CENTRAL CONTROL OF POSTURAL REACTIONS 927 



is held upright, for example, the wrist is flexed under the weight of the 

 hand. If the joints are passively bent, their movement is not checked 

 by the action of the muscles, but continues until the articulations cause 

 them to lock. The characters which the disturbance in tone imposes on 

 the knee jerk have been described on page 921. 



Asthenia expresses the fact that the muscles of the injured side are 

 weaker than on the normal side. Not only are the patients conscious 

 that these muscles feel weaker, but measurement with a dynamometer 

 shows that they can exert in some cases only 50 per cent of the force 

 of which the normal muscles are capable. The limbs in which the mus- 

 cles are asthenic are also unusually subject to fatigue. It should be 

 emphasized that the muscles which have been deprived of a cerebellar 

 influence are never paralysed: the asthenia is not a failure of function, 

 but rather an expression of inefficiency in the act of voluntary contraction. 

 Closely associated with the asthenia is the discontinuity or irregu- 

 larity of a maintained muscular contraction known as astasia. The af- 

 fected muscles frequently tend to give way under the load they bear, so 

 that objects held in the hand are liable to be dropped, or the leg on the 

 injured side may suddenly collapse under the body's weight and thus 

 cause the patient to fall as he attempts to walk. A tremor may occur 

 in maintaining an attitude, if it requires the exertion of some force, par- 

 ticularly as the muscles begin to tire. It is a less prominent symptom in 

 man, however, than in animals. The destruction of the cerebellum seems 

 to disturb the mechanism by which the individual twitches of the con- 

 tracting muscle become fused so as to maintain a constant tension. We 

 have pointed out on page 917 that the continuity of muscular movement 

 is facilitated by the reinforcement of the primary response by an adjust- 

 ment in the postural tone of the muscle, and it would seem that this is 

 the mechanism whose disturbance gives rise to the astasia of cerebellar 

 injuries. 



Under ataxia may be grouped those symptoms which are due to ir- 

 regularities in the voluntary movements of the body. The limbs of the 

 affected side are ill directed when an attempt is made to perform some 

 precise movement. The finger may strike the eye, when its true objec- 

 tive is the mouth and consequently the patient fears to hold his cigarette 

 in the affected hand while smoking. It is impossible to stop the move- 

 ment at the right point, with the result that in trying to reach some ob- 

 ject, the hand strikes it forcibly or else falls short of its objective. When 

 voluntary movements are made they are interrupted in their progress 

 by a tremor, particularly as they approach their objective, and greater 

 need for accuracy becomes necessary. This jerky character of the mo- 

 tion is obviously closely associated with the static tremor described as 

 astasia. Certain abnormalities in movement indicate that difficulty ex- 



