24 THE MECHANISM OF LIFE 



are striated, while in those over which we have no control (the 

 muscles of the heart, alimentary canal, and bloodvessels chiefly — 

 the involuntary muscles) the fibres are " plain," or non-striated, 

 or are. otherwise different from the ordinary striated fibres. 



The Sensory Mechanisms. 



So much for the apparatus of mobility. The reader can 

 easily supplement the above very general summary from the 

 textbooks on physiology. Next we have to consider how this 

 apparatus is set in motion, and that leads us to the study of 

 sensation. There are certain mechanisms, mainly those of the 

 heart and respiratory organs, which are automatic — that is, 

 they work in the absence of any external cause (though their 

 rate of working is affected by outer events) — but in most of 

 the muscular mechanisms of body and limbs a stimulus is 

 necessary in order that they may be set in motion — that is, 

 something analogous to the pressing of the spring in our 

 imagined model must occur. 



Sensation, then, is the preliminary to movement, and here we 

 mean by " sensation " only the physical and chemical events 

 that occur in the organs of sense and in the nerves and brain, 

 and not any affection of consciousness; when the latter occurs 

 we have to do with ferception, and this we consider later. Things 

 that happen in the environment, then, affect the organs of sense : 

 the movements of material bodies are known to us by vision, 

 for the light reflected from those things enters the eyeballs and 

 forms a picture on the retina, which is the essential organ of vision ; 

 vibrations in the atmosphere are set up by other material move- 

 ments outside ourselves, and these vibrations are received by 

 the auditory organs ; the particles of chemical substances floating 

 in the air are drawn into the nose or mouth, and so affect the 

 olfactory and gustatory organs; while contact of material sub- 

 stances with the skin similarly affects the nerves that terminate 

 there. These sensations we call sight, hearing, smell, taste, and 

 touch. 



Exteroceptive and Proprioceptive Sensation.— Changes that 

 occur outside the body thus affect the organs of sense, or, as we 

 shall call the latter, the receptors. The eyes and auditory organs 

 are therefore exteroceptive organs, or distance receptors, for 

 they are affected by events that may happen at considerable 



