CHAPTER IV 

 REGIONAL ANALYSIS 



SINCE the brain of Amblystoma presents a generalized structure 

 which is probably close to the ancestral type from which all 

 more highly specialized vertebrate brains have been derived, the 

 salient features of internal organization are here summarized in 

 schematic outline. Tlie accompanying diagrammatic figures 1-24 

 give the necessary topographic orientation, and the details may be 

 filled in by reference to the corresponding sections of Part 11. What 

 is here described may be regarded as the basic organization of the 

 brains of all vertebrates above fishes, that is, the point of departure 

 from which various specialized derivatives have been differentiated. 

 Amblystoma possesses the equipment of sensory and motor organs 

 typical for vertebrates at a rather low level of specialization and in 

 evenly balanced relations. All the usual systems are present, and none 

 shows unusual size or aberrant features. The great lateral-line system 

 of sense organs so characteristic of fishes is preserved, though some- 

 what reduced after metamorphosis. On the motor side the organs of 

 locomotion and respiration have advanced from the fishlike to the 

 quadrupedal form, but in very simple patterns. In early phylogeny 

 the specialization of the motor systems seems to lag behind that of 

 the sensory systems because the aquatic environment of primitive 

 forms is more homogeneous than that of terrestrial animals, and, 

 accordingly, fewer and simpler patterns of behavior are needed. 



Our search in this inquiry is for origins of human structures and 

 for an outline of the history of their evolution. From this standpoint 

 it is evident that in the central nervous systems of all vertebrates 

 there is a fundamental and primary difference between the cerebrum 

 above and the rhombencephalon (and spinal cord) below a transverse 

 plane at the posterior border of the midbrain (for further discussion 

 of this see chaps, viii and xiii). 



The spinal cord and rhombic brain contain the central adjustors of 

 the basic vital functions — respiration, nutrition, circulation, repro- 

 duction, locomotion, among others. This apparatus is elaborately 

 organized in the most primitive living vertebrates, as also no doubt 

 it must have been in their extinct ancestors. The cerebrum, on the 



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