Nervous System 



359 



Despite this qualification, it seems fairly 

 obvious that pioneer fibers often do take di- 

 rective courses toward growing organs, which 

 by their very growth activity have become 

 hubs of pathway systems. It is in line with 

 this view that nerves can be made to con- 

 verge vipon a transplanted limb bud only if 

 the operation is performed prior to their first 

 outgrowth; once established, their coixrse can 

 no longer be redirected (Detwiler, '24b). 



Intracentral Connections. There are hardly 

 any systematic investigations on the manner 

 in which the different intracentral nerve 

 tracts are laid down, the routing of which 

 is of such paramount importance to orderly 

 function. In attempting a causal analysis, it 

 is well to keep in inind two basic factors. 

 First, there is no microprecision, in the sense 

 of rigidly determined connection patterns, on 

 the level of the individual neuron. Only 

 the gross group characters of the various 

 tracts are determined, while the details are 

 merely statistically defined, the individual 

 elements conforming to some "norm" but 

 being otherwise indeterminate. This consider- 

 ably reduces the number of relevant patterns 

 to be accounted for. Second, the incredibly 

 complex structure of the adult brain owes 

 its intricacy to the fact that it is the com- 

 pound result of innumerable elementary pat- 

 terns laid down one after another in a long 

 succession of separate ontogenetic steps, each 

 one in itself rather simple. If we assume that 

 intracentral fibers, like peripheral fibers, 

 trace oriented pathways in their colloidal 

 surroundings, then each pathway system that 

 temporarily dominates a given embryonic 

 period will leave a permanent record behind 

 in those neuron systems which happen to 

 grow out during that period. As conditions 

 change, the colloidal matrix will adapt to 

 the change, assume new orientation and thus 

 establish new nerve courses, often unrelated 

 geometrically to the earlier ones. However, 

 actually to resolve central nerve patterns into 

 such simple constituent steps is still largely 

 a task for the future. 



That it promises success is indicated, for 

 instance, by the observation that fiber tracts 

 tend to develop between central neuron 

 groups that develop contemporaneously (Cog- 

 hill, '29). This looks like the "two-center 

 effect" at work again. Reasonable guesses 

 are also possible regarding the pathway struc- 

 tures underlying the longitudinal fiber tracts, 

 the commissural fiber systems and the early 

 internuncial connections of the cord. The 

 longitudinal tracts appear to be determined 

 by the longitudinal stretch to which the 



neural tube is subjected by the growth in 

 length of the surrounding body (see below). 

 This assumption finds support in the early 

 appearance of axial birefringence in the 

 neural tube (Hobson, '41), indicating polar 

 orientation of the substratum. The arched 

 transverse pattern of the ventral commissures 

 might be attribvitable to transverse stresses 

 which the median strip of the medullary 

 plate suffers diiring the bending of the plate 

 into the tube. The early internuncial fibers 

 travel along an interface clearly demarcated 



Fig. 134. Deflection and recombination of nerve 

 fibers (plexus formation) along intersecting sys- 

 tems of ultrastructural pathways. 



between the dense neural epithelium sur- 

 rounding the central canal and the more 

 loosely packed mantle zone. 



All these preneural guide structures, how- 

 ever, can only account for the initial orienta- 

 tion of nerve patterns. Other sets of factors 

 determine their further elaboration with re- 

 gard to numbers, size, and connections (see 

 below). 



Deflection and Plexus Formation. Many 

 nerve courses show a sudden angular de- 

 flection from one direction into another. This 

 may be the result of a passive distortion — 

 for instance, by sudden change of course of 

 a towing organ or by wedging in of another 

 organ — or it may be a sign of actual angular 

 outgrowth. The latter condition is realized 

 whenever pioneering fiber tips proceeding 

 along one pathway system come upon another 

 one running crosswise (Fig. 134). Depending 

 on how completely the intersecting system 

 obliterates the original system, fewer or more 

 of the tips will be diverted into the new 

 direction; a tight barrier (e.g., membrane) 

 will produce total deflection. Intersecting 

 structures of this kind arise, for instance, in 



