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 

 stimuli that act upon the nerve endings 

 in the nose. We can readily convince 

 ourselves of this by trying to distinguish, 

 without the use of sight or smell, the taste 

 of various substances having distinct 

 flavors. A blindfolded person, holding 



FIG. 90. Vorticella 



This one-celled animal lives in 

 water, attached by its stalk to a 

 rock or twig. When disturbed 

 the animal contracts suddenly. 

 Running through the stalk is a 

 strand of highly contractile sub- 

 stance, shown in the figure by the 

 dark area 



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 



