614 THE HUMAN BODY. 



from the general position of the limbs, or visual, or others 

 (equilibrium sensations, see below) from the semicircular 

 canals. But the oftener a given group of sensations has been 

 followed by a given muscular contraction the more close be- 

 comes the association of the two ; the path of connection be- 

 tween the aiferent and efferent fibres becomes easier the more 

 it is travelled, and at last the afferent impulses arouse the 

 proper movement without volitional interference at all, and 

 while hardly exciting any consciousness; we can then walk or 

 skate without thinking about it. The will, which had at first 

 to excite the proper muscular nerve-centres in accordance with 

 the felt directing sensations, now has no more trouble in the 

 matter; the afferent impulses stimulate the proper motor 

 centres in an unconscious and unheeded way. Injury or dis- 

 ease of the cerebellum produces great disturbances of locomo- 

 tion and insecurity in maintaining various postures. After a 

 time the animals (birds, which bear the operation best) can 

 walk again, and fly, but they soon become fatigued, probably 

 because the movements require close mental attention and 

 direction all the time. 



Sensations of Equilibrium. In order to make proper 

 movements of balancing or locomotion we need a knowledge 

 of the space relations of the Body to its surroundings. When 

 eyes, muscles, and skin send in concordant afferent impulses, 

 movements are precise ; if sensations of any one of these groups 

 are wanting (excluding blind persons who have learned to do 

 without some of them) or abnormal the whole mechanism is 

 thrown out of gear. Persons who have lost muscular or tactile 

 sensibility stand 'and walk with difficulty; those who have 

 nystagmus (jerking unconscious movements of the eyeballs 

 which cause the visual field to seem to move in space) do the 

 same and feel giddy ; and, if a person be rapidly rotated with 

 his eyes open he soon becomes giddy ; the succession of retinal 

 images suggests that he is moving in space, but the muscular 

 and tactile afferent impulses are in conflict with that ; and 

 though this discordance hardly comes into direct consciousness 

 as a definite contradiction between sensations, the want of 

 harmony in the afferent impulses throws co-ordinating motor 

 mechanisms out of gear, with resulting uncertainty in loco- 

 motion. An important group of afferent impulses concerned 

 with the maintenance of bodily equilibrium in addition to 

 those above referred to is probably derived through the semi- 



