FUNCTIONS OF CEREBELLUM. 587 



of the corpora quadrigemina in man are uncertain. The 

 cerebellum is the chief organ of combined muscular move- 

 ments; it is the main seat of what we may call acquired 

 reflexes. Every one has to learn to stand, walk, run, and 

 so on; at first all are difficult, but after a time become easy 

 and are performed unconsciously. In standing or walking 

 very many muscles are concerned, and if the mind had all 

 the time to look directly after them we could do nothing 

 else at the same time; we have forgotten how we learnt to 

 walk, but in acquiring a new mode of progression in later 

 years, as skating, we find that at first it needs all our atten- 

 tion, but when once learnt we have only to start the series of 

 movements and they are almost unconsciously carried on for 

 us. At first we had to learn to contract certain muscle 

 groups when we got particular sensations, either tactile, 

 from the soles, or muscular, 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 becomes the associ- 

 tion of the two; the path of connection between the 

 afferent and efferent fibres becomes easier the more it is 

 traveled, and at last the sensations arouse the proper move- 

 ment 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 disease of the cerebellum produces great disturbances 

 of locomotion and insecurity in maintaining various pos- 

 tures. 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. The semicircular canals 

 have probably nothing to do with hearing. An old view was 

 that, lying in three planes at right angles to one another, 



