144 THE WORK OF THE DIGESTIVE GLANDS. 



on the part of its digestive apparatus, is much less than that for other 

 foods. How admirably, therefore, the food prepared by nature dis- 

 tinguishes itself when compared with all others ! 



The facts just related bring forward a new aspect from which the 

 relative nutritive values of different foods may be judged. The older 

 criteria must frankly make room for the new or else be displaced 

 by them. Experiments upon the utilisation of food-stuffs, in which what 

 remains undigested is determined, as well as what is absorbed into the 

 body fluids, cannot alone be trusted to solve the question in a satisfactory 

 manner. Suppose, for instance, that in the digestion of a given food the 

 alimentary canal has been given a certain work to perform ; if it be in 

 health, the work will be accomplished in the best possible manner, that 

 is to say, with complete abstraction of everything nutrient. You will 

 thus learn how much nutrient material was contained in the food, 

 but the question of its digestibility remains as obscure as before. In 

 your experiment you do not know how great an effort it has cost 

 the alimentary canal to extract all the nourishment from the food. Kor- 

 ean artificial digestion experiments settle the question of digestibility, 

 for experiments in which food is normally partaken of are quite 

 different from those in the test-tube, where we have to deal with only 

 one juice, and not with the interaction of different juices and different 

 food constituents. That one must here, as a matter of fact, make a dis- 

 tinction, is clear from the observation of Dr. Walther in our laboratory. 

 Fibrin, which is regarded by all as the most easily digested proteid, 

 proved, when compared with a nitrogen equivalent of milk, to be 

 a much stronger excitant of the pancreas, although milk contains, 

 in addition to nitrogenous substances, a good deal of other non-nitro- 

 genous material. The digestibility and nutritive value of foods must 

 obviously be decided by an estimation of the real work which they 

 entail upon the digestive apparatus, both in regard to the quantity and 

 quality of the juices poured out on a given amount of nutrient material. 

 The energy used up in gland metabolism must be deducted from that of 

 food taken in. The remainder will then indicate the value of the food 

 to the organism, that is to say, will give the amount available for use by 

 all the other organs exclusive of the digestive apparatus. From this 

 point of view those materials must be taken as less nourishing and le?s 

 digestible, which are in large part used up to make good the expenditure 

 entailed by their digestion on the part of the alimentary canal ; that is 

 to say, those food-stuffs are less useful whose nutritive value little 

 more than covers the cost of their digestion, consequently it is of great 

 practical importance to compare from this aspect the same foods, 

 differently prepared for example, boiled and roast meat, hard and soft- 

 boiled eggs, boiled and unboiled milk, &c. 



