270 THE SPECIAL SENSES. 



and located. This deep sensibility to touch or pressure must 

 be mediated through the sensory nerves of the muscles, and it 

 constitutes one form of the conscious reaction of the nerve fibers 

 of 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 

 enough to make a distinct and peculiar impression upon our con- 

 sciousness. We designate this feeling as fatigue, but there is no 

 question apparently that this sensation is mediated through the 

 same nerve fibers that ordinarily give us our muscular sensibility. 



Sensations of Hunger and Thirst. Hunger and thirst are 

 typical interior (or common) sensations. We feel them as changes 

 in ourselves. Neither sense has been the direct object of much 

 experimental investigation, and what knowledge we possess is there- 

 fore derived largely from accidental or pathological sources. Hunger 

 in its mild form is designated as appetite. It occurs normally at a 



