PAET IV 



THE EXTEROCEPTIVE SYSTEM 



The following description of the higher centres is based 

 upon Sherrington's division of the sensations into three 

 main classes — exteroceptive, proprioceptive and interoceptive. 

 We shall first consider the exteroceptive sensations — that 

 is, those arising from changes in the outside world — and 

 the manner in which the animal reacts to them. Secondly, 

 we shall consider the proprioceptive sensations, or those 

 which give impressions of bodily position, and the reactions 

 which they induce. Finally, we shall deal with the intero- 

 ceptive system, which relates to the gut and the structures 

 derived from it. 



The exteroceptive sensations are those changes in its 

 surroundings to which the animal responds, which rise 

 into its consciousness, and to which, if it is in a normal 

 condition, it pays attention. They may be classified as 

 follows : — ■ 



1. Those due to direct contact of a body with the skin 

 {cutaneous and deep sensation). 



2. Light. 



3. Sound-waves. 



4. Chemical stimuli produced by vapours of substances 

 situated at a distance {smell), and by substances actually 

 in contact with the mouth {taste). 



These disturbances are appreciated because they stimulate 

 certain nerves specially adapted to receive them. Each 

 kind of sensory nerve conveys to the brain only one kind 



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