PHYSIOLOGICAL INTERPRETATIONS 83 



will be found. Clearly, the nervous system does not operate, even in 

 the case of the simplest known reflex, on the mechanical principles 

 of an automatic telephone exchange. We get only confusion by over- 

 simplification of the problem. On the other hand, there are no dis- 

 embodied functions, and the apparatus that performs functions has 

 locus in space and time. Our problem is, first, to observe what is done 

 and then to find out wliere and when it is done and how. 



The observed spatial arrangements are not meaningless, and their 

 functional interpretation is possible and fruitful, as evidenced by 

 their practical utility in medical diagnosis and treatment. These 

 structural patterns are stable and heritable. Their phylogenetic de- 

 velopment can be traced, and in broad outline this has been done. 

 But as these patterns are followed backward in the evolutionary 

 series they become, not more simple and sharply defined, but less so, 

 until in the most primitive and generalized vertebrates they tend to 

 disappear in a more nearly homogeneous matrix. This would seem to 

 support the view that localization of function is a secondary acqui- 

 sition, derived from a primitive equipotentiality. With certain im- 

 portant qualifications, this is probably true; and if we follow in phy- 

 logeny the differentiation of local centers and their connecting tracts 

 in correlation with types of function performed, the significance of 

 localization appears. The problems of cerebral localization have 

 usually been attacked in mammals and especially in man, where 

 clinical applications are vitally important. Let us approach the sub- 

 ject from the other end of the phyletic series and look for the incep- 

 tion of localization patterns in primitive animals. 



In the simplest known organisms localization of function is mini- 

 mal and transient. In ameba any part of the cytoplasm may on oc- 

 casion perform any function of which the organism is capable. There 

 is a local differentiation of nucleus from cytoplasm, but in some bac- 

 teria even this localization disappears. A surface-interior pattern is 

 always present, but the physical substance may shift from one to the 

 other of these zones. In primitive multicellular species, ectoderm and 

 entoderm were early differentiated — a specialization which persists 

 throughout the animal kingdom as manifested in the basic distinc- 

 tion between somatic and visceral organs, a structural differentiation 

 that has physiological meaning. Further specialization advanced 

 more rapidly in somatic organs than in visceral, and in the former 

 more rapidly on the sensory side than on the motor side. This again 



