138 ANIMAL SENSE ORGANS 



the sense is roused by insufficient moisture in that region. 

 This insufficiency of moisture may be due to the drying 

 out of the exposed surface, as when one sleeps with his 

 mouth open ; or may be due to a lowering of the percent- 

 age of water in the throat in consequence of loss of water 

 from the whole body. Thirst due to the first cause can 

 be allayed by swallowing saliva, but thirst caused by in- 

 sufficient water in the body can be relieved only by taking 

 in water from outside. It is interesting to note that 

 human beings may through accident or disease lose all 

 their other senses, but very few cases have ever been 

 observed in which the sense of thirst is lost. 



Pain. — There are organs for pain, both within the 

 body and on its surface, so that pain is in a manner both 

 an internal and an external sense. It is ordinarily classed 

 with the internal senses because in the experience of hu- 

 man beings it is recognized as a change in bodily state, 

 and not as an environmental change. For instance, if 

 the edge of a knife is pressed against the skin, we think 

 of the knife ; but as soon as it penetrates to the point of 

 stimulating the pain organs it is no longer the knife that 

 we think of but the body that is hurt. Pain is a warning 

 sense, it is aroused only under conditions which imply 

 harm. The adjustments to pain are obviously adjust- 

 ments whose purpose is the avoidance of injury. 



Muscle Sense. — This is a sense about which most of 

 us know very little, but which is of the very greatest im- 

 portance in the activities of mankind and animals. Its 

 organs are located in the muscles and about the joints 

 and they furnish information as to the degree of contrac- 

 tion of the muscles, and the position of the joints. This 

 information is important in that it is the basis of success- 

 ful locomotion. Any animal that moves from place to 

 place by muscular effort must contract its muscles in cor- 

 rect sequence and with the proper degree of force, with 

 reference to one another. Otherwise the activities which 

 constitute locomotion will not be properly coordinated. 



