Nervous System 



351 



nently to a peripheral receptor or effector 

 cell. Further elongation becomes essentially 

 a matter of passive extension, the fiber being 

 in tow by the terminal tissues, which are 

 subject to considerable migi-ations (e.g., 

 muscle buds) and displacements during the 

 subsequent phases of growth (Fig. 128). 

 Because of this "towing" process, the primary 

 growth pattern of nerves becomes greatly 



problem, and a variety of "tropisms" and 

 "attractions" of chemical, electrical, me- 

 chanical or undefined physiological nature 

 have been suggested as the orienting agents 

 (see Harrison, '35a,b). At present, we are ap- 

 proaching a rather unified concept of the 

 mechanism of nerve fiber orientation, which 

 is summarized in the following account 

 (condensed from Weiss, '41c, '44, '50c). 



a o c u e 



Fig. 127. Five consecutive phases in the advance of an axon tip (semidiagrammatic). Arrows indicate 

 directions of flow, thrust and drain of neuroplasm. In e, dichotomous branching of fiber has been initiated. 

 Dotted portions represent the location of earlier protrusions that have been sucked back by the draining 

 force of the axial stream. 



distorted. This explains why ontogenetic 

 shifts, hence phylogenetic relations, of in- 

 dividual muscles can often be traced through 

 their nerve supply, as in the pelvic fins of 

 fishes that have migrated far forward until 

 they have come to lie ahead of the pectoral 

 fins. 



During its period of free advance, the 

 orientation of the nerve fiber is of course 

 determined by the course which its roving 

 tip takes. The early "pioneer" or pathfinder 

 fibers thus lay down the primary nerve con- 

 nections to the nerveless tissues which they 

 invade. Since later fibers simply follow the 

 course of the earlier ones, the problem of 

 nerve orientation concerns primarily the 

 pioneers. Much experimental work, and even 

 more speculation, has been devoted to this 



Mechanism of Orientation. Without recount- 

 ing the trials and errors of the past, it is yet 

 instructive to point to one basic fallacy of 

 earlier concepts, namely, the tacit assump- 

 tion that nerve fibers can penetrate structure- 

 less space in the manner in which plants 

 can grow into air or water — an impression 

 strengthened by the selective nerve stains, 

 which impregnate nerve fibers to the exclu- 

 sion of their surroundings. The suggestiveness 

 of plant growth as a model of nerve growth 

 is clearly reflected in the widespread use of 

 botanical similes in neurological terminol- 

 ogy; e.g., "dendrites," "rami," "roots," "ar- 

 borization," "sprouts." Actually, however, 

 according to the best available experimental 

 evidence, nerve processes, like most animal 

 tissue cells, are unable to push freely into 



