266 THE SPECIAL SENSES. 



latter suggestion is true it seems to be beyond doubt that adequately- 

 controlled voluntary movements depend for their adaptation upon 

 the inflow of sensory impulses along the fibers of muscle sense. 

 We have a certain consciousness of the condition of our muscles at 

 all times, and if we were deprived of this knowledge we should be 

 unable to control them properly, perhaps unable to use them 

 voluntarily. 



The Quality of the Muscle Sense. Our conscious realization 

 of muscular sensibility is not distinct. Under ordinary conditions 

 the untrained person is unaware of the presence of such a sense; 

 but physiological analysis enables us to realize its existence. What 

 we designate as the feeling of resistance and of weight depends 

 usually partly upon the pressure sense, but largely upon the muscle 

 sense. In estimating the difference in weight between two bodies 

 our judgment is much more exact if the bodies are lifted by muscular 

 effort, as is our custom, than if they are simply allowed to press 

 upon the skin; and in all calculations of resistance to effort it is 

 the amount of muscular contraction exerted that furnishes us with 

 the chief sensory basis for our judgments. So also in the judg- 

 ments of distance based upon visual impressions it is believed that 

 for close objects, particularly, the muscle sense connected with the 

 extrinsic and intrinsic musculature of the eyeballs plays a funda- 

 mental part. Doubtless also this sense takes an essential part 

 in the primitive formation of our conceptions of space, since it 

 may be assumed that the continual movements of the extremities 

 in connection with our visual and tactile impressions furnish essen- 

 tial data upon which we build our perceptions of distance and 

 size, our judgments of spatial relations. As is explained in the 

 chapter on the physiology of the ear, the sensations from the semi- 

 circular canals and vestibular sacs co-operate in giving data for 

 these fundamental conceptions, and it is not possible for us to 

 disentangle the parts taken by these senses separately in building 

 up our knowledge of the external world. The muscle sense is 

 reckoned usually among the internal (or common) senses, that is, 

 those which are projected to the interior of the body and are felt 

 as changes in ourselves. A little reflection, however, demonstrates 

 that, like the temperature sense, it may under conditions be pro- 

 jected to the exterior and be interpreted as a quality of external 

 objects. Weight and resistance, for example, are attributed to 

 the objects giving rise to the feeling of muscular effort, and it may 

 be said, perhaps, that, as in the case of temperature, the feeling 

 is projected more or less clearly to the exterior when it is combined 

 with the pressure sense which acts as the predominating or guiding 

 factor in the projection. In excessive muscular effort the quality 

 of the muscle sensation undergoes a change and becomes strong 



