i CUTANEOUS SENSIBILITY 11 



mean liy the term " sensation " the simplest and indivisible state of 

 consciousness, l>y which \ve appreciate any alteration, c.tj. light, 

 colour, a sound, a taste, etc., without associating with it any 

 relation tu internal or external causes. 1'urc and simple sensations, 

 as such, exist only in the new-liorn, in whom the sensory centres 

 are incompletely developed. In adults, sensations are converted 

 by a psychical process into perceptions, which are a complex of 

 co-ordinated elementary sensations, by which we not only perceive 

 the changes in our state of consciousness but are able to interpret 

 and to objectify them. A simple tactile sensation, for instance, 

 is inevitably connected with an external body coming into contact 

 with the skin; a sensation of bitterness with the presence of 

 something in the mouth; a sensation of sound or colour with the 

 presence of a sounding or a coloured body in the outer world, and 

 more or less remote from us. Each of our sensory perceptions, 

 though composed of a complex of elementary sensations which are 

 more or less distinct from each other, nevertheless presents itself 

 as a kind of unit in our consciousness. In the physiology of the 

 senses it is often no easy task to distinguish in apparently simple 

 sense-perceptions the elementary sensations of which they are 

 composed. 



The objectifying of perceptions, by which we refer the changes 

 in our senses to external causes acting on them, is a fundamental 

 characteristic common to all perception. The tendency to project 

 our perceptions externally varies in the different senses. It is 

 strongest in the higher senses of vision and hearing. Common 

 visual and auditory perceptions appear unmistakably as properties 

 attaching to external objects, more or less remote from us, apart 

 from any appreciable sensation of change in our visual or auditory 

 organs. The perceptions of the lower senses, touch, temperature, 

 taste, and smell, have less tendency to projection. Tactile per- 

 ceptions are, as a rule, projected to the place where the object that 

 excites the cutaneous sense-organ is situated, and we are clearly 

 able to distinguish the sensation of the external object that comes 

 into contact with the skin from the change in the sensory surface. 

 In the sensation produced by a warm body we may be uncertain 

 whether we feel the heat of our skin or of the external object. 

 So too in sensations of taste or smell, it is doubtful whether we 

 arc most aware of the changes in the tongue and nasal mucous 

 membrane, or of the presence of the sapid or odorous substance. 



More important, however, than the greater or less degree to 

 which normal sensory perceptions are projected beyond us, or to 

 the peripheral sense-organs, is the fact that both subjective and 

 hallucinatory perceptions, and also the effects of experimental or 

 pathological stimulation of the sensory nerve-trunks, are pro- 

 jected externally : we refer them not to the place at which they 

 are really excited, but to that to which we are accustomed to 



