VARIETIES OF NERVE FIBRES. 273 



everywhere, so that some nerves are coarser and others 

 finer. 



Generally, we may say, that the primitive fibres usu- 

 ally become finer in the terminal portions of nerves, and 

 that the ultimate ramifications of these latter are wont to 

 contain comparatively the finest fibres ; still this is not 

 an absolute rule. In the optic nerve we commonly find 

 from the very moment of its entrance into the eye only 

 very .narrow, pale fibres (Fig. 79, a), whilst the tactile 

 nerves of the skin present quite up to their terminations 

 comparatively broad and dark bordered fibres (Fig. 83). 

 It has not yet been found possible to arrive at any cer- 

 tain opinion with regard to the import of the different 

 kinds of fibres from their breadth and the proportion of 

 medulla they contain. For a time it was believed that 

 a distinction of this sort could be established between 

 them, namely, that the broad fibres were to be regarded 

 as derived from the real cerebro-spinal parts, the fine 

 ones as parts of the sympathetic ; but this is not found 

 to be borne out by facts, and all that can be said is, that 

 the ordinary peripheral nerves certainly are abundantly 

 provided with broad fibres, whilst the sympathetic nerves 

 contain a comparatively larger portion of fine ones. In 

 many places, as for example in the abdomen, grey, broad 

 fibres predominate (Fig. 78, A), with regard to the ner- 

 vous nature of which doubts are still entertained by 

 some. For the present, therefore, no definite conclu- 

 sions can be drawn as to any difference in the functions 

 of a nerve from its mere structure, although it can 

 scarcely be doubted that such differences must exist, and 

 that a broad fibre must exhibit other properties, even if 

 only quantitatively different, than a fine one, a medul- 

 lated fibre others than a non-medullated one. However 

 concerning all this nothing is at present known with cer- 

 tainty ; and since it has been demonstrated by more 



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