FEEDING AND METABOLISM EXPERIMENTS. The analysis of plants as above shows the total amount of the constituents present ; but as these are not all in form to be digested by the animal. digestion experiments are made by the chemist on live subjeet F.. these experiments the and composition of the feed consumed by the animal, and of the corresponding excreta voided, are definitely determined for a short period, and by ealeulation the percentages of protein. fat. fibre, and nitrogen-free extract tic. tually digested are ascertained. These percent ages are called the coefficients of digestibility, and have to lie determined for each feeding stuff separately. A method of artificial digestion has been worked out. in which the feed is treated with pepsin and hydrochloric acid and with pan creas solution. but it is not generally considered to he as reliable as the natural method. In re cent years the bomb calorimeter has come into quite extensive use in the study of feeding stuffs, for measuring the fuel value, or the capacity of the feed for furnishing heat and energy for work. In this apparatus a sample of the material is ignited with oxygen in a platinum-lined bomb, and the heat evolved is measured by means of very accurate thermometers, suspended in a water-jacket surrounding the bomb. For fur ther details regarding fuel value and the sub ject of animal nutrition in general, see FEEDING STUFFS. and FEEDING FARM ANIMALS.
Very refined and delicate methods and appara tus have also been elaborated for studying the functions of the different nutrients in animal feeding, and the way in which they are used in the animal body in producing animal heat and energy for work. repairing the waste of the body, and 111:1 king growth or meat or milk. In the ordinary feeding experiment with cattle, or sheep, or pigs, the herd is divided into equal lots, and fed in periods varying from a few weeks' to several namtlis' duration, account be ing kept 1 if the amounts of food of known com position which each lot, or individual eats, and of the changes in weight. At the conclusion the animals are often slaughtered and the com position of the carcass determined. Til this way
the feeding value of nearly every available feed ing stuff, and of a great variety of compound rations, has been studied. and the specific effect of nutrients from different sources on the qual ity of the beef, pork. butter, etc. Mtueli of this beding work has also been directed toward the relation of live stock to the problem of mainte native of soil fertility, and the development of a rathmal system of agriculture. It has been shown that by feeding the crops largely on the farm where they are produced, and applying the ma nure to the soil, the fertility of the latter can the supply. of humus kept up. and the expense for cnnunercial fertilizers reduced to a minimum. The reason of this is that when crops are fed to growing stock or milch cows, from GO to 90 per cent. of their fertilizing in gredient:, voided in the solid and liquid manure. are retained upon the farm.
In studying the fundamental principles of nu trition, and the function of the nutri ents, the total Moline and outgo of the body during the experimental period must he deter mined. This includes the carbonic acid and other gases given off, as well as the excreta. For measuring these gases a. respiration apparatus is employed, which usually consists of a closed chamber in which the animal or pers,01 is placed, with means for measuring and sampling auto matically the air as it enters and leaves the chamber. With such a respiration apparatus Henneberg, Kfihn. and others have worked out the functions and relative values of the different nutrients, and many of the underlying princi ples of nutrition. Atwater and Rosa have com bined a calorimeter with the respiration appa ratus, and made many improvements in the accuracy of the latter. In their apparatus the chamber in which the subject is placed is a calorimeter, with very delicate arrangements for registering the heat given off by the subject. With the aid of this highly sensitive respiration calorimeter, it has been possible to demonstrate the absolute conservation of matter and energy in the body. A similar apparatus for use with animals has been &instructed by Armsby.