154 THE BRAIN OF THE TIGER SALAMANDER 



The sensory root fibers are related peripherally with end-organs 

 which seem to be physiologically as specific as those of man, though 

 the specificities are of different types; but at the first synapse this 

 specificity is no longer preserved in terms of localized gray centers or 

 pathways of conduction. A neuron of the second order may have 

 functional connection with afferent fibers of tactile, vestibular, 

 lateral-line, or gustatory roots, singly or in any combination. 



Since most of the sensory root fibers span the entire length of the 

 medulla oblongata and have numberless collateral branches through- 

 out this length, evidently there is little provision for localization of 

 function in terms of fore-and-aft relations in space. Mass responses 

 of the entire musculature are readily evoked by excitation of any or 

 all components of this sensory complex, but how are selective local 

 responses effected? When the arrangement of the secondary connec- 

 tions of these sensory root fibers is examined, it is evident that this 

 question cannot be answered in terms of any mechanism of switch- 

 board type. The axons of these secondary neurons do not take ran- 

 dom courses. They tend to be assembled in bundles of fibers, which 

 have a common direction and destination. Such bundles as ascend to 

 higher levels (the lemniscus systems) evidently have some kind of 

 specificity, for they retain their identity up to termination in definite 

 and different places. This specificity may be determined by the 

 source of their excitation — the modality of sense and the location in 

 the body of the receptive end-organs — or it may be determined by 

 the nature of the response to be evoked — muscular contraction, secre- 

 tion, and the location of the effector organs activated. Probably both 

 these factors are operative in establishing the pattern of arrangement 

 of the higher centers of adjustment and their connecting tracts of 

 fibers. 



The problem of the apparatus employed in making discriminative 

 responses to specific types of sensory excitation is not simplified by 

 these observations. In higher animals the specificity of the various 

 modalities of sense is preserved in central pathways of conduction 

 leading up to higher centers of adjustment in the thalamus and the 

 cerebral cortex. But somewhere in the course of this transmission 

 these diverse, localized, functional systems are brought into relation 

 with one another and are integrated in such a way as to result in 

 appropriate responses to the total situation. In these amphibians the 

 process of integration begins at the first synapse. The organization of 

 the neurons of the second order is such as to facilitate activation of 



