STIMULATION THE SENSES 10; 



spots sensitive to temperature, and, a rather curious thing, there are 

 different receptors for heat and cold (E., p. 21 1). The nature of a 

 specialised receptor may, to some extent, be realised by stimulating 

 with an electrical current a spot sensitive to cold, for example. A 

 sensation of cold, and no other, is produced, but the strength of the 

 stimulus necessary is very much greater than when it is the normal 

 one of cold. This means that the mechanism is specially adjusted 

 to be affected by the withdrawal of heat. In what way this is 

 done we cannot say. It has been suggested that it may be by 

 some chemical reaction which is very sensitive to change of 

 temperature, or some effect on volume may be concerned. 



Taste and Smell. These may be called "chemical senses," 

 because they depend on the properties of substances acting in 

 watery solution on the receptors. But it must be remembered that 

 the properties are not ordinary chemical ones, since there are a 

 number of compounds which taste sweet, although there is nothing 

 in common in their chemical nature. 



The skin of fishes has a kind of generalised chemical sense, 

 such as would naturally be expected to make its appearance at an 

 early stage in evolution, in response to the variety of chemical 

 substances given off to water by other animals and plants. It 

 seems probable that the senses of smell and of taste of the higher 

 animals have developed from this. It should be remembered also 

 that the sense of smell plays a large part in the life of water animals. 



In one case, that of acid substances, the taste is definitely in 

 relation to the hydrogen-ion concentration. 



With the exception of smell, the senses hitherto described 

 require the actual contact of objects with the surface of the body, 

 and they give us no warning of the approach of distant influences. 

 Although touch gives valuable information of the properties of 

 objects, and guides us in muscular movements, while smell, 

 especially in certain organisms, is of value in warning of distant 

 occurrences, it is by sight and hearing that accurate information is 

 obtained of such things. It is to these " distance receptors " that we 

 owe the greater part of our higher intellectual life. The mode of 

 action of the receptors in these cases is a complex one, but, never- 

 theless, it may be said that we know more about it, up to a certain 

 point, than about the apparently simpler cases. 



Hearing or the Receptor for Sound. The phenomenon in the 

 outer world that arouses in us the sensation of sound is an alternate 

 condensation and rarefaction of the material of which bodies are 

 composed, transmitted in the form of waves. If we confine our 

 attention to one point in the air, for example, we notice that the 

 air becomes alternately denser and rarer. What is known as \bzpitch 

 of a note is the number of times per second that this process takes 



