THE NEURONE 469 



number of neurones whose complex fibre paths place all portions of 

 the body in communication with all other portions. 



Nerve cells are unequally distributed throughout the central 

 nervous system; they therefore occur in more or less distinct 

 groups or nuclei, from each cell of which a neuraxis is frequently 

 distributed along the same path. The larger bundles thus formed 

 are called tracts; the smaller ones, funiculi, fasciculi, or fibre 

 bundles. 



Since each fibre of such a tract is dependent for nutrition upon 

 the nerve cell from which it arises, the tract as a whole must de- 

 pend upon its nucleus of origin for its nutrition. Each nucleus 

 therefore becomes the trophic center for the fibre tract to which it 

 gives origin. 



It may be readily demonstrated that if any such group of neu- 

 raxes be cut or otherwise separated from its trophic center, that 

 tract will promptly degenerate. If these neuraxes happen to be 

 the axis cylinders of medullated nerve fibres, as is often the case, 

 their myelin sheaths become rapidly altered in composition and 

 acquire a tendency to disintegrate into small globular granules, 

 which stain deeply with osmic acid when used according to the 

 method of Marchi. For the experimental demonstration of this 

 form of partial cell death occurring in that portion of the neurone 

 which has been cut off from its cell of origin, we were originally 

 indebted to the eminent English physiologist Waller ; the result- 

 ing changes are therefore called Wallerian degeneration. 



Obviously that portion of a neurone or of a fibre tract which, 

 after injury or disease involving its path, still retains its connec- 

 tion with its cell body or trophic center, will not degenerate. This 

 part of the neurone is called its central portion, in contradistinction 

 to its distal portion, the latter of which has been severed from its 

 trophic center and is consequently degenerated. 



To the study of the various types of Wallerian degeneration we 

 are indebted for many of the facts by means of which the intri- 

 cate tangles of neuraxes composing the various fibre tracts of the 

 central nervous system have been partially unraveled. 



The Anatomic Relations of the Neurone. The many neurones in 

 the nervous system are in relation with one another through their 

 neuraxes, collaterals, and dendrites, and by their peripheral proc- 

 esses are closely connected with all the organs and tissues of the 

 body. Our former conception regarded the processes of any one 

 neurone as being nowhere in direct anatomical connection with 



