286 EXPERIMENTAL GENERAL SCIENCE 



apart on the small of the back. The end organs which per- 

 ceive differences in temperature are scarcely to be distinguished 

 from those of touch, though they react to a different set of 

 stimuli. The organs of smell come to the surface in the 

 mucous membrane in the upper part of the nose, and are very 

 similar to ordinary cells surrounding them. They are so 

 located that they can perceive odors in the air currents as 

 they pass into the body and thus notify us when the sur- 

 rounding air is losing its purity. The sense of smell is very 

 easily tired and, after reporting an odor for a short time, ceases 

 to be stimulated by it. This accounts for the fact that people 

 who work with ill-smelling materials soon cease to be annoyed 

 by the odors. Though the sense of smell is easily fatigued, 

 the impressions made by smells on the memory are very lasting, 

 and the sense itself is exceedingly keen. One part of vanilla 

 in eight million parts of air can be detected by this sense. 

 The sense of taste is located in certain taste buds, mostly on 

 the upper surface of the tongue. There are three forms of 

 these taste buds; those on the back of the tongue are relatively 

 few in number and consist of elevations each surrounded 

 by a circular depression; those on the rest of the tongue are 

 either thread-like bodies embedded in the other cells or are 

 mushroom-like structures scattered here and there. These 

 latter may be clearly seen with the unaided eye. The tip of 

 the tongue best perceives sweets, the sides are most affected by 

 acids, and those at the back of the tongue by bitter substances. 

 These three classes of flavors are all that the tongue really 

 perceives, as may be easily seen by holding the nose while 

 testing other substances. Great heat and cold paralyze the 

 sense of taste. 



247. Sight and Hearing. Sight and hearing, concerned 

 with vibrations in the ether and vibrations in the air re- 

 spectively, have much more complicated organs for receiving 

 impressions than have those of the other senses. The organs 



