192 RECEPTORS 



the proprioceptors to report on work done but it is obvious that 

 they have to form some opinion of the relation of the organ to 

 its environment. They thus have a function closely allied to 

 that of the true exteroceptors which are stimulated under ordinary 

 conditions by forces outside the organism. 



It is a good thing to bring in the aid of comparative physiology 

 when about to study a fresh group of organs. One may then see 

 how various modifications arise and how the simple undifferentiated 

 cell becomes specialised and fitted to act as a receptor for one 

 particular form of external energy. Time and space do not permit 

 such a digression here, but the student would do well before 

 reading further to revise his knowledge of the comparative zoology 

 of the sense organs. 



The sense organs or receptors may be considered as points of 

 least resistance, gateways through which the manifestations of 

 external forces may reach the internal structures. They are 

 much more than that. They are specialised outposts of an 

 intelligence service which pick up minute alterations in the energy- 

 complex of the environment, make a rapid but incomplete analysis 

 of these changes, and send in a report to departmental head- 

 quarters for complete analysis and transmission to headquarters. 



Only by specialisation can efficiency be obtained. The 

 organism is subject to stimulation from various forms of energy 

 which may be classified into vibratory and chemical. 



A. Vibratory Energy. 



1. Mechanical impacts received by the tactile corpuscles of the 

 skin. They may be perceived as separate stimuli even when 

 they arrive as rapidly as 1552 per second. 



2. Slow vibrations especially in air are received by the ear. The 

 human ear may be stimulated by vibrations ranging from 16 to 

 40,000 per second. Practice may extend this range. 



3. Rapid vibrations in ether. 



(a) Radiant heat. Vibrations with a frequency of between 

 3 billions and 400 billions per second stimulate the temperature 

 receptors of the skin. 



(b) Light. The retina is capable of receiving as light, ether 

 waves, the frequency of which varies between about 400 billions 

 and 800 billions per second. 



B. Chemical Energy. 



The various chemical stimuli to which the organ is exposed, 

 have receptors in the skin, giving rise to sensations of pain or 



