DISCOVERY 



161 



If the entire cerebellum has been removed, the animal 

 cannot maintain its balance either in a state of rest or 

 of motion ; and if one-half of the organ has been re- 

 moved, the animal has tremor of the muscles on that 

 side and tends to fall towards the side of the defect. 



In man. injury to the cerebellum, according to its 

 gravity, leads to more or less extensive inco-ordination 

 of muscles, or ataxia, especially' in the legs, so that the 

 equilibrium of the body becomes unstable, whether the 

 body be at rest or in motion. His muscles are not 

 jiaralysed. and he can move them voluntarily as 

 before ; but if the cerebellar disease be at all serious, 

 he walks with a staggering gait like a drunken man 

 and strives to keep his legs far apart because this 

 widens his base of support. This, of course, he does 

 consciously ; his straddling gait is the conscious com- 

 pensation for the results of the subconscious irregu- 

 larity in the cerebellar mechanism for the maintenance 

 of the equilibrium of the body. The gait in cerebellar 

 ataxia is characteristic, the head lags behind the torso 

 and the torso behind the legs. Although, then, the 

 cerebellum is not the seat of awareness of bodily states, 

 and not the organ which originates bodily actions, yet 

 it seems to be an important organ in the co-ordination 

 of muscles which steady the body whether at rest or 

 in motion (static or dynamic equilibrium). 



The cerebellum sends no motor impulses to muscles 

 or to any other " effector " organs, as far as is known. 

 Efferent tracts do, however, leave it, not for the muscles 

 directly-, but for them indirectly via the cerebrum. 

 The cerebrum controls the muscles, decides whether 

 they shall merely leave the body at rest or balance it 

 as it moves about ; but all the time the cerebellum, in 

 its turn, apparently controls or influences the cerebrum. 

 For instance, it has been noticed that when the cere- 

 bellum has been injured, the cerebrum is apt to put 

 forth either too little or too much nerve-energy, and so 

 produce ataxia by deficiency (adynamia), or ataxia 

 by excess (over-action, over-compensation). 



The study of the behaviour of a large intelligent 

 animal which has suffered the loss of one-half of its 

 cerebellum is instructive. Let us suppose that an 

 adult dog has had its left cerebellar hemisphere re- 

 moved ; for the first few days it cannot stand properly, 

 but falls over on the left side. The eyes are not at rest, 

 but oscillating ; the muscles of the left side are tre- 

 mulous and rather weaker than usual, though by no 

 means paralysed. By degrees the dog gets better ; 

 the tremors are less severe, and finally he learns that 

 he can prevent himself falling over if he walks towards 

 the wall and leans up against it. This is intelligent 

 compensation for the defect of impaired maintenance 

 of equilibrium ; it is possible only because the animal's 

 cerebrum is still intact. 



Again, a dog with only half of .the cerebellum intact, 



if thrown into the water, will swim more or less like a 

 normal animal. It is noticed, however, that the dog 

 does not go in a straight line, but in a curve which 

 bends towards the sound side. The reason for this is 

 interesting: dogs swim not like men, but by "grab- 

 bing " the water with their fore-paws. The muscles 

 on the sound side, having more tone than those on the 

 other, allow the paws to catch hold of the water better, 

 with the result that if it is the left cerebellar half that is 

 gone, the dog swims away towards his own right. 



Muscles in human cerebellar disease are not paralj'sed, 

 but they are tremulous and to greater or less extent 

 show diminished tone and force (adynamia). The 

 same facts may be brought out by another device : if 

 the dog above referred to be now made to walk on the 

 hard ground, he is seen to be deviating towards his 

 own left. This is because the right-sided muscles, 

 ha\ing the better tone, get a better push-off from the 

 ground than do the opposite ones, and so in time 

 cause the animal to walk away from the straight line. 



A curious thing was noticed about the dog made to 

 swim. After a certain number of trials it was learning 

 to swim straight. The dog found that it could steer 

 itself by means of its tail. It used its tail as a rudder, 

 curved in this case towards the left in order to bring 

 its body over to the side from which it was drifting. 

 A dog, one of whose cerebral hemispheres has been 

 removed, cannot do this. 



But the cerebellum receives impulses from regions 

 other than the periphery ; it receives them also from 

 the cerebrum. WTien the cerebrum is sending down 

 nerve-energy to the muscles to maintain their tone or 

 put them into activity, it sends simultaneously some 

 energy over to the opposite half of the cerebellum. 



The meaning of these various paths into and out of 

 the cerebellum is probably somewhat as follows : 



The cerebellum is informed of the state of tone or of 

 contraction of the entire musculature of the body at 

 any given instant ; if, for any reason, it is necessary to 

 increase that tone, the cerebrum has to emit fresh im- 

 pulses, but just how intensely it must do so will depend 

 on the amount of innervation which is sent over to it 

 from the cerebellum (augmentor function) ; if, however, 

 from any cause the cerebrum has been innervating the 

 muscles too intensely, then it receives impulses from 

 the cerebellum to restrain the intensity of its energy 

 (inhibitory function). 



The cerebellum is, as regards the cerebrum, a kind of 

 " centrifugal governor " : if the cerebrum is not doing 

 enough in the way of innervating muscles, the cere- 

 bellum intensifies its activity ; if it is doing too much, it 

 cuts itdown, and all this below the level of consciousness. 



The cerebellum is, then, not so much an organ for 

 maintaining equilibrium as an organ corrective or 

 regulative of the intensity of central motorial innerva- 



