728 CUTANEOUS SENSATIONS [CH. LII. 



The Muscular or Kinsestlietic Sense. 



By the muscular sense we become aware that movement is taking 

 place in some part of the body. We are especially conscious of willed 

 muscular action, and the muscular sense has thus been confused and 

 identified with the "feeling of innervation," or "sense of effort," 

 which accompanies volitional movements. By some this feeling has 

 been attributed to a direct discharge from the motor to the sensory 

 cells of the cerebral cortex occurring at the very birth of the effer- 

 ent impulse. But most physiologists of the present day regard the 

 sense of effort as due to afferent impulses peripherally generated by 

 the accompanying respiratory and other strains ; and they no longer 

 consider it as a very important factor in effecting or estimating move- 

 ment. It is in the estimation of weights that the value of peripheral 

 sensations to the muscular sense can be most clearly seen. 



When a weight is first handled, the amount of force necessary to 

 lift it is estimated in the light of past experience. As it is being 

 lifted, sensations from the moving limb guide the expenditure of force : 

 a weight which flies up too fast or does not move, at once calls for less 

 or more muscular force. Similarly the muscular sense is invoked 

 when we estimate the extent to which we have moved our limbs, or 

 to which they have been passively moved by others. 



These guiding sensations are not merely of cutaneous origin. 

 Persons whose skin has been rendered insensitive by cocaine, or by 

 certain diseases, yet retain the power of estimating weights and the 

 extent of their movement. In locomotor ataxy the muscular sense 

 may be destroyed while the skin retains its usual sensitiveness to 

 touch. On the other hand, we must remember that it is not at all 

 certain that the muscles are solely or even predominately the seat of 

 these peripheral sensations, and that therefore " kinsesthetic sense" 

 is a term preferable to "muscular sense." It is true that sensory 

 end-organs and nerve-fibres occur in muscles and tendons, which pre- 

 sumably transmit impulses upon change of muscular form or of 

 tendinous strain. But we have experimental evidence that the 

 pressure and movement of joint-surfaces are important factors in the 

 development of kinaesthetic sensations. The "muscular sense" is 

 thus of very complex origin. 



