Nervous System 



353 



it has been possible to direct them into a 

 straight oriented course by forcing the un- 

 derlying matrix into pai'allel alignment. 

 This has been achieved both in the gelati- 

 nous fin tissue of larval amphibians (Weiss, 

 '50a) and in the blood clots binding severed 

 nerve stumps of advilt mammals (Weiss, '44; 

 Weiss and Taylor, '43). In the latter case 



tion. If the nerve course depends on preneural 

 guide structures in the colloidal matrices, our 

 attention must therefore turn to the factors 

 producing structural orientation. Tension 

 being presumably the commonest orienting 

 agent, let us examine first the potential 

 sources of tensional stresses in the body. 

 Oriented tensions arise from external stretch- 



a h 



Fig. 129. Advance of nerve fibers in fibrous media of different degrees of ultrastructural organization 

 (randomness in center turning into prevailing horizontal orientation in left part, and strict vertical orienta- 

 tion in right part of diagram). Along random meshes of center strip, the course of fibers a, b and e is 

 tortuous, with frequent branching; in the more orderly parts of the medium, fiber courses become corre- 

 spondingly aligned; in a rigorously oriented mediimi, fibers (c, d) run straight and remain undivided. 



there is a primary phase, during which longi- 

 tudinal tensions orient the fibrin of the blood 

 clot in a prevailing direction from stump to 

 stump, followed by a secondary phase, dur- 

 ing which fibrinolytic agents discharged in 

 the wound dissolve all remaining disoriented 

 crosslinks between the longitudinal fibrin 

 strands. This gives a good illustration of the 

 multiplicity of factors involved in orienta- 

 tion. 



In view of the ubiqviitous presence of 

 fibrous elements in the tissue spaces, these 

 examples may be considered to be fair models 

 of the normal mechanism of nerve orienta- 



ing or internal shrinkage of a cohesive sys- 

 tem. Differential growth, resulting in exten- 

 sive displacement of body parts relative to 

 one another, is an ample sovuxe of stretch 

 effects. Nerve growth may thus be expected 

 to trail actively advancing organs even prior 

 to being taken in tow by them. Localized 

 shrinkage is perhaps even more important 

 as a source of stress. Such shrinkage occurs, 

 for instance, around any intensely proliferat- 

 ing area as a result of a peculiar dehydrating 

 effect which proliferating cells exert on sur- 

 rounding colloids (Weiss, '29, '34a; Grossfeld, 

 '34). The resulting local contraction of the 



