430 THE TISSUES. 



the white (fibrous) portion of the encephalon and the spinal cord; 

 while they also sometimes form even one-half of the gray substance 

 of these two organs and of the ganglia. 



It has already been shown that there are at least eight or nine 

 times as many coarse as fine fibres in muscles (p. 415); while the 

 fine fibres greatly predominate in the ganglionic nerves. 



Chemical Composition of the Nerve- Fibres. 



It appears that the neurilemma and the axis-fibre very much 

 resemble elastic tissue, and are albuminous compounds ; while the 

 medulla is rather a fatty compound. It is not, however, to be sup- 

 posed that either of these three has a chemical composition precisely 

 identical with that of any other tissue whatever. The nerve-tissue 

 has vital properties which are sui generis, and doubtless its chemical 

 composition is peculiar. "When the immediate principle peculiar to 

 this tissue is identified, it may, from analogy, be called nervine (p. 

 426). A quantitative analysis of the nerve-fibres occurring to form 

 masses, will be given in connection with the fibrous substance of 

 the brain. 



The vessels of the nerve-fibres will also be described further on. 



Functions of the Nerve-Fibres. 



It is very certain that the nerve-fibres, in all cases, merely minister 

 to the central parts of the nervous system ; being merely 1 the media 

 by which impressions are transmitted to and from the latter. 



From the statement of Yolkmann (p. 415), we must infer that many 

 at least of the coarser fibres, if not all, are employed in transmitting 

 impulses to the muscles, and are therefore motor nerve-fibres. On 

 the other hand, the fine fibres are not motor, since in muscles they 

 are sent to the vessels especially (p. 416). They also abound in the 

 ganglionic nerves. The inference, therefore, is, that the fine fibres 

 must include both the sensory and the ganglionic, and both of 

 which are afferent i. e. they conduct impressions to the centres and 

 ganglia; while the coarse fibres are efferent. Still, we must not 

 assert that a motor fibre is coarser throughout its entire extent, 

 since they sometimes become very fine at their distal extremity. 



In regard to the office of each of the three component parts of 



1 In the performance of this function, -however, "the whole extent of the fibre 

 between the point stimulated and its peripheral and central connection is the seat 

 of change " (Todd and Bowman, p. 235.) 



