140 NERVOUS TISSUES 



degenerate. If these axons happen to be the axis cylinders of medullated 

 nerve fibers, 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 neuron which 

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

 the eminent English physiologist Waller; the resulting changes are there- 

 fore called Wallerian degeneration. 



Obviously that portion of a neuron or of a fiber tract which, after 

 injury or disease involving its path, still retains its connection with its 



c. rod. post. 



FIG. 153. TRANSECTION OF THE SPINAL CORD OF AN EMBRYO CHICK. 



c. rod. ant., axons to the ventral roots; c. rod. post., axons to the dorsal roots; 

 col, collateral from an axon back to the gray matter; gg, dorsal root ganglion; roc. ant., 

 ventral root; roc. post., dorsal root. (After van Gehuchten.) 



cell body or trophic center, will not degenerate. This part of the neuron 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 intricate tangles of 

 axons composing the various fiber tracts of the central nervous system 

 have been partially unraveled. 



The contiguous relationship of different neurons within the nervous 

 system occurs in any one of several ways. The terminal arborizations or 

 telodendrions of one neuron may interlace with : 



a. the telodendrions of axons belonging to other neurons, 



b. the telodendrions of collaterals of other neurons, 



c. the dendrons of other neurons, or 



d. the terminal arborization may surround, basket-like, the cell 



body of other neurons. 



