COURSE OF NERVE-FIBRES. 371 



the whole of the branches of the olfactory nerves, the principal 

 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 J or as large in their course within the trunks and 



FIG. 134. 

 A 



Gray, 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 ; tt, a, two 

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

 fibres. B. From the sympathetic nerve. 



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

 in their contents being apparently uniform ; and in their 

 having, when in bundles, a yellowish-gray hue instead of the 

 whiteness of the cerebro-spinal nerves. These peculiarities 

 make it probable 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 

 of the substance corresponding with the central portion, or 

 axis-cylinder of the larger fibres. Yet since many nerve-fibres 

 may be found which appear intermediate in character between 

 these two kinds, and since the large fibres, as they approach 

 both their central and their peripheral end, gradually dimmish 

 in size, and assume many of the other characters of the fine 

 fibres of the sympathetic system, it is not necessary to suppose 

 that there must be a material difference in the office or mode 

 of action of the two kinds of fibres. 



Every nerve-fibre in its course proceeds uninterruptedly from 

 its origin at a nervous centre to near its destination, whether 

 this be the periphery of the body, another nervous centre, or 

 the same centre whence it issued. 



Bundles, or fasciculi of fibres, run together in the nerves, 



