838 IMPORTANCE OF DIGESTIBILITY. [BOOK n. 



alcohol, various other allied substances, whose action is even more 

 potent than that of the ethylic alcohol itself, and whose presence 

 very markedly determines the total effect of the drink. Such 

 articles of diet as tea and coffee stand upon very much the same 

 footing as alcohol. 



The quantity of fluid which a man drinks or should drink 

 daily, or more correctly the quantity of water which he should 

 daily add to the dry solids of his diet, must vary widely according 

 to circumstance. It will differ according as he is perspiring greatly 

 or not, according to the nature of the dry solids of the diet, whether 

 largely carbohydrate or not, and so on. A lower limit, below 

 which excretion is impeded, and a higher limit, above which 

 digestion and metabolism are injuriously affected, probably exist ; 

 but we have as yet no adequate data which will enable us to fix 

 either of them. 



552. In the selection of articles of food to supply the food- 

 stuffs and other constituents of a normal diet, regard must of 

 course be had in the first place to the amount of potential energy 

 present in the material. The articles chosen for the daily fare 

 must contain between them so much proteid, fat, and carbohydrate 

 representing so much available energy. But it is no less important 

 to secure that the energy potential in the material should be really 

 available for the economy. The material must have such qualities 

 that it is digested within the alimentary canal, and further that its 

 digestion and absorption do not give rise to trouble either in the 

 alimentary canal or in that secondary digestion carried on by 

 means of the various metabolic events which we have discussed in 

 preceding sections. A really nutritious substance is one which 

 not only contains in itself an adequate supply of energy, but is of 

 such a nature that its energy can be appropriated by the economy 

 with ease or at least with as little trouble as possible. We have 

 approximate data for determining how far an estimate of the 

 relative usefulness of various articles of food must be corrected by 

 allowing for the proportion of each which after an ordinary meal 

 merely passes through the alimentary canal and the energy of 

 which is not in any way available for the body's use. Thus a 

 number of observations carried out on healthy individuals gave in 

 the case of the following articles of food, the following figures as 

 the percentage, reckoned in each case on dry material, which 

 could be recovered from the faeces, and was therefore not digested 

 and not used by the body : Meat 5 p.c., Eggs 5 p.c., Milk 9 p.c., 

 Bread (white) 4 p.c., Black Bread 15 p.c., Rice 4 p.c., Maccaroni 

 4 p.c., Maize 7 p.c., Peas 9 p.c., Potatoes 11 p.c. It must however 

 be remembered that the actual correction to be made in any case 

 will depend on the mode of cooking of the material, on the 

 character of the meal of which it forms part and on the individual 

 capabilities of the consumer, the latter too varying under different 

 circumstances. 



