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HAND-BOOK OF PHYSIOLOGY. 



only about \ or \ as large in their course within the trunks and 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 yel- 

 lowish grey hue instead of the whiteness of the cerebro -spinal nerves. 

 These peculiarities depend on their not possessing the outer layer of 



FIG. 306. Grey, pale, or gelatinous nerve-fibres. 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. B. From the sympathetic nerve. X 450. (Max Schultze.) 



medullary nerve-substance; their contents being composed exclusively of 

 the axis-cylinder. 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 diminish in size, and assume many of the other characters of 



FIG. 307. Several fibres of a bundle of medullated nerve-fibres acted upon by silver nitrate to 

 show peculiar behavior of nodes of Ranvier toward their reagent. The silver has penetrated at the 

 nodes, and has stained the axis-cylinder for a short distance. (Klein and Noble Smith.) 



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

 that there is any material difference in the two kinds of fibres. 



It is worthy of note, that in the foetus, at an early period of develop- 

 ment, all nerve -fibres are non-medullated. 



