Nervous System 



391 



like all development, it follows a trend from 

 the general to the specific and from more 

 widespread involvement of elements to more 

 restricted and differential activation. Cog- 

 hill's principle of "individuation" from a 

 background of mass reaction is based on this 

 realization. It still holds true as designating 

 a trend of events, even if the initial per- 

 formance under consideration has never 

 been a total activity of the whole body. In 

 some cases and for some functions the pri- 

 mordial activity undoubtedly involves all 

 the neioral apparatuses capable of fimction- 

 ing at the same time (Hooker, '52; Barron, 

 '50), while in other cases and for some 

 other functions, it seems equally clear that 

 activity is territorially localized from the 

 beginning (Windle, '50). 



To give a clear-cut example of the latter 

 type, we need only refer to the appearance 

 of the lid-closure reflex in amphibians (Koll- 

 ros, '42, '43b). This reflex appears only at 

 metamorphosis after having been completely 

 absent in the otherwise fully functional 

 larva (p. 389). But from its very onset it 

 constitutes a strictly localized and circum- 

 scribed act, which has never been part of 

 a "total pattern" of central functions. Evi- 

 dently, individuation from mass action does 

 not apply to this type of response, but this, 

 in turn, does not invalidate the principle for 

 other performances. The sobering lesson 

 from all this work has been that we cannot 

 find a key formula for the development of 

 behavior which will save us the trouble of 

 investigating each component of behavioral 

 development in its own right. 



THE NEURAL BASIS OF 

 INDIVIDUATION 



The development of behavior shows clearly 

 two phases — an early expansive and a later 

 restrictive one (Carmichael, '33). During 

 the expansive phase, wider and wider areas 

 of the body come under neural control. This 

 seqvience has been clearly correlated with 

 the gradual expansion of intracentral nerve 

 connections and pathways (Coghill, '29). In 

 a sense this correlation is obvious, as it 

 merely expresses the fact that where there 

 is no neural pathway, there can be no neural 

 function. Reactions during this early phase 

 are remarkably stereotyped, indicating ab- 

 sence of discriminative response mechanisms. 

 However, the more the nervous system ap- 

 proaches structural completion, the more 

 prominent becomes its ability to activate 

 restricted portions and patterns of the exist- 



ing network independently in selected and 

 coordinated combinations. It is this restric- 

 tive "individuation" for which the proper 

 neural correlate is still to be revealed. 



It had been thought that the development 

 of limb innervation in amphibians could 

 serve as an exquisite model of "individua- 

 tion" in strictly anatomical terms (Cog- 

 hill, '29; Yoimgstrom, '40). The limbs move 

 at first only in association with trunk move- 

 ments, which was explained by the fact 

 that their early innervation consists of col- 

 laterals from the motor neurons of trunk 

 muscles. Later "dissociation" from the trunk 

 appeared linked to the development of a 

 secondary separate fiber system from the 

 limb segments of the cord (Youngstrom, '40). 

 Other studies (Taylor, '43), however, suggest 

 that the so-called "primary" associated limb 

 movements are, in reality, passive movements 

 effected through the trunk muscles of the 

 shoulder, while the intrinsic true limb move- 

 ments do not appear until after the limb 

 muscles have received the independent set 

 of segmental "secondary" neurons. Hence 

 the intrinsic limb function arises as a sepa- 

 rate and individualized activity from the 

 very first rather than as an "individuated" 

 offshoot of an earlier mass response. Al- 

 though some aspects of the situation are still 

 in doubt (Herrick, '48, p. 128), it is quite 

 evident that this singular instance cannot 

 possibly serve as the key model for "individu- 

 ation" in general, as originally proposed. 

 The neural correlate of individuation thus 

 remains as obscure as ever, nor is it very 

 likely to reveal itself in gross microscopic 

 features. Moreover, the progressive refine- 

 ment and localization of coordinated function 

 are only in part attributable to improvements 

 in the central action systems, as the progres- 

 sive muscle-specific modulation of effector 

 neurons (see p. 384), establishing more dis- 

 criminatory relations with the central action 

 systems, undoubtedly has a share in the 

 process (Weiss, '36; Barron, '50). 



CENTRAL ORIGIN OF COORDINATION 



Many modern concepts of neurophysiology 

 attempt to derive the properties of the out- 

 put of the nervous system directly from the 

 pattern of the sensory input. Such a concept 

 is clearly contradicted by the stvidies on 

 development. The fact that the appearance 

 of motor performance antedates sensory con- 

 trol has often been stressed (see Coghill, '29; 

 Herrick, '48). Even more striking is the evi- 

 dence of animals in which the development 



