[25] NUTRITIVE VALUE OF FISH AND INVERTEBRATES. 457 



2. Refuse: e. ^., bones of meat and fish, shells of oysters. 



The edible substance consists of — 



1. Water. 2. Nutritive substances or nutrients. — Thie refuse may, for 

 our present purpose, be left out of account, and our attention confined 

 to the edible substance. And, as the water which forms a part of the 

 edible substance, though indispensable, is nevertheless inexpensive and 

 distinct from the nutritive ingredients, we may consider simply the 

 nutrients. 



Speaking as chemists and physiologists, we may say that our food 

 supplies, besides mineral substances and water, albuminoids, carbo-hy- 

 drates, and fats; and that these are transformed into the tissues and 

 fluids of the body, muscle and fat, blood and bone, and are consumed 

 to produce heat and force. Viewed from a chemico-physiological stand- 

 point, then, the nutritive ingredients of food can be classified as follows: 

 Of the actually nutritive substances or nutrients of foods the most im- 

 portant groups (exclusive of water) are — 



1. Pro^ew (proteids, albuminoids, &c.) : e.g., albumen ("white") of 

 egg, fibrin of blood, "lean" of meat, gluten of wheat. 



2. Fats: e.g., fat of meat, butter, olive«oil. 



3. Carbo-hydrates: e. g., starch, sugar, glycogen. 



4. Mineral matter or ash: e. g., calcium and potassium phosphates and 

 chlorides. 



The terms protein, proteids, and albuminoids are applied somewhat 

 indiscriminately, in ordinary usage, to several or all of certain classes 

 of compounds characterized by containing nitrogen. The most impor- 

 tant are the proteids or albuminoids, of which albumen, the white of 

 egg, and myosin, the basis of muscle, are types. Allied to these, but 

 occuiring in smaller proportions in animal tissues and foods, are the 

 nitrogenous compounds that make the basis of connective and other 

 tissues. Gelatin is derived from some of these tissues, and may be 

 taken as a type of the compounds of this class. As these constituents 

 are of similar constitution, and have similar or nearly similar uses in 

 nutrition, it is customary to group them together as protein. The mus- 

 cular tissues of animals, and hence the lean portions of meat, fish, &c., 

 contain small quantities of so-called nitrogenous extractives — creatin^ 

 carnin, &c. (contained in extract of meat, &c.) — which contribute mate- 

 rially to the flavor and somewhat to the nutritive eifect of the foods 

 containing them. They are not usually deemed of sufficient importance, 

 however, to be grouped as a distinct class in tabular statements of tlie 

 composition of foods. Concerning their chemical comjjosition, it will 

 suffice to state that the compounds classed together as protein contain 

 carbon, oxygen, hydrogen, and nitrogen, while the carbo-hydrates and 

 fats contain no nitrogen, but consist chiefly of carbon, oxygen, and 

 hydrogen. The fats are much richer in carbon than the carbo-hydrates. 

 Animal foods, as meats, fish, &c., contain but little of carbo-hydrates, 

 their cliief nutrients being protein and fats. Milk, however, and some 

 shell fi«h, as oysters, scallops, &c., contain more or less of carbo hy- 



