90 'IIIE IJHAIN OF THE TKiEK SALAMANDER 



syntlu'tic rtillicr than Jiiuilylic, and they arc usually descrihod in 

 dynamic terms in which a fourth dimension a time factor — plays 

 an important pail. Yet this ef|nilil)rated dynann'c system is not dis- 

 embodied, and its component parts have locus in space and time. 

 These loci, or fields, may have some degree of permanence with char- 

 acteristic structural organization, as in the so-called "association 

 centers" of the cortex, or they may fluctuate as ever changing pat- 

 terns of linkage of the o})erating neui-ons. As Papez ('44) expresses 

 it: "The anatomical structures are stable, the function is labile, de- 

 pending on numbers of cells and their excitable or refractory states 

 at any particular time." Many of these synthesizing patterns are 

 repetitive, as in habit and memory. Though the actual structures 

 involved may not be identical in successive repetitions, the i)attern 

 of performance is similar, that is, it recurs in conformity with an 

 enduring "schema" (to employ Henry Head's term), or the engram 

 of conventional teiininology. Doubtless the engram has a structural 

 (or chemical .f*) counterpart, but we do not know what it is. 



The stable localization of the structural fields is contrasted with 

 the evanescent localization of the i)attern of their combination at 

 each repetition of the schema. It is possible to find out where the 

 tissue is that yields these dynamic schemata and to delimit it; but 

 these limits cannot be circumscribed on the surface of the brain in 

 simple mosaic patterns. "^I'he manifestation of any schema at a par- 

 ticular time is always a function of a configuration of nervous ele- 

 ments, which has location in space. But a very similar schema may at 

 another time be exhibited by a different structural configuration, 

 whose locus in space is by no means identical with the first ('30a). 



It has recently been shown by Lashley and Clark ('46) that cor- 

 tical structure is variable to a degree not hitherto appreciated in 

 different individuals of the same species of monkey, and it is prob- 

 able that the range of this variability will be found to be still greater 

 in any human population. They conclude that "marked local vari- 

 ations in cell size and density among individuals of the same species 

 may constitute a basis for individual difi'erence in behavior"; but 

 they challenge the validity of the criteria in current use for parcella- 

 tion of the cortex into functionally specific areas, except for rather 

 large areas of projection. This is supported by the experiments of 

 Murphy and Gellhorn ('45) and the observations of Bailey and von 

 Bonin ('4(5). 



