 apparently uniform ; and in their having, when in bundles, 

 a yellowish-grey hue instead of the whiteness of the 

 cerebro-spinal nerves. These peculiarities make it pro- 

 bable that they differ from the other nerve-fibres in not 

 possessing the outer layer of white or medullary nerve- 

 substance ; and that their contents are composed exclusively 



* Fig. 134. Grey, pale, or gelatinous nerve-fibres (from Max 

 Schultze), magnified between 400 and 500 diameters. A, from a 

 branch of the olfactory nerve of the sheep ; a, a, two dark -bordered 

 or white fibres from the fifth pair, associated with the pale olfactory 

 fibres. 1>. from the sympathetic nerve. 



