152 FOODS 



volatile substances of smoke. The brewing of beverages, such as coffee, 

 tea, cocoa, and beers, consists either in the making of a decoction (as in 

 tea and coffee) ; of a solution which is then cooked (cocoa and chocolate) ; 

 or in the fermentation of the sugars in many vegetable substances (of 

 which barley is typical) under the influence of yeast (Saccharomycetes 

 cerevisice). Freezing is a culinary method which has come into common 

 use only of late years, but it affords many desserts nourishing and grateful 

 to the well, and often very valuable in febrile disorders. The use of ices 

 is rapidly increasing in Europe as well as in America, where the art 

 originated. 



Quantitative Adaptation to its Service is Essential. Another require- 

 ment in an ideal diet is that it should be adapted quantitatively in some 

 degree to the conditions under which it is employed. As has already 

 been emphasized, a food provides energy for the warming and the mov- 

 ing of the body and materials for the replacement of wornout and excreted 

 tissue. The greater then (a) the amount of muscular and neural 

 exercise, the greater (6) the atmospheric opposition for keeping warm, 

 and the greater (c) the need of new tissue, whether for growth or for 

 tissue-waste, the greater the amount of food required. This is axiomatic, 

 but it is important in dietetics. The only reason, in practice, that there is 

 observed no greater differences in the amount of food consumed by, say, 

 the idle "society woman" and the street-laborer is that the former prob- 

 ably eats far too much, while the latter, owing chiefly to his small income, 

 perhaps eats somewhat too little. The status of mental labor as regards 

 the amount of food required is at present in a somewhat uncertain con- 

 dition theoretically. We do not know exactly how much energy and 

 tissue the brain's activity actually consumes. Researches now being 

 performed on men with balances large enough to support a man and yet 

 exceedingly delicate, will doubtless determine the amount of tissue lost 

 in mental work. Calorimetric measurements of the energy expended in 

 mental activity have given thus far no important results. So far, then, 

 we cannot make as close a correlation between the amount of brainwork 

 and the food-requirements as is made between muscle-work and the 

 necessary amount of food. The cortex cerebri in the average man weighs 

 about 17 gm., while the muscles in an average sized man weigh about 

 33,000 gm. This, obviously, is the cause of the difference noted. It is 

 likely that the nervous system per gram wastes faster and gives out more 

 energy than does muscle. (For an estimate of the work done by the 

 human heart see page 279.) 



The colder the atmosphere in which an organism lives the more heating 

 fuel it will require, and the quantity of food needs adaptation to this 

 condition to some extent. Part of the body-heat comes from muscular 

 exercise, so that the metabolism in general tends to be more active in a 

 cold atmosphere, and tissue-waste is thus increased. Again, heat is 

 lost by radiation and conduction from the skin more rapidly on a cold 

 day than on a warm day, and this excess of loss the food must supply. 

 Of the 2500 calories of energy which an average man daily receives from 



