1932] Poliak: Afferent Fiber Systems, Primate Ceredral Cortex 221 



The relationship of the individual to the environment and to its 

 changing' conditions is achieved in lower stag-es of the animal scale 

 exclusively or preponderantly by an immediate contact with the 

 medium wherein the animal lives. Here it is almost the entire surface 

 of the body of the animal which plays the role of the receptive or 

 sensitive surface for the various stimuli. The stimuli to which lower 

 animals react are mostly of a short range, tactual, chemical, and 

 thermic, while others, notably light and sound, play an inferior or 

 negligible role. For orientation to these stimuli a ''diffuse," poorly 

 specialized nervous system and corresponding primitive receptors 

 suffice. Only gradually certain parts of the nervous system acquire 

 the ability to react to special forms of radiant energy, especially to 

 those whose source lies at a greater distance. To receive, to conduct, 

 and to utilize stimuli arising from the latter forms of radiant energy, 

 special parts of the nervous system are ' ' condensed ' ' from the origin- 

 ally diffuse system into special paths, and special peripheral organs 

 facilitating the reception of the special stimuli are evolved. In connec- 

 tion with this, special small portions of the surface of the body or of 

 the nervous system itself are transformed into highly sensitive receptor 

 surfaces (retina, cochlea, vestibular apparatus, and so forth), con- 

 taining a greatly increased number of neurons (these changes going 

 hand-in-hand with structural modifications). 



Still another fact has to be considered. The increase in the num- 

 ber of neurons itself in the afferent paths and even their structural 

 and chemical perfection without further changes would merely signify 

 facilitation of the reception of stimuli — for instance, increased inten- 

 sity and other quantitative changes. The properties of stimuli, 

 notabty those which could be named "spatial" or "dimensional" 

 would be of no use since the "diffusely" organized afferent paths 

 would always react "in toto" (as for example, the olfactory and 

 gustatory paths). No or little "spatial" or "dimensional" dis- 

 crimination of external stimuli would in fact be possible. The emi- 

 nently "spatial" character of visual, auditory, tactile, and vestibular 

 stimuli, being in itself a superior quality, serving for orientation in 

 space, requires evidently an organization of sense organs and of the 

 aiferent paths different from that which suffices for the reception of 

 stimuli void of a "spatial" character. 



The changes which occurred in some of the highly specialized 

 afferent complexes of neurons and enabled the utilization of the 

 "spatial" properties of certain stimuli might be called the "rearrange- 



