THE RECEPTOR SYSTEM 207 



eludes 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. 



Not Included in this Classification are a group of feelings which 

 in consciousness have features in common with the senses, although 

 from the standpoint of physiology they seem not to fall in the same 

 category. Examples are fatigue, nausea, and the general state of 

 ill-feeling called malaise. While these are well-marked sensations 

 there is reason to doubt whether they are mediated by definite 

 receptors as are the senses. They are more probably induced by 

 general bodily states in some manner not now understood. 



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

 thirst 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 

 hold 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 them- 

 selves, or in the tendons or joints with which the muscles are con- 

 nected. 



