262 



NERVE FIBRES OR TUBES. 



.Fig. 115. 



Subdivision of nerve fibres in the frog, magnified 350 diameters. 



Of such fibres, arranged parallel to each other in bundles, the bundles 

 united by fibro-cellular tissue, nerves are composed, the tissue not only 

 accomplishing that mechanical object, but also affording a nidus for blood- 

 vessels, which run in a course parallel to the nerve fibres. Though we 

 Form and size have spoken of those fibres as cylinders, they, in reality, ap- 

 of nerve fibres, proach more nearly to the figure of acute cones, since, though 



their diameter is from the 2^0 o" 



Wo 



an 



* n 



nerve 



trunks, they diminish to the 10 ^ QO or the T4 ^ OQ of an inch as they reach 

 the nerve centres, and, in the same manner, their diameter becomes less 

 as they branch off in their peripheral distribution. In the brain, as they 

 pass through the medulla to the cortical part, they exhibit a similar dim- 

 inution. 



The sympathetic fibres differ from the preceding in appearance. Being 

 Character of ^ a y e U wis h-g ra y color, and only about half as large, they 

 sympathetic do not show the separation into an axis cylinder and white 

 investment after death, as is the case with cerebro- spinal 

 fibres ; they may therefore be regarded as being more homogeneous in 

 their construction, or possessing a constitution like that of the other kind 

 of fibres when they undergo diminution and approach their central or 

 peripheral termination. Even in the cerebro-spinal fibres the quantity 

 of white substance present is very variable ; the retina, the olfactory or- 

 gan, and the Pacinian corpuscles furnish instances of its absence. The 

 sympathetic, gray, or gelatinous fibres, as they are indifferently called, 

 contain many nucleated corpuscles, which may be rendered very distinct 

 by the action of acetic acid. 



