534 PHYSIOLOGY 



comparison among them becomes impossible. Thus in the skin and 

 underlying parts we have, as a result of stimulation, sensations of 

 touch and pressure, sensations of heat and of cold, and sensations of 

 pain. The contact of certain dissolved substances with the end-organs 

 of the gustatory nerves excites in us a sensation of taste. Other sub- 

 stances diffused in the air and carried by it to the olfactory termina- 

 tions give us sensations of smell. Vibrations of a certain frequency, 

 transmitted by the air and by the auditory ossicles to the endings of 

 the auditory nerve, produce sensations of sound, while light falling 

 on the retina evokes visual sensations. 



Besides these sensations resulting from stimulation of the extero- 

 ceptive system of nerves, we are aware of the existence of a number of 

 organic sensations some derived from the viscera (enteroceptive). 

 others caused by stimulation of the proprioceptive system. As 

 examples of the latter we may mention the muscular sense, by which 

 we judge of the amount of tension exerted by a contracting muscle ; 

 the sense of position of the limbs ; and the sense of position of the head, 

 resulting from stimulation of the labyrinthine organ. 



How the physiological excitatory process in nerve fibres, with 

 its concomitant chemical and electrical phenomena, is able on arrival 

 at the brain to excite a conscious sensation we are unable to decide ; or 

 even to discuss, since we are dealing here with processes of two different 

 orders. We should not arrive any nearer to the solution of this 

 riddle if we were able to follow out the whole of the events occurring 

 in the body as the result of the application of any given stimulus to its 

 surface. We might under these circumstances be able to predict with 

 certainty the behaviour of any animal, if we knew its past history 

 and the comparative resistance of every path in its central nervous 

 system which might possibly be traversed by any given nerve impulse. 

 Such knowledge would be purely objective and could not be used to 

 explain the ' epi- phenomenon ' of consciousness. One might in 

 fact imagine a machine which would react like a living animal, but 

 would be perfectly devoid of self-consciousness, and we should be 

 unable in such a case to decide whether consciousness were or were 

 not present. Each one of us only knows consciousness as it exists in 

 himself. 



Before therefore we can employ our conscious sensations as a 

 means of throwing light on the conditions of action of the receptor 

 organs of the body, we must have some idea as to how far our sensa- 

 tions correspond to the stimuli, i.e. the physical events by which 

 they have been evoked. Appeal to our own experience shows the 

 existence of two kinds of difference between various sensations. 

 The greatest difference is found between those sensations which 

 are normally evoked by different sense-organs. Thus we are all 



