172 THE HUMAN BODY 



body; the second includes the projecting senses which tell us of the 

 surroundings not immediately touching us. 



The group of contact senses includes the cutaneous senses, touch, 

 temperature, and pain, the latter being both external and internal, 

 and the sense of taste. The group of projecting senses includes 

 hearing, smell, and sight. 



It is not desirable to follow this classification exactly in the 

 discussion of the various senses, but it represents in the main the 

 order of their consideration. 



The Internal Senses. Of these only muscle sense, hunger, thirst, 

 and fatigue will be considered here. The sense of pain is treated 

 more satisfactorily in connection with the cutaneous senses. The 

 equilibrium sense requires an account of the structure of the ear 

 and will be given in connection with the sense of hearing. The 

 functions of these senses are to inform the body of its own con- 

 dition. They are recognized in consciousness as bodily states, 

 being in this respect very different from the external senses, which 

 we interpret altogether in terms of the sources from which the 

 stimuli arise. The difference in consciousness between internal and 

 external senses may be illustrated by supposing that a knife is 

 held in the hand. The sensations we have are referred in our 

 consciousness to the knife. It is hard, cold, etc. Let the knife now 

 cut through the skin. The stimulus arises from the knife as much 

 as before, but it is to the hand and not to the knife that we refer 

 the feeling of pain. 



The Muscular Sense. From the muscles arise sensations of great 

 importance, although they do not often become so obtrusive in 

 consciousness as to arouse separate attention. They are due to the 

 excitation of sensory nerves ending within the muscles themselves, 

 or in the tendons or joints with which the muscles are connected. 



We have at any moment a fairly accurate knowledge of the 

 position of various parts of our Bodies, even when we do not see 

 them; and we can also judge fairly accurately the extent of a 

 movement made with the eyes shut. The afferent nerve impulses 

 concerned in the development of such judgments may be various; 

 different parts of the skin are pressed or creased; different joints 

 are subjected to pressure; different tendons are put on the stretch 

 and different muscles are in different states of contraction, and it 

 is by no means easy to determine the part played in each case by 



