444 PHYSIOLOGY CHAP. 



sta'nt trembling of the head, which in this posture is the only 

 unsupported part of the body, its position being maintained by the 

 active contraction of the muscles of the neck. When the animal 

 stands it can be seen that the tremor is not limited to the head, 

 but involves the whole body, which oscillates slightly either in the 

 transverse, oblique, or diagonal direction. When it moves slowly 

 this tremor is exaggerated ; the movements of the limbs on the 

 operated side and of the vertebral column show a characteristic 

 defect in continuity and stability, owing to the intermittent nature 

 of the contractions, as though the summation of single impulses 

 were imperfect. This defective co-ordination and unsteadiness is 

 known to clinicians as titubation, since it gives the impression 

 that the patient hesitates to decide, or has difficulty in transmitting 

 the voluntary impulse to the muscles. 



This titubation, however, disappears when the animal spon- 

 taneously, or compulsorily, accelerates its gait. No signs of 

 ataxy are then perceptible other than those which depend on 

 hemiasthenia and hemiatonia, and on the abnormal compensatory 

 acts by which the animal endeavours to escape the effects of these. 

 This proves that the tremulousness does not depend on delay in 

 the development of the voluntary impulses, or on difficulty of 

 transmitting them to the muscles ; but solely on the incomplete 

 summation of the single impulses, owing to which the movements 

 become slightly tremulous. 



On the other hand, the tremor increases and assumes the 

 character of marked rhythmical oscillations when the animal eats 

 some favourite food. There are also true pendulum movements of 

 the head in the diagonal direction, due to the alternate functional 

 predominance of its depressor and levator muscles, which are 

 partially transmitted over the whole trunk. The animal is unable 

 to check or arrest them, so that its nose may hit the bottom of 

 the dish or the floor on which the food is placed. 



To this group of phenomena, which includes tremor, titubation, 

 and rhythmical oscillating movements, we gave the name of astasia 

 for the sake of brevity and owing to their probable common 

 origin. 



The ataxy in apes deprived of half their cerebellum is 

 fundamentally identical in its main features. Generally speaking, 

 compensation sets in more rapidly and in a very varied form in these 

 animals. We have already seen that monkeys can overcome the 

 effects of vertigo soon after the operation. On the disappearance 

 of the dynamic disturbances they are almost always able to avoid 

 falling to the affected side ; in walking the limbs of this side are 

 strongly abducted ; in sitting upright they support themselves by 

 placing one or both hands to the ground or by holding on to the 

 leg of a table. They can also avoid the swaying of the head and 



