104 INTRODUCTION TO GENERAL PHYSIOLOGY 



There must be channels along which the messages are conveyed 

 to the nerve centres and back again to the muscles. These are 

 the white threads called " nerves " (E., p. 209), which consist of 

 a number of separate fibres, each carrying its own message apart 

 from the rest. Nothing can be seen to happen either in the nerve 

 or in the nerve centres. In the nerve-muscle preparation which 

 we made previously, a stimulus applied at the far end of the nerve 

 caused the muscle to contract, although there was no sign of 

 anything passing along the nerve. 



The student should examine the general arrangement of the 

 central nervous system in a frog or rat (E., p. 209). But, at the 

 present stage, details are unnecessary. 



Let us next see what are the different kinds of sensations we 



receive from various external agencies. If the skin is pinched, we 



feel pain. If touched gently, there is no pain, but a sensation 



of a different kind. If a warm object is held near the skin, we have 



a sensation of heat. A cold object produces a sensation which is 



distinct from that of heat. All these are from the skin. By the 



eyes we perceive light. By the ears, sound. By another receptor, 



anatomically associated with that for the perception of sound, but 



having no physiological connection with it, we are informed of 



changes in our position in space, or our relationship to the direction 



of gravity. By the nose we smell, and by the tongue we taste. 



There are, thus, nine different kinds of sensation, each corresponding 



to some distinct property of external nature. The receptors which 



enable these sensations to take place must therefore each possess 



a structure which is appropriate to some particular form of external 



energy, so that a change may be effected in it by that form of 



energy when it obtains access to the receptor. A structure 



sensitive to light would be unaffected by sound waves, and so on. 



A not inappropriate illustration, as we shall see later, would be 



a photographic plate, in which chemical changes are produced 



by light, but not by sound. The change brought about in the 



receptor must be of such a nature and magnitude as to act as 



a stimulus to the ends of the nerves which arise from this receptor. 



We saw that pressure is able to stimulate a nerve when applied 



directly, but, in order to do so, it must be far greater than the 



degree of pressure involved in the sense of touch. It would seem, 



in this case of touch, that all that is necessary is some form of 



mechanical magnification of the action of the external agent. In 



other cases, as those of sound and light, the nerve itself is unaffected 



by them (E., p. 210), and it is necessary that they shall set into 



activity some, mechanism which has the result of producing a form 



of stimulus to which the nerve is sensible. It appears that the 



energy value of an actual stimulus to which a sense organ can 



