142 ANIMAL SENSE ORGANS 



is breathed in through the nose. In the higher animals, 

 then, we find one organ located in the mouth and the 

 other in the nose ; the former is called the organ of taste, 

 the latter the organ of smell. The taste organ is affected 

 only by materials which are actually present in the 

 mouth. Contrary to conmion belief it has a very limited 

 range of perception, there being only four true tastes, 

 namely, sweet, sour, salty and bitter. Flavor, which is 

 usually confused with taste, is really part of the sense of 

 smell (see below). 



The significance of taste is obviously to aid in forming 

 a final judgment as to the desirability of food that has 

 been captured or grasped, before swallowing it. It is 

 widely but by no means invariably true, that materials 

 which are of agreeable taste are fit for food, while ma- 

 terials that do not taste good are unfit for food. 



The Distance Receptors. — Under this head we in- 

 clude the three senses of smell, hearing, and sight. The 

 advantage which they confer on the organism possessing 

 them is obvious in connection with all or nearly all of its 

 activities. For example, to learn of the presence of food 

 at a distance, or of the threat of harm at a distance, 

 greatly facilitates the response of the organism. 



Smell. — The organ for smell, as stated above, is 

 stimulated by materials that are present in air in gaseous 

 form. It is located in the mucous lining of the nose, 

 where the air that is breathed in and out passes over it, 

 and the gaseous constituents of the air are thus able to 

 stimulate it. One or two points about this sense may be 

 mentioned. The first is its extraordinary delicacy. 

 When we think of the amazingly minute amounts of odor- 

 ous material which suffice to arouse the sense of smell in 

 a good dog, for example, we realize the extraordinary sen- 

 sit ivcMioss of the organ. A second point is the ease with 

 which the sense is fatigued. The continued presence of 

 any odorous substance in the air that is being breathed 

 quickly fails to be perceived. It is this which enables per- 



