SENSATIONS OF MOVEMENT AND POSITION 599 



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, 

 individual 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 gusta- 

 tory impressions from the tongue have a very full representation in conscious- 

 ness. 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 move- 

 ment. 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 in- 

 tegrity of a special sense-organ contained in the labyrinth of the internal 

 ear. It is therefore sometimes spoken of as the labyrinthine sense. 



THE SENSE OF RELATIVE POSITION, INCLUDING THE 

 MUSCULAR SENSE 



Without using our eyes we are able at any moment to tell the position of 

 our limbs. If one arm be moved passively into any position we can without 

 difficulty move the other arm into an exactly similar position. We thus 

 know the extent to which we move the limb and the static position attained 

 as the result of the movement. If the movement is resisted, we are able to 



