18 THi: NKKVOI S SYSTEM AM) ITS C( (NSKKVATIOX 



RECEPTORS; ADJUSTORS; EFFECTORS 



These three words have recently been used to develop 

 a most simple and helpful outline of the general work 

 of the nervous system. Whether we consider a worm, an 

 insect, a reptile, or a mammal, three departments of 

 organi/ation with functions of three orders can he indi- 

 cated. The receptors, as the name implies, constitute 

 that department of the system which is suhject to ex- 

 ternal influences. Receptors are wrought upon hy press- 

 ure, by changes of temperature, hy chemical agents in 

 solution. In the more highly developed animals they 

 include particular sense organs like the eye and the ear. 

 Such organs translate vibratory energy of the ether and 

 the air respectively into the form of energy which nerves 

 convey. 



It is to he noted that receptors may he strictly part 

 of the nervous system, as where naked endings of the 

 fibers lie exposed to contacts, or they may he mechanisms 

 made of other tissues serving to transmute and apply the 

 energy of the stimulus. AYhere the receptor does not con- 

 stitute a part of the nervous system the path from it to 

 the central axis must be given a special name, and it is 

 spoken of as a sensory or afferent path. 



The effectors upon which the nervous system plays 

 include 1 the contractile and glandular tissues. The effec- 

 tors are always distinct from the nervous system, and the 

 libers stretching out to them form efferent paths. The 

 third class of working units, the adjustors, are those 

 within the nervous system which mediate between the 

 receptors and the effectors, or. more accurately, between 

 afferent and efferent pathways. It is in the realm of the 

 adjustors that the utmost complexity of Organization is 

 possible. The variety of reaction so typical of animals 

 which we call "high in the scale" has it- physical basis here. 



I'arker, 1 of Harvard, has written ino-t suggestively of 



'('. II. I'.-irkcr, "The Origin and Significance of the Primitive 

 Nervous System," Proceedings of the American Philosophical 

 Society, lUll, vol. 1, No. 1'J'J. 



