CHAPTER XIX 

 THE CEREBRAL HEMISPHERES 



SUBDIVISIONS OF THE HEMISPHERE 



LITTLE need be added here to the general descriptions given in 

 ^ chapters iv and vii. For details the reader is referred to the 

 paper of 1927. That description was based mainly on a survey of a 

 small number of well-preserved specimens cut in the transverse plane. 

 There are in our collection many more instructive Golgi sections cut 

 in longitudinal planes which have not been critically studied, though 

 preliminary surveys have been made. It is deemed unprofitable at 

 this time to continue the study of these sections because their inter- 

 pretation should be based on physiological experiments correlated 

 with the anatomical analysis. 



At the present time our knowledge of the details of the internal 

 structure of the cerebral hemispheres of Necturus ('336) is more com- 

 plete than of any other amphibian. This brain is not only larger than 

 most of the others, but it is less compact and its great elongation is 

 favorable for accurate localization of experimental studies by a wide 

 variety of methods. This generalized brain provides a norm or basic 

 pattern for the vertebrate phylum as a whole. The other urodeles and 

 the anurans present a series of progressively more differentiated 

 brains, and the sequence of stages of this process of specialization 

 can readily be followed. That such a program of correlated histologi- 

 cal and experimental work is practicable was demonstrated by Cog- 

 hill in a restricted field of embryological research. With the refined 

 experimental methods now at our disposal and with some reorienta- 

 tion in the fields of developmental mechanics, localized experimental 

 extiipations, and electrical excitations, supplemented by oscillo- 

 graphic records, the steps in progressive phylogenetic differentiation 

 of structure can be correlated with changes in overt behavior. For the 

 completion of such a program frogs will probably prove to be more 

 serviceable animals than the more sluggish salamanders (p. 98). 

 These data will enable the comparative psychologists to identify and 

 interpret prodromal stages of some of the basic patterns of human 

 mentation. 



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