Nervous System 



361 



brain and spinal cord could account for the 

 regular emergence of collaterals at those 

 sites. Possibly different fiber types might 

 even react in different degrees to the same 

 irritant so that one type would give off a 

 branch while another type would not. These 

 are merely some pointers to future work. 

 Concrete information is scanty. 



In view of the labile state of the neuron 

 (see later), we must expect it to spring 

 minor leaks in its stu-face all the time, es- 

 pecially in its unsheathed terminal branches 

 and at the nodes of Ranvier. Whether such 

 weak spots will be repaired or become the 

 source of a collateral branch will presum- 

 ably depend on the vigor and rate of growth 

 of the neuron and the competitive strength 

 of the main stem of the fiber, which counter- 

 acts accessory outgrowths. Agents capable 

 of either loosening the axonal surface or in- 

 vigorating neuronal growth or merely re- 

 ducing the drain into the main axis of the 

 fiber (as after amputation) should, there- 

 fore, automatically increase the frequency 

 of collateral branching. The compensatory 

 sprouting of peripheral collaterals after par- 

 tial denervation of muscles or after injection 

 of substances from degenerated muscles (Hoff- 

 man, '50; Edds, '53), as well as the "para- 

 sitic" branches forming from severed or 

 otherwise truncated neurons (Nageotte, '22), 

 seem to bear out this expectation. 



Nerve Patterns Within Peripheral Organs. 

 Usually, the factors that guide nerves to a 

 given organ are not the same that will deter- 

 mine the distribution pattern within the or- 

 gan (Hamburger, '29). For instance, when 

 limb buds are transplanted to the head 

 region and innervated by foreign cranial 

 nerves, the latter assume a distribution pat- 

 tern which is essentially a typical limb 

 pattern. It is from this very observation that 

 Harrison ('07b) first deduced the structural 

 guidance of nerve fibers, a conckision which 

 was soon also adopted by Braus ('11), who 

 had previously interpreted his own similar 

 observations as evidence of a peripheral 

 (autonomous) origin of nerves. Evidently, 

 the growing limb can impose a limb-specific 

 arrangement upon nerve fibers coming from 

 whatever source (see also Hamburger, '29; 

 Piatt, '41; Weiss, '37a). This plainly contra- 

 dicts the gratuitous contention (Ruud, '29) 

 that nerves from a given source contain the 

 geometry of their future distribution in 

 themselves. Rather, the "limb pattern" of 

 distribvition is determined by a complex, 

 multifactorial chain of events, roughly divis- 

 ible into two phases — a primary one governed 



by the structural and chemical properties of 

 the preneural pathways in the limb blastema, 

 and a secondary one of elaboration of the 

 primary pattern by towing, fasciculation 

 (see below, p. 366), and the differential sur- 

 vival, growth, and resorption of fibers, de- 

 pending on the physiological adequacy of 

 their terminal connections. The decisive 

 patterning effect of the primary phase is 

 revealed by the observation that virtually 

 all nerve branches of the mature limb (in 

 the frog) are already recognizable as such 

 in the early limb bud at a very primitive 

 stage of morphogenesis (Taylor, '43). The 

 principal nerve paths are thus laid down 

 by factors in the early limb blastema. 



If, on the other hand, a limb is kept nerve- 

 less ("aneurogenic") during its differentia- 

 tion and is then grafted to a normal host 

 body from which it can derive belated in- 

 nervation, the invading nerves follow quite 

 irregular and aberrant courses (Piatt, '52). 

 A certain predilection for some major in- 

 vasion routes at times creates some gross 

 resemblance to a limb pattern (Piatt, '42), 

 but this could be dvie simply to trivial ana- 

 tomical features, offering only limited spaces 

 between skin, muscles and skeleton for the 

 massive advance of nerve fibers. In this in- 

 stance, major blood vessels may also play a 

 leading role (Hamburger, '29), although in 

 normal development the noted parallelism 

 between vascular and nerve trvmks is more 

 likely to be a sign of common guidance of 

 both systems by the same ultrastructural 

 pattern in the common matrix. Whether the 

 nerve distribution pattern within the limb 

 follows normal or aberrant lines is of no 

 consequence, however, as far as the later 

 functional activity is concerned. As will be 

 described below (p. 384), functional coordina- 

 tion between the central nervous system and 

 receptor and effector organs remains orderly 

 even if the anatomical nerve connections are 

 utterly confused. The relative stereotypism of 

 peripheral nerves is presumably significant 

 only as a means of insuring ubiquitous in- 

 nervation of adequate quantity. 



PERIPHERAL CONNECTIONS 



Specificity of Preneural Pathways. Nerve 

 fibers of a given kind can penetrate foreign 

 organs with ease. After heterotopic trans- 

 plantations or other deviations, cranial nerves 

 have been followed into limbs (see above; 

 Harrison, '07b; Braus, '11; Nicholas, '33; 

 Piatt, '41), midbrain fibers into trunk muscles 

 (Hoadley, '25) in the chick (not observed 



