52 THE NERVOUS SYSTEM 



a separate existence or to regenerate when their continuity with the cell body 

 has been lost. This is what is meant by the statement that the neuron is the 

 trophic unit of the nervous system. 



Degeneration and Regeneration of Nerve-fibers. As has already been stated, 

 that portion of a divided fiber which has been separated from its cell of origin 

 degenerates. The axon breaks up into granular fragments, the myelin under- 

 goes chemical change and forms irregular fatty globules. Later the degenerated 

 axon and myelin are entirely absorbed. The neurilemma cells of a degener- 

 ated peripheral nerve-fiber increase in number, their cytoplasm increases in 

 quantity, and they become united end to end to form nucleated protoplasmic 

 bands or band-fibers. These changes in the nerve-fiber are known as Wallerian 

 degeneration. 



In regeneration new axons grow out from the old ones in the central unde- 

 generated portion of the nerve. These grow into the distal degenerated stump 

 and find their way along the nucleated protoplasmic bands, mentioned above, 

 to the terminals of the degenerated nerve. These band-fibers serve as conduits 

 for the growing axons and from them the new neurilemma sheaths are differ- 

 entiated. Thus, while the neurilemma cells and the band-fibers derived from 

 them appear to be incapable of developing new nerve-fibers by themselves in 

 the peripheral stump, they play an important part in nerve regeneration in 

 co-operation with the new axons from the central stump (Cajal, 1908; Ranson, 

 1912). It is important to note that the nerve-fibers of the brain and spinal 

 cord, which, as has been stated before, are devoid of neurilemma sheaths, are 

 incapable of regeneration. 



The neuron concept, which is based on such facts as have been presented 

 in the preceding paragraphs, was first clearly formulated by Waldeyer in 1891, 

 who was also the first to use the name neuron for the elements under considera- 

 tion. The neuron doctrine may be summarized as follows: 



1. The neuron is the genetic unit of the nervous system each being derived 

 from a single embryonic cell, the neuroblast. 



2. The neuron is the structural unit of the nervous system, a nerve-cell with 

 all its processes. These cellular units remain anatomically separate, i. e., while 

 they come into contact with each other at the synapses there is no continuity 

 of their substance. 



3. The neurons are the functional units of the nervous system. They are 

 conduction units and the conduction pathways are formed of chains of such 

 units. 



