346 PHYSIOLOGY 



section of the corresponding branches of both fifth nerves, and was interpreted 

 by him as indicating a possible motor function for these nerves. 



In these phenomena of sensory paralysis we are dealing with the effects 

 produced by the deprivation of two distinct classes of afferent impressions, 

 viz. those from the skin and those from the deep structures and muscles. 

 The phenomena due to these two factors may be 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 move- 

 ment 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 loco- 

 motion, the impressions from the muscles play a more important 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 abnor- 

 mal 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 con- 

 tracted, and the impressions are wanting which should cut short the con- 

 traction 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 other muscles. In such 

 a patient therefore walking finally becomes impossible and, with well- 



