836 NUTRITION AND HEAT REGULATION. 



The Stimulants. Under this head we include alcohol, tea, coffee, 

 chocolate, or cocoa, and meat extracts (beef tea, etc.). Regarding 

 the last mentioned substance, its physiological value has been made 

 clear by the work of Pawlow (p. 709). Meat extracts of various 

 kinds contain secretogogues which stimulate the gastric glands to 

 secretion. In themselves they may contain very little actual 

 foodstuff. Liebig's extract contains some protein, gelatin, and gly- 

 cogen, which form an actual nourishment, but its specific value 

 as a gastric stimulant depends upon other constituents, possibly the 

 nitrogenous extractives, creatin, xanthin, carnin, etc. Coffee 

 and tea owe their well-known stimulating action to the presence 

 of an alkaloid, caffein or trimethyl-xanthin. It may be considered 

 as xanthin in which three of the hydrogen atoms have been re- 

 placed by methyl (CH 3 ) groups, as is indicated in the following 

 structural formulas: 



HN CO CH 3 N CO 



CO C NH CO C N< CH3 



\ P TT I || ^CH 



//^' CH 3 N C N 



HN-C N 



Xanthin. Caffein. 



This alkaloid has a diuretic action on the kidneys and a stimulating 

 effect on the nerve centers, as is illustrated by its effect in raising 

 blood-pressure by an action on the vasoconstrictor center. The 

 influence of tea and coffee in preventing sleepiness may be referred, 

 to this action on blood-pressure. The use of these substances, 

 according to general experience, augments muscular energy and 

 diminishes the sense of fatigue. Cocoa, or the chocolate made 

 from it by the addition of sugar, contains considerable nourish- 

 ment in the form of fats, carbohydrates, and proteins, but its stimu- 

 lating effect is referred to the alkaloid theobromin or dimethyl- 

 xanthin, and to some extent possibly to the essential oils developed 

 in roasting. The theobromin exerts stimulating effects similar to 

 those of the caffein. The methylxanthins are in part oxidized in 

 the body and in part (one-third) excreted in the urine. 



Alcohol. The physiological effects of alcohol are of peculiar 

 interest to mankind, owing to its widespread use, and especially 

 to the disastrous results following its intemperate consumption. 

 Those who employ it in excess are in danger of acquiring an alco- 

 holic thirst or habit toward which the body possesses no counter- 

 acting regulation. When food is eaten in excess we experience a 

 feeling of satiety which destroys the desire for more food, and the 

 same regulation prevails in the case of water. With alcoholic 

 drinks, however, the desire may continue long after the alcohol 

 taken has begun to exert an injurious action upon the tissues. 

 The evil effects of excessive use of alcohol are so continually demon- 



