lo Nov., 1908.] Elements of Animal Physiologx. 677 



detail. The various receptors of the body present certain broad outlines 

 of similarity which may be given as follows : — 



1. Each receptor is readily responsive to one particular form of 

 stimulus and responsive with difficulty to all other forms. The more 

 delicate receptors are as a. rule shielded in great part from stimuli other 

 than those to which they are attuned. 



2. If a receptor, or the nerve coming from it. be excited bv anv form of 

 stimulation, the same sensation is always experienced. Thus if a receptor 

 in the skin responsive to warmth be stimulated by electricity the sensation is 

 warmth ; if it be a receptor for cold then the sensation is that of cold. 

 A blow on the eve stimulating the retina or optic nerve gives the sensation 

 of light. 



3. A certain intensity of stimulus is necessary with each receptor before 

 it can start a nerve impulse. If the stimulus be increased a limit is 

 reached at which no further increase of sensation can be produced by in- 

 creasing the stimulus. Between these limits sensations vary in intensity 

 according to the proportion that exists between the stimuli. Thus if the 

 human eye cannot perceive any difference in the illumination of a room lit 

 by ten candles and the same room lit by eleven candles but can distinguish 

 if twelve candles are employed, then the same eye regarding the same room 

 lit by a hundred candles could not detect anv change until twenty more 

 candles were added. 



4. Each type of receptor, except that for pain, can readily be fatigued 

 by its particular stimulus, especiallv when this is intense. Thus a person 

 entering a room can easily detect the presence of musk or iodoform, but 

 after a few sniffs the sensation vanishes. 



The receptors of the animal bodv can be grouped under the following 

 classes — proprioceptors, interoceptors, skin-receptors, telereceptors, pain 

 receptors, visceral receptors. 



PROPRIOCEPTORS.— These are receptors which are set in action by 

 stimuli caused bv the animal's movement or position. Impulses arising 

 from them give information as to the position of different parts of the body, 



Fig. 56. Senii-circular canals and cochlea. The smaller figure shows the natural 

 size in man. 3, 4 and 5 the semi-circular canals ; 6, 7 and 8 the cochlea. "2, elastic 

 window leading to -scaki veMihuU ; 9, elastic window leading to scala fympani. (After 

 Sommering.) 



the extent of movement and the resistance to muscular action. These recep- 

 tors are found in muscles, tendons and ligaments (.see Chapter III.) and 

 are probably excited by strain in these tissues. They are also present in 

 joints being here responsive to pressure and mo\ement. In the head, and 



