482 PHYSIOLOGY CHAP. 



The solution of these is important to the general physiology of 

 the cerebellum and the functional localisation within it, since 

 they not only afford new evidence for the reinforcing action of the 

 cerebellum upon the cerebrospinal axis, but further throw light 

 on the mechanism by which the motor area of the brain gradually 

 becomes capable of compensating the effects of cerebellar deficiency. 

 The most important problems raised are : 



(a) What change occurs in the normal excitability of the 

 cerebral motor area of dogs that have previously .been deprived of 

 half or the whole of their cerebellum ? 



(b) Is electrical stimulation of the cerebellum capable of alter- 

 ing the threshold value of the motor area ? 



(c) Is there a definite functional relation between the cerebellar 

 lobules that have been electrically excited and the centres in the 

 central motor area, the excitability of which is affected ? 



We instituted experiments directed to solving the first 

 question, and published the results in our Monograph (1891). 

 In dogs, some months after the removal of half or of the 

 whole cerebellum, excitability was increased in both motor 

 areas of the cerebral cortex, both to electrical and to mechanical 

 stimulation. In two dogs in which one-half of the cerebellum 

 had been extirpated a year previously, both sigmoid gyri containing 

 the motor centres for the limbs were removed. During the 

 operation the mechanical excitation of these centres produced 

 intense and general reactions in both limbs, which were equal on 

 the two sides. In a third dog, which had lost the median and 

 right lateral lobe of the cerebellum fourteen months earlier, the 

 same results were obtained with faradisation of the two motor 

 areas. Cortical excitability was increased on both sides, and we 

 were unable, even with weak induced currents, to provoke move- 

 ments limited to one limb ; they were always diffuse and involved 

 either the two limbs of the opposite side or all four limbs. 



This increased excitability of the cerebral motor centres agrees 

 perfectly with our explanation of the compensation of cerebellar 

 deficiency, as due to an exaggerated functional activity initiated 

 by their greater excitability. 



In 1893 Russell obtained a diminution of excitability of the 

 motor area of the cerebral cortex in dogs and apes some weeks 

 after removal of the opposite half of the cerebellum. 



This result is a new argument in favour of our theory that the 

 cerebello-cerebral relations are principally crossed, so that the 

 reinforcing action which each half of the cerebellum exerts on the 

 cerebral motor centres mainly affects those of the opposite side. 

 On the other hand it can readily be understood that the ablation 

 of one-half of the cerebellum, by eliminating this reinforcement, 

 must in the early period which may last lor some weeks- 

 produce a diminution in the excitability of the motor centres of the 



