670 PHYSIOLOGY 



Consciousness we have seen to be developed in proportion to the 

 differentiation of the educatable association centres, which are 

 responsible for our powers of ideation, and by means of which the 

 different reflex movements which we call volitional are carried out, 

 guided, augmented, or inhibited, according to the past experience 

 of the individual. Volitional movement is therefore a movement 

 determined by previous neural events, of which a part at any rate is 

 represented in consciousness as feeling, emotion, or desire. Where an 

 act is involuntary, i.e. does not need the guidance of experience, indi- 

 vidual or racial, for its performance, the afferent impulses which arouse 

 it are also, as a rule, devoid of representation in consciousness. Thus 

 we have no sensation of the passage of a bolus along the oesophagus. 

 The proprioceptive impulses also only affect consciousness where 

 they are necessary for the guidance of volitional movement. The 

 tactile and gustatory impressions from the tongue have a very full 

 representation in consciousness. Volition, however, only interferes 

 for the rejection or acceptance of the food taken into the mouth, 

 and is not required for the minute direction of the movements of 

 mastication and deglutition. The muscular sensibility of the tongue, 

 and therefore our voluntary control of its movement, is extremely 

 slight, although there must be a continual flow of afferent impressions 

 from the tongue to the lingual motor centres to guide the complex 

 movements both of mastication and deglutition. In the case of the 

 palate muscles, as of the oesophagus, muscular sensibility is entirely 

 wanting. 



It has been suggested that afferent impressions from the muscles 

 can play only a subordinate part in our sensations of movement, since 

 we are not aware of the part taken by each individual muscle in any 

 given movement. Such a statement is absurd. We have no objective 

 phenomenal experience of our muscles. All that we are aware of 

 and can judge of by our other senses is the movement as a whole, 

 and our sensation of movement is therefore referred to the whole 

 movement and not to the individual muscles. 



The sensations arising in the proprioceptive system can be 

 divided into two main classes : 



(1) The sensation of the relative positions of parts of the body. 



(2) The sensations which inform us of the position of the head, 

 with regard to its surroundings, i.e. with regard to the direction 

 of the pull of gravity. (It must be remembered that ' downwards ' 

 always means towards the centre of the earth, ' upwards ' away 

 from the centre of the earth, i.e. against the gravitational forces.) 

 This orientation sense depends on the integrity of a special sense- 

 organ contained in the labyrinth of the internal ear. It is therefore 

 sometimes spoken of as the labyrinthine sense. 



