2 28 BIOLOGY AND HUMAN LIFE 



oil." In both cases the reaction depends upon some relation 

 between the composition of one body and the composition of 

 the other body or substance. Water does not choose to dissolve 

 sugar and to leave sand undissolved; neither can we be sure 

 that a simple organism chooses its food, although it does take 

 some kinds and reject other kinds of objects or materials. It 

 is only when we come to the higher animals that we may speak 

 of choice, and even among the highest animals most of the 

 selecting and rejecting depends upon reflexes and instincts 

 rather than upon thought and feeling ; that is, it depends upon 

 the structure of the organism and upon the composition of cer- 

 tain organs. Even in our own bodies, there are reactions to 

 chemical stimuli similar to those of the ameba, as in the way 

 the white corpuscles react to the presence of various kinds of 

 bacteria that invade the body (see page 178). In addition, 

 we have two special chemical senses, taste and smell. 



186. Organs of taste. On the upper surface of the tongue, 

 and in other parts of the lining of the mouth and of the pharynx, 

 there are little projections called papillce. These contain the 

 nerve endings of the neurons connected with the brain cells 

 that are aware of taste. The wry face that you make on tasting 

 something disagreeable is a reflex of which the arc is formed 

 by (i) the afferent nerves of taste and (2) the efferent nerves 

 controlling the muscles of the lips, tongue, and cheeks. Another 

 reflex started by taste stimuli is the watering of the mouth. 



A blindfolded person, holding his nose to prevent currents of air from 

 passing through it, cannot distinguish ground coffee, for example, from 

 sawdust, or vanilla flavor from raspberry. When we speak of the taste 

 of good food, we usually mean the odor. Our taste system can distin- 

 guish four classes of tastes : sweet, sour, salt, and bitter. 



187. Organs of smell. Some of the nerve endings in the lin- 

 ing of the nose, and of the air passages extending back from the 

 nose into the pharynx, are sensitive to touch ; others are sensi- 

 tive to odors. This specialized chemical sense is more highly 

 developed in many of the lower animals than it is in man. 



