MULLER'S LAW 193 



discomfort, and in the special end organs to those of taste and 

 smell. 



The function of the receptors is to receive that form of energy 

 for which they are fitted and to transform that energy into nervous 

 energy. We have previously shown that any one form of energy 

 can be transformed into any other form. It has been found 

 convenient, for instance, to calculate all forms of energy mani- 

 festations found in the body in heat units -calories. Nervous 

 energy may be electrical energy. It can readily be demonstrated 

 that any form of energy can be converted into the electrical form. 



As receptors for these various manifestations of energy we have 

 the so-called five senses. That is, five different means are 

 employed for the purpose of orientation, viz. touch, hearing, 

 sight, smell and taste. These senses come into contact with the 

 external forces through the skin, ear, eye, nose and tongue. 

 But some of these are composite end-organs. The skin, for 

 instance, includes not only touch corpuscles but the end-organs 

 for pain and temperature. The ear not only analyses sounds but 

 contains organs for the static and dynamic senses. In all there 

 are over twenty different kinds of receptors and sense-organs in 

 the body. 



Specific irritability. In the preceding chapter we have alluded 

 to Miiller's law of specific irritability, and an electrical model 

 was discussed as illustrative of this idea. A very little con- 

 sideration suffices to show (a) that the same physical stimulus 

 applied to various receptors will give rise to absolutely different 

 sensations and (,8) that different physical stimuli applied to the 

 same end-organ, if they arouse any response at all, give rise to 

 the specific sensation of that organ or to pain. 



Stimulus and Sensation. Each receptor has a certain functional 

 inertia and will not respond to stimulation until the energy of 

 the stimulus has reached a minimal value which is specific for 

 each receptor and for each form of stimulus. This threshold 

 value is lowest, as has been said above, for the form of stimulation 

 specific for that organ. Once this value is gained, the resulting 

 sensation bears a definite relationship to the incident stimulus 

 until an upper limiting value has been reached, after which 

 increase of stimulation is of no avail. In fact, fatigue rapidly 

 sets in, and the resulting sensation is sub-maximal (Fig. 41). 



Touch is the sense by which mechanical force is appreciated 

 Mere contact is gentle pressure, a greater amount of applied force 

 causes a feeling of resistance referred to the skin, a. still greater 

 B. u. 13 



