TROPISMS AND THE BEGINNINGS OF SENSE 227 



and of the pharynx there are little projections called papilla, 

 which contain the nerve endings of the neurons connected 

 with the cells that feel taste. The wry face that one makes 

 on tasting something disagreeable is a reflex in which the 

 afferent nerves of taste and the mus- 

 cular nerves controlling the muscles 

 of the lips, tongue, and cheeks form 

 the arc. 



There are also associated in this kind 

 of reflex other neurons that stimulate 

 the salivary glands. Your mouth waters 

 on tasting something sour, but this 

 watering is not related to the digestive 

 process. It may not be unreasonable to 

 consider this excessive watering as useful 

 in the sense that it helps to dilute the 

 acid, or to neutralize it (since normal 

 saliva is alkaline), or to wash it away. 



The nerves are capable of perceiving 

 four distinct tastes: sweet, sour, bitter, 

 and salty. When we perceive the vari- 

 ous flavors in substances that we place 

 in our mouths, we are really getting 



FIG. 90. Vorticella 



**%% 

 dark area 



Stimuli that act upon the nerve endings This one-celled animal lives in 



in the nose. We can readily convince wat f r ' attached ** lts s ^ lk * * 



3 rock or twig. When disturbed 



Ourselves Of this by trying tO distinguish, the animal contracts suddenly. 

 without the USe of Sight or Smell, the taste Running through the stalk is a 



of various substances having distinct 



flavors. A blindfolded person, holding 



his nose to prevent currents of air pass- 



ing 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 generally mean the odor. 



266. Organ of smell. The nerve endings in the lining of 

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

 nose into the pharynx, are of two kinds : some are sensitive to 

 touch ; others are sensitive to odors. This sense of smell is a 



