THE RECEPTOR SYSTEM 171 



is proportional to the strength of the stimulus already acting; for ex- 

 ample, the heavier a pressure already acting on the skin the more 

 must it be increased or diminished in order that the increase or 

 diminution may be felt. Examples of this, which is known as 

 " Weber's " or " Fechner's psychophysical law " will be hereafter 

 pointed out, and are readily observable in daily life; we have, for 

 example, a luminous sensation of certain intensity when a lighted 

 candle is brought into a dark room; this sensation is not doubled 

 when a second candle is brought in; and is hardly affected at all by 

 a third. The law is only true, however (and then but approxi- 

 mately), for sensations of medium intensity; it is applicable, for 

 example, to light sensations of all degrees between those aroused 

 by the light of a candle and ordinary clear daylight: but it is not 

 true for luminosities so feeble as only to be seen at all with diffi- 

 culty, or so bright as to be dazzling. 



Besides their variations in intensity, dependent on variations 

 in the strength of the stimulus, our sensations also vary with the 

 irritability of the sensory apparatus itself; which is not constant 

 from time to time or from person to person. In the above state- 

 ments the condition of the sense-organ and its nervous connections 

 is presumed to remain the same throughout. 



Classification of Receptors. It is possible to group the sense- 

 organs in several different ways according to the properties upon 

 which the classification is based. If we group them according to 

 the forms of energy to which they respond they fall into four 

 classes: 1, the senses aroused by mechanical stimulation, touch, 

 pain, muscle sense, equilibrium, and hearing; 2, those aroused by 

 chemical stimulation, taste, smell, and probably the sensations of 

 hunger, thirst, and fatigue; 3, the temperature sense, aroused by 

 thermal stimuli; 4, the sense of sight, aroused by stimuli of light. 



Another classification, and a more convenient one to follow in 

 describing the receptors, is based upon their position in the body. 

 This classification gives us two main groups: 1, the internal senses, 

 whose receptors lie within the body; here belong muscle sense, 

 equilibrium, pain, hunger, thirst, and fatigue; 2, the external 

 senses, whose receptors are on the surface of the body and which 

 therefore obtain information of the outside world. These senses 

 fall again into two subgroups; the first includes the contact senses 

 which are stimulated only by things in immediate contact with the 



