viii THE HIND-BRAIN 447 



walk, and tin- cerebellar ataxy is pronounced, the animal does not 

 always correct the abnormal positions in which its limbs are 

 placed, and when it does there is a certain delay in the limbs of 

 the side operated on, as compared with the normal side. Ducceschi 

 and Sergi drew attention to the fact that during this period the 

 dog with half a cerebellum in many cases does not correct the 

 abnormal postures given to the limbs of the operated side, and 

 sometimes, though more rarely, not even those of the limbs on the 

 healthy side, in which there is no reason to suspect any disturb- 

 ance of the muscular sense. 



If, lastly, the muscle sense is investigated during the long 

 period in which the cerebellar ataxy has become stationary and 

 permanent, anomalous positions of the limbs of the operated, as 

 well as of the sound, side are corrected as in normal dogs. 



These facts show that absence of the cerebellum is com- 

 patible with integrity of the muscle sense. It is evident that the 

 frequent failure to react in the early stage and afterwards has no 

 value as evidence of sensory disturbance ; in this kind of research 

 the maxim that one well-estal dished positive proof is worth more 

 than any number of negative proofs holds good. 



If the behaviour of a dog in which the cortex of one side of 

 the so-called sensory -motor area (sigmoid gyrus) has been removed 

 is compared with that of the dog with only half a cerebellum, 

 the conclusion that the muscular sense is seriously disturbed in 

 the former and has not perceptibly suffered in the latter is 

 inevitable. In both the defect phenomena disappear in time, but 

 in the former the failure to correct the abnormal postures of the 

 limbs persists for months, while in the latter it disappears entirely 

 as soon as the animal has acquired the power of walking, although 

 extreme ataxia persists. 



But the most cogent proof of the integrity of the muscular 

 sense in decerebellated dogs is the retention of power, when the 

 animal lies at rest, of scratching the skin of the abdomen, thorax, 

 and neck with one or both hind-feet, with perfect adaptation to 

 the purpose of removing disagreeable stimuli. This is such a common 

 occurrence that it may altogether escape the careless observer. 

 But this action, on the one hand, necessitates integrity of cutaneous 

 sensibility, and on the other capacity for rightly exciting, directing, 

 measuring, and therefore being aware of muscular contractions in 

 a word, integrity of the muscle sense. 



V. A critical analysis of the ataxia due to unilateral lesions of the 

 cerebellum will greatly facilitate our task of analysing the second 

 typical form of cerebellar ataxy that which results from bilateral 

 lesions. Speaking generally, the absence of the whole cerebellum 

 produces the same symptoms as the loss of one-half, only they 

 affect both sides, and do not predominate in one alone. 



This spread of the defect phenomena to both sides produces a 



