Chap. IV.] GENERAL PHYSIOLOGY. 57 



ordination, and eventually the initiation, of successively more 

 complex activities. But in what way they are able to do this 

 what is the exact nature and order of the changes which go on 

 within them these are points which it must be left to the 

 physiology of the future fully to determine. 



The activities of which we have spoken involve sense-organs, v 

 nerves, nerve-centres, and muscles. It is the function of the 

 sense-organs to be the recipients of stimuli. It is the function 

 of afferent nerves to transmit the waves of stimulation from the 

 sense-organs to the nerve-centres. It is the function of the 

 nerve-centres to receive the waves of stimulation brought by the 

 afferent nerves, to organise them, and to transmute them into 

 outgoing waves of stimulation. It is the function of the efferent 

 nerves to convey these outgoing waves to the muscles which are 

 thus stimulated to orderly contraction. The common property 

 of nerves and muscles is irritability. But the irritability is 

 differently manifested in these two different tissues. 



It is not necessary here to dwell longer on this aspect of 

 general physiology. It will be sufficient to note that the whole 

 surface of the body is, so to speak, in communication with the 

 surrounding world through the sense of touch ; while the organism 

 is to some extent made acquainted with the state of its own 

 organs by means of stimuli arising within the body (e.g. hunger). 

 To these are added special senses which might perhaps be termed 

 organs of prediction. Taste predicts what materials taken into the 

 mouth are suitable for allaying hunger. Smell still further pre- 

 dicts, even before they are taken into the mouth, what materials 

 are fitted for this purpose. Hearing predicts or recognises from 

 afar pleasurable or painful sensations, and thus enables the 

 organism to seek the one or to avoid the other. Still more does 

 Sight predict the pleasurable and the painful, and enable the 

 organism to act betimes. It must be noticed that, in speaking 

 of the pleasurable and the painful, we have drifted into the 

 region of feeling. It is almost impossible, in speaking of the 

 physiology of the sense-organs and nervous system, not to do so. 

 And although we know nothing, except by inference, of the 

 feelings of animals, we shall probably not be far wrong in 



