MOTOR SENSATIONS 471 



Likewise the thyroarytenoid muscle, whose finely graduated contractions de- 

 termine the pitch of the vocal tones, appears to possess a very delicate muscular 

 feeling produced by the afferent nerves; there is, however, no feeling of move- 

 ment connected with this. 



According to the view which Goldscheider in particular has worked out, 

 the most important nerves for the perception of passive movements are the 

 afferent nerves of the joints. The sensations of pressure and tension in the 

 soft parts of the limbs not only do not produce the sensation of movement, 

 they even interfere with it. Again, since the threshold value of the sensation 

 i. e., the smallest passive movement which one can perceive is not influ- 

 enced in any way by the degree of contraction of the muscles when the passive 

 movement begins, cooperation of the muscular sensibility as a factor appears 

 to be excluded. Finally the distance described by the moving force bears no 

 relation to the amount of sensation which one experiences; the determining 

 factor is the amount of rotation at the joint. 



Lewinski made some experiments on ataxic patients (cf. page 472) by mov- 

 ing their limbs very slowly and very slightly at the ankle, knee and hip joints, 

 part of the time pressing the parts together at the joint, part of the time not. 

 When the parts were thus pressed the patients always perceived the movement 

 very exactly, when not they could form no idea of the motion. 



The perception of active movements likewise results from the rotation of 

 the joints. To this are to be added, however, as contributing factors the 

 sensations connected with the tension of the tendons and their epiphyses, 

 possibly also the sensations aroused through the sensory nerves of the mus- 

 cles. These sensations concern not merely the tendons, etc., of the active 

 muscles, but also their antagonists. In a passive movement the tendons simply 

 follow the pull; some are stretched, others are only under the tonic resistance 

 of their own muscles. 



In active movements, especially if the joint to be moved is loaded, we 

 also experience sensations of weight and of resistance. The nerves of the 

 joints and of the tendons are again of the first importance, the pressure on 

 the surface of the joints and the tension of the tendons varying according to 

 the resistance or the weight. 



Jacobi has called attention to still another circumstance to which he ascribes 

 great importance in the determination of the size of a weight, namely, the com- 

 parison of the amount of nerve force employed with the latent period of the 

 movement i. e., the time which elapses between the act of volition and the incep- 

 tion of the movement. The latent period, in his opinion, depends upon the 

 amount of nerve force employed, and with the same amount of nerve force is 

 proportional to the size of the weight. 



The sensory nerves of the skin appear commonly to be of but slight im- 

 portance in any kind of motor sensations. The sensation of weight remains 

 unchanged after the skin is rendered insensitive to touch. And yet cutaneous 

 sensations appear to contribute something to the quantitative refinement of 

 a sensation of resistance as well as to the localization of the sensation and 

 so to the formation of a clearer total impression (Goldscheider and Blecher). 

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