222 University of California Puhlications in Anatomy [you 2 



ment of neurons." This rearrang-ement of the receptive and con- 

 ducting neurons, gradually evolved, culminates finally in a peculiar 

 mutual relationship of individual neurons of each of the aifected 

 afferent paths, an arrang^ement which renders possible the reception, 

 conduction, and utilization of stimuli affecting a very reduced portion 

 of the peripheral sensitive surface and involving a small number of 

 neighboring neurons, or even a single neuron. The most perfect 

 example of this in the higher mammals, is the visual system. 



In this way, (a) by "condensation" and "isolation of neurons" 

 from the originally diffuse nervous system into special "systems" 

 of neurons or afferent paths, and (&) by the "rearrangement of 

 neurons ' ' in each of the above mentioned systems with respect to their 

 mutual relation (besides other consequential changes in the central 

 nervous system), the exploitation of the superior "spatial" or "dimen- 

 sional" properties of certain stimuli is made possible, this utilization 

 being limited only by the absolute size of individual neurons (for 

 example in the fovea centralis of the human retina) . 



According to this viewpoint it is the rearrangement of afferent 

 neurons which, besides those minute changes in the neuroplasm itself 

 which facilitate its function, signifies and conditions a perfection of 

 highly specialized afferent paths, especially those connected with dis- 

 tance receptors (except the olfactory system). The real causes of 

 this rearrangement, however, are unknown, and it is not my intention 

 at present to go beyond the simple statement of facts. One might 

 consider the tendency of neurons which usually act together to take 

 neighboring positions (compare Ariens Kappers, 1920-21, pp. 70 and 

 898 ; see also Bok, 1915, p. 536, Hanstrom, and Veit) ; yet this alone 

 would not explain the actual causes of the "isolation" nor the addi- 

 tional hypothesis that neurons not acting together have the ability 

 to separate from one another. 



The above mentioned rearrangement of afferent neurons and their 

 adaptation to the spatial or dimensional nature of certain stimuli is 

 variously advanced in different stages of the animal scale and is not 

 equally well expressed in different systems in the same animal. "While 

 it is most evident in the visual system in most of the higher animals, 

 especially in mammals and in birds, it is less developed or at any rate, 

 less striking in the somato-sensory and in the auditory systems, being 

 apparently quite absent in the olfactory and in the gustatory systems. 

 Even in one and the same system the "spatial arrangement" of 

 neurons reaches a different degree of perfection in different parts. 



