Ill 



SOME FUNDAMENTAL FACTS AND CONCEPTIONS CON- 

 CERNING THE COMPARATIVE PHYSIOLOGY 

 OF THE CENTRAL NERVOUS SYSTEM i 



1. The understanding of complicated phenomena depends 

 upon an analysis by which they are resolved into their simple 

 elementary components. If we ask what the elementary com- 

 ponents are in the physiology of the central nervous system, 

 our attention is directed to a class of processes which are called 

 reflexes. A reflex is a reaction which is caused by an external 

 stimulus, and which results in a coordinated movement, the 

 closing of the eyelid, for example, when the conjuctiva is 

 touched by a foreign body, or the narrowing of the pupil under 

 the influence of light. In each of these cases, changes in the 

 sensory nerve endings are produced which bring about a change 

 of condition in the nerves. This change travels to the central 

 nervous system, passes from there to the motor nerves, and 

 terminates in the muscle-fibers, producing there a contraction. 

 This passage from the stimulated part to the central nervous 

 system, and back again to the peripheral muscles, is called a 

 reflex. There has been a growing tendency in physiologj^ to 

 make reflexes the basis of the analysis of the functions of the 

 central nervous system, and consequently much importance 

 has been attached to the processes underlying them and the 

 mechanisms necessary for reflex. 



The name reflex suggests a comparison between the spinal 

 cord and a mirror. Sensory stimuli were supposed to be 

 reflected from the spinal cord to the muscles; destruction of the 

 spinal cord would, according to this, make the reflex impossible, 



1 Reprinted from Loeb, J., Comparative Physiology of the Brain and Compara- 

 tive Psychology (1899). By courtesy of G. P. Putnam's Sons of New York and 

 London. 



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