THE CEREBELLUM, MEDULLA AND MIDBRAIN 163 



the position of the parts of the Body as indicated by the muscle 

 sense, and the nature of the footing as shown by the sense of sight. 



We can safely conclude, then, that the cerebellum is an organ 

 through which are carried on complicated muscular reflexes. 

 The reflexes which it- performs are all continually subject to 

 volitional modification; they are of a sort which the cerebrum is 

 competent to carry on. We may view the cerebellum, therefore, 

 as an organ which by taking up complicated but not highly in- 

 tellectual tasks leaves the cerebrum free for higher forms of ac- 

 tivity. 



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 for- 

 gotten 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 'attention, 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 equilibrium sensations 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 association of 

 the two ; the path of connection between the afferent and efferent 

 fibers becomes easier the more it is traveled, and at last the af- 

 ferent 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 motor neurons in accord- 

 ance with the felt directing sensations, now has no more trouble 

 in the matter; the afferent impulses stimulate the proper motor 

 centers in an unconscious and unheeded way. Injury or disease 

 of the cerebellum produces great disturbances of locomotion 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, perhaps because the move- 

 ments require close mental attention and direction all the time. 



