410 GENERAL STRUCTURE AND FUNCTIONS 



bundles ; as for example in the sensitive and motor roots of the fifth 

 pair of cranial nerves, and in the anterior and posterior roots of the 

 spinal nerves generally. The fibres belonging to the facial nerve are 

 all motor fibres, making this exclusively a motor nerve. The three 

 branches of the fifth pair, on the other hand, which are distributed to 

 the integument and mucous membranes of the face, are composed exclu- 

 sively of sensitive fibres ; while the branch of the same nerve distributed 

 to the muscles of mastication is made up principally or entirely of motor 

 fibres. 



No essential distinction has been discovered in the anatomical char- 

 acters of sensitive and motor nerve fibres. In nerves and nervous 

 branches which perform a motor function, the nerve fibres, as a rule, are 

 of comparatively large size, averaging 15 mmm. in diameter ; while in 

 those performing a sensitive function they are smaller, averaging not 

 more than 10 mmm. in diameter, and many of them being considerably 

 less. But this difference is only one of proportion in numbers between 

 the larger and smaller fibres ; since both large and small fibres are found 

 in both motor and sensitive nerves. Even in the motor nerves, the 

 large fibres become reduced to the size of the smaller ones before their 

 termination in the muscular tissue; and the nerve fibres generally are 

 diminished or increased in diameter on passing into or out of the gray 

 substance of the nervous centres. No absolute distinction therefore can 

 be made between sensitive and motor nerve fibres as regards their size ; 

 and in regard to the essential details of their structure, namely, the 

 tubular sheath, the medullary layer, and the cylinder axis, they are to 

 all appearance completely identical. 



Effect of Division upon the Nerve Fibres. The immediate effect of 

 dividing or seriously injuring the nerve fibres is a suspension of their 

 physiological function. The physical communication being cut off be- 

 tween their extremities, the sensitive fibres can no longer transmit an 

 impression from the skin to the nervous centre, and the motor fibres 

 can no longer convey the stimulus of voluntary motion from the nervous 

 centre to the muscles. In addition to this result, when the divided 

 nerve fibre is permanently separated from its central connections, there 

 also follows a change in its texture, which is propagated mainly in 

 one direction, and which consists in an atrophy or degeneration of 

 the nervous substance. The most distinct effects of this degeneration 

 of a divided nerve fibre are to be seen in its medullary layer. According 

 to the observations of Yulpian and Philippeaux, the alteration in struc- 

 ture, which takes place from the point of division toward the periphery, 

 begins to be perceptible in mammalians, by microscopic examination, at 

 the end of five days. The transparency of the fibre is first diminished, 

 its contents having a more or less cloudy appearance. At the end of 

 eight or ten days, the double contour of the fibre has become irregular 

 and at various points partially or completely interrupted ; and the sub- 

 stance of the medullary layer is broken up into separate masses of 

 varying size, presenting the appearance of a coagulation and dislocation. 



