CHAPTER IV 



REFLEXES 



WE have said repeatedly, in one form and another, 

 that it is the duty of the nervous system to produce 

 timely adjustments to changing environmental condi- 

 tions. These adjustments are called reflexes. The 

 statement has already been made that a receptor neuron 

 linked to an effector is the simplest possible "reflex arc." 

 It is now time to enlarge upon these matters. 



The word "reflex" readily suggests the word "reflec- 

 tion." The elementary conception is of an influence 

 brought to bear upon the central nervous system from 

 without and rebounding to contractile or glandular 

 structures which are aroused to activity. This is a de- 

 cidedly crude notion and must be dismissed in favor of a 

 more accurate description. Descartes, in the seventeenth 

 century, attempted to account for the familiar reactions 

 of animals to stimuli in terms like the following: "Nerves 

 are made up of fine tubular conductors connected with 

 reservoirs of a peculiar fluid in the brain. These tubes 

 contain slender threads which are attached to valves 

 controlling the outflow of this fluid. When there is a 

 disturbance at the exposed ends of some of these con- 

 ductors the threads are pulled and the valves opened. 

 Thereupon the energizing fluid runs down the same tubes 

 and enters the substance of the muscles, which are at 

 once thrown into contraction." 1 



This is not a correct exposition of the mechanism, but it 

 is a creditable effort. It can be paraphrased in such a way 

 as to lose much of its fantastic aspect. We can recognize 



1 Foster, "Lectures on the History of Physiology," Cambridge 

 University Press, 1901, chap. x. 



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