GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 167 



is that while the latter are instinctive, born in us, the former are 

 not. 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 per- 

 formed 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 incoming and out- 

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

 sensory 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 accordance with 

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

 the sensory impulses stimulate the proper motor centers in an 

 unconscious and unheeded way. Injury or disease of the cerebel- 

 lum produces great disturbances of locomotion and insecurity in 

 maintaining various postures, as well as marked loss of endurance. 

 The functions normally performed by it are transferred to other 

 parts of the brain, and these, which are less fitted for the task, do it 

 less well and with more fatigue. 



Postural Reflexes. In a previous chapter (p. 126) the depend- 

 ence of posture on extensor tonus was described. This tonus is 

 maintained reflexly, and to the extent that it involves equilibrium, 

 as in the erect posture in man, is to be looked upon as belonging to 

 the class of cerebellar reflexes. The most striking fact about 

 postural tonus, perhaps, is the completeness with which it dis- 

 appears in the presence of any active movement which would 

 conflict with it. This is a striking example of the adaptive char- 

 acter of the nervous system. So long as one is maintaining any 



