472 ORGANIC SEiNSATIONS 



In the case of the face muscles and the levator palpebrce superioris distinct 

 sensations which inform us of the displacements suffered by the soft parts 

 accompany the contractions (Goldscheider). 



The sensations which make us aware of the position of the extremities 

 have their origin in the skin, tendons, and probably the joints. By combina- 

 tion with optical memory pictures they give us our idea of position. Sensi- 

 bility of the muscles appears to have little to do with the perception of 

 position in the case of the extremities, but in the case of the eye muscles it 

 plays a very important part. 



2. PHYSIOLOGICAL SIGNIFICANCE OF THE MOTOR SENSATIONS 



Taken in their broadest sense, the motor sensations are of very great im- 

 portance for the regulation of all bodily movements. Whenever any part 

 of the body suffers loss of the motor sensations or a decline in their intensity 

 and fineness, that part of the body exhibits disturbances in the coordination 

 of its movements. In this way are brought about those pathological symptoms 

 which are described as ataxia and which are denned in brief as a disturbance 

 in the harmonious and purposeful cooperation of the muscles. 



One of the most frequent forms of ataxia arising from lesion of afferent 

 pathways is locomotor ataxia occurring in tabes dorsalis. It is characterized by 

 the peculiar way in which the legs are swung and the feet planted in walk- 

 ing. Instead of the slightly flexed position of the normal leg as it is swung 

 forward, the knee is extended, sometimes excessively, and the leg is thrust for- 

 ward, the heel being planted on the ground with a sudden stamp. At the same 

 time the legs are kept far apart, the trunk sways back and forth, and the body 

 is in momentary danger of losing its equilibrium. 



Coordination of the muscles being necessary to hold the body erect no less 

 than to carry out movements of the limbs, ataxia is at times noticeable in stand- 

 ing. Thus ataxic persons are inclined to place the legs far apart in order to 

 increase the area of support. If the feet are placed close together the body sways, 

 or may fall, especially if the eyes be at the same time closed so as to shut out 

 control by visual impressions (Leyden and Goldscheider). 



There are many clinical observations to support this dependence of exact 

 movements upon different impulses, and they are confirmed in the most beau- 

 tiful way by experiments on animals. 



Thus, after section of the afferent roots which supply one hind leg, a dog 

 is unable to run on the ataxic leg when the sound leg is tied up (Hering, Jr.). 

 When the afferent roots to both hind legs are cut, a dog is utterly unable to 

 walk, and can only pull himself along on the abdomen by means of the fore legs, 

 the hind parts dragging. Gradually, however, the dog can learn to walk again, 

 and at the end of three to four weeks but few signs of the original disturbance 

 are left (Bickel). There is therefore a means of compensating the loss of these 

 afferent impulses. 



J. R. Ewald observed that in cases of this kind the animal could call into 

 play certain aids not previously used for regulation of his movements, and in 

 fact Bickel observed that a dog which has recovered the use of his legs after 

 an operation, exhibits again the original symptoms when he is taken into a dark 



