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THE NERVOUS SYSTEM. 



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 -J or -f as large in their 

 course within the trunks and branches of the nerves ; in 

 Fig. 127.* 



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 

 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, 



* Fig. 127. 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 ; at a, a, two dark-bordered or white fibres 

 from the fifth pair, associated with the pale olfactory fibres. B. From 

 the sympathetic nerve. 



