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 o\ir 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 



