THE MECHANISM OF CO-ORDINATED MOVEMENTS 391 



studied separately. If in the monkey all the afferent brachial roots 

 except the last cervical, which supplies cutaneous sensations to the 

 whole hand, be divided, the monkey uses the arm and hand both in 

 climbing and in taking food. A marked ataxy of the movement is, 

 however, observed. Whereas the normal monkey, in taking grains 

 of rice out of the observer's hand, exhibits perfect precision of 

 movement so that he rarely touches the hand on which the grains 

 are lying, the monkey with only cutaneous sensibility remaining 

 grasps clumsily with the whole hand, and the arm sways as it is 

 put out, often missing the object aimed at altogether. Cutaneous 

 insensibility of the hind limb causes very little disturbance of 

 locomotion, the alternate movements of which seem to be started 

 by the stretching of the structures at the front of the thigh. On the 

 other hand, a patient affected with such a loss may be the subject 

 of ' static ataxy,' i.e. he is unable to stand with his feet together and 

 his eyes shut. The afferent impressions from the skin of the feet 

 appear therefore to be necessary for the maintenance of static 

 equilibrium. 



In the carrying out of co-ordinated movements, such as those 

 of locomotion, the impressions from the muscles play a more im- 

 portant part. Division of the afferent nerves from the muscles gives 

 rise to a condition of tonelessness, and the passive mobility of the 

 joints is greater than usual, so that the hip with the limb extended 

 at the knee may be flexed to an abnormal extent. The effect of 

 this loss of tone is more apparent in the case of certain muscles. 

 The disturbance of co-ordination resulting from the cutting off of 

 afferent muscular impressions is well seen in cases of tabes dorsalis, 

 or locomotor ataxy, in man, and to a slighter extent in cases of 

 peripheral neuritis affecting chiefly the sensory nerves of muscles. The 

 ataxic gait of such patient is characteristic. There is no loss of 

 power in the muscles, but there is loss of control. The patient is 

 unaware of the position of his limbs and has to guide his walk by 

 visual impressions ; even then the movements are inco-ordinated. 

 The contraction of every muscle is exaggerated, so that in walking the 

 leg is first raised too high and then is brought down on to the ground 

 with a stamp. As the disease progresses the loss of control becomes 

 more and more pronounced, so that attempts to walk simply give 

 rise to a profusion of disordered movements, the legs being thrown in 

 all directions with the patient's efforts, but with no effective result. 

 The centres are no longer informed of the degree to which each muscle 

 is contracted, and the impressions are wanting which should cut short 

 the contraction of a muscle when it has attained its optimum, and 

 which should inhibit the antagonists during the contraction and induce 

 activity of the antagonists in successive alternation to those of the 



