134 METABOLISM AND NUTRITION 



We may reckon among flavors not only the substances commonly under- 

 stood by the word in its strictest sense, such as that which gives the character- 

 istic taste to roast beef, or that developed in the baking of bread, the spices, 

 extracts, etc., but also coffee, tea, alcoholic drinks, tobacco, etc. in short, any- 

 thing which adds to a meal an element of pleasure. In this sense we might 

 include also the various extraneous means of making a meal enjoyable, like 

 neat service, lively conversation, etc. 



Some of the foodstuffs themselves serve at the same time as flavors e. g., 

 sugar and salt. The body requires NaCl; but in the quantities in which we 

 ordinarily eat salt, it is really a flavor. 



The physiological importance of flavors consists in the stimulus they afford 

 for the secretion of the digestive fluids. Sight or smell or even the thought 

 of an appetizing dish makes the mouth water i. e., makes the salivary glands 

 secrete profusely. As we shall see later under digestion, the same can be 

 demonstrated, for the gastric glands. If a meal or its accompaniments are 

 not pleasant, these reflex effects are not forthcoming (cf. Chapter VII). 



If one eats too much or too frequently of a dish once palatable, it becomes 

 distasteful or even " turns against " him ; and the more pronounced the taste 

 of the dish, the more quickly will it become distasteful. On this account there 

 are only a few articles of diet, such as bread, which we can eat every day or in 

 large quantities. Herein lies the importance of variation in diet, even different 

 modes of preparing the same articles being advantageous. For example, the 

 people who live mainly on flour or meal of cereals do not eat these substances 

 exclusively in the form of bread, but use them also in the preparation of 

 dumplings, noodles, pancakes, etc. 



13. ON THE THEORY OF METABOLISM 



In order to comprehend fully the processes of metabolism it is needful 

 that we inquire to what extent, if any, the organized substance of the body is 

 broken down in combustion. 



When we consider how very much the destruction of proteid depends upon 

 the amount ingested (page 99), how the N-excretion after meals is very 

 closely connected with the absorption of proteid into the blood (page 101), 

 and when we remember that the nonnitrogenous foodstuffs do not essentially 

 alter the destruction of proteid (page 105), and that physical work with 

 plenty of N-free foodstuffs supplied does not increase the proteid combustion 

 (page 111), we are almost compelled to suppose with Voit that it is the 

 proteid of the food and not the living protoplasm which first breaks down in 

 metabolism. 



Since the living protoplasm is derived from proteid, and upon its degrada- 

 tion after death again forms proteid, it is quite common in the physiology of 

 nutrition to apply to it the name of tissue proteid, and to the dead proteid 

 coming from food and found in the fluids of the body, the name " circulating 

 proteid." In order to avoid misunderstanding we shall use here the terms: 

 living substance, in whatever tissues it may occur, and food proteid i. e., the 

 proteid absorbed from food but not transformed into living substance. 



