REGENERATION OF NERVE FIBERS 145 



Thus the optic nerve is formed. The optic stalk undergoes changes and 

 constitutes merely a supporting medium for these nerve fibers. 



Regeneration of nerve fibers 



The fact that a growing nerve fiber tends to follow solid objects is impor- 

 tant in nerve regeneration. When a nerve is cleanly cut, the part which is 

 severed from the cell body undergoes degeneration. The part which is con- 

 nected with the cell body regenerates. In this process the regenerating nerve 

 fiber tends to follow its former pathway in the degenerating section of the 

 nerve. This is illustrated in Figure 85, A and B. 



If a section of the nerve is removed by injury instead of merely being 

 severed, however, there is a gap between the regenerating nerve fibers and 

 the old pathways in the degenerated section. Since we know from tissue- 

 culture studies that the nerve fiber follows a random path in the absence of 

 any formed structures, it is not surprising that regeneration in these cases 

 does not lead to a normal nerve (Fig. 85, C) . If we could devise some means 

 of bridging this gap between the two cut ends of the nerve, we could get 

 the nerve fibers to grow out in parallel fashion and the regenerated nerve 

 would have the structure of the original one. One means of bringing about 

 this situation is to transplant into the gap a piece of frozen, dried nerve 

 (Fig. 85, D). A nerve is frozen at very low temperatures and dried in a very 

 high vacuum. This treatment results in a preservation of much of its original 

 structure, and thus the frozen and dried nerve fibers are parallel to one 

 another. If these nerve fibers are then placed between the two cut ends of the 

 nerve, the living nerve fibers will tend to follow the parallel paths of the 

 dead fibers and thus bridge the gap. The dead fibers are absorbed by the 

 adjacent tissues. 



Another way of providing these parallel pathways is to exert tension on 

 blood plasma (Fig. 85, E) . If we fill a tube with blood plasma, place the two 

 cut ends of the nerve within this tube, and exert some tension on the clot, 

 the particles in the clot will become oriented in parallel fashion. As with 

 the frozen, dried nerve, the living fibers will enter these parallel pathways 

 and grow across to the other cut section of the nerve. By these means the 

 chaotic growth of nerve fibers is avoided and more normal regeneration and 

 more normal function is attained. Tubulation, or the joining of the two ends 

 of a nerve by a tube, is practiced with some success in surgery. 



