71 



of this quantity of food, except the small portion that passes 

 through the alimentary canal undigested, must be either dis- 

 solved or divided into such minute particles as to be able to get 

 through the microscopic passages that permeate the walls of 

 the canal and thus find their way to the blood. To judge 

 accurately of the nutritive value of our food, then, we must 

 know not simply how much of the different nutritive ingredients, 

 the protein and fats and carbohydrates, it contains, but how 

 much of each of these nutrients will be digested. This is a 

 matter that can be determined more or less accurately by ex- 

 periment. But a great deal of labor is needed to make the 

 experiments accurate, the line of research is new, the methods 

 are not yet perfectly matured, and the results thus far obtained, 

 though extremely interesting and valuable, are still far from 

 complete. The side questions, such as differences in the diges- 

 tive apparatus of different persons ; the effects of exercise and 

 rest, or mode of preparation of the food, and of the flavoring 

 materials and beverages taken with it, tend to complicate the 

 problem of digestibility, yet even here experimental research 

 has something to tell us. In brief, we have to-day a tolerably 

 fair idea as to what proportions of the ingredients of a good 

 many of the more common kind of animal and vegetable food- 

 materials, meats, milk, butter, cheese, eggs, bread, potatoes, 

 are ordinarily digested by healthy people. But the list of 

 materials the digestibility of which has been accurately tested 

 is far from including all the more common kinds of food, 

 and more experiments are needed, even with the foods that 

 have been tested, to show the variations in digestibility by 

 different classes of people, and under different conditions. The 

 only direct experiments on the digestibility of fish by men or 

 other animals, so far as I know, are those described in this 

 paper. 



THE CONSTITUENTS OF FOOD. 



But before going farther I ought, perhaps, to say a few 

 words about the nutritive ingredients of fish and other food 

 materials and the technical terms which we are coming to apply 

 to them in the chemical laboratory. Fish, like meats and other 



