140 ANIMAL SENSE ORGANS 



which they are entirely absent is the front of the eyeball, 

 which has the sense of pain but no sense of touch. The 

 total number of touch organs in man probably mounts 

 into the millions. Touch is obviously a basis for in- 

 formation in connection with nearly all kinds of animal 

 activity. The suitability of objects for food, or their 

 possibility as sources of injury are determined largely 

 through this sense. 



The Temperature Sense is the means by which an 

 anunal perceives differences of temperature between its 

 skin and objects with which it is in contact. From the 

 standpoint of the organs involved, there are really two 

 temperature senses, since we have a set of organs for the 

 perception of warmth and another for the perception of 

 cold. The temperature sense is undoubtedly present in 

 all warm-blooded animals, viz., the mammals and birds; 

 whether it exists in the cold-blooded animals is uncertain. 



The significance of the temperature sense is obviously 

 to stimulate the animal to adjust itself to the most favor- 

 able possible temperature. The stimulus of cold leads 

 to activities which either will make the animal warm by 

 reason of increasing the amount of functional metab- 

 olism, and so the production of heat (see Chap. VIII), 

 or will bring the animal into an environment in which it 

 is protected from the cold. Warmth, on the other hand, 

 influences the animal to remain comparatively quiet, and, 

 if extreme, to seek a cool spot. 



The Chemical Senses are the means whereby animals 

 are affected through chemical substances in the environ- 

 ment. Only substances dissolved in water can act as 

 stimuli to protoplasm, and so the chemical sense reacts 

 only to flissolvefl material. The sense is limited in scope 

 in that not all dissolved substances affect it, although the 

 number of substances which do not arouse the sense is 

 small indeed in comparison with the enormous number 

 by which the sense is aroused. 



In aquatic animals the organ for the chemical sense 



