76 THE BRAIN OF THE TIGER SALAMANDER 



The excitation of a peripheral sense organ may be followed by an 

 efferent discharge back to the receptor. An instructive illustration of 

 this is seen in the auditory apparatus of mammals. Excitation of the 

 cochlea is followed by an efferent return to the tensor tympani and 

 stapedius muscles and also to the cochlea itself (the latter pathway 

 recently demonstrated by Rasmussen, '46). Almost all contracting 

 muscles report back to the center by a system of proprioceptive 

 fibers. The central nervous system is full of similar reciprocating sys- 

 tems. Many of the fasciculated tracts of Ambly stoma are two-way 

 conductors, transmitting in both directions, and there are number- 

 less illustrations of a circular type of connection, efferent fibers of one 

 center activating another, which has a return path, perhaps by a 

 devious route, back to the first center. A neuropil may be interpo- 

 lated in any of these types of circuit. The thalamo-cortical connec- 

 tions of the human brain are of this sort, exhibiting what Campion 

 and Elliot Smith ('34) have aptly named a "thalamo-cortical circula- 

 tion," a circulation not of blood but of nervous transmission. All 

 parts of the cerebral hemispheres are in similar reciprocal intercon- 

 nection, as has recently been emphasized and illustrated by Papez 

 ('44). 



Here reference may be made to Dewey's ('96) stimulating analysis 

 of the reflex-arc concept or, as he prefers to say, the "organic circuit" 

 concept. This he elaborated in terms of psychology, and nearly 

 twenty years later I made this comment about it ('13«) : 



"Let us see how it may be applied to biological behavior. The simple reflex is 

 commonly regarded as a causal sequence: given the gun (a physiologically adaptive 

 structure), load the gun (the constructive metabolic process), aim, pull the trigger 

 (application of the stimulus), discharge the projectile (physiological response), hit 

 the mark (satisfaction of the organic need). All of the factors may be related as 

 members of a simple mechanical causal sequence except the aim. For this in our 

 illustration a glance backward is necessary. An adaptive simple reflex is adaptive 

 because of a pre-established series of functional sequences which have been biologi- 

 cally determined by natural selection or some other evolutionary process. This gives 

 the reaction a definite aim or objective purpose. In short, the aim, like the gun, is 

 provided by biological evolution and the whole process is implicit in the structure- 

 function organization which is characteristic of the species and whose nature and 



origin we need not here further inquire into The aim (biological purpose) is 



so inwrought into the course of the process that it cannot be dissociated. Each step 

 is an integral part of a unitary adaptive process to serve a definite biological end, and 

 the animal's motor acts are not satisfying to him unless they follow this predeter- 

 mined sequence, though he himself may have no clear idea of the aim. These re- 

 actions are typically organic circuits Always the process is not a simple se- 

 quence of distinct elements, but rather a series of reactions, each of which is shaped 

 by the interactions of external stimuli and a preformed or innate structure which 



