448 PRINCIPLES OF ANIMAL NUTRITION. 
§ 1. Utilization for Tissue Building. 
Under this head we have to consider almost exclusively ex- 
periments upon the fattening of mature animals. While the growth 
of young animals and the production of milk are both forms of 
tissue building, the experimental data available seem too scanty 
to justify including them in the scope of the present work. For 
convenience we may first bring together the recorded results and 
later discuss them in their more general bearings. 
One difficulty, however, is encountered at the outset in our 
inadequate knowledge of the net availability of nutrients and 
feeding-stufis, as pointed out in the foregoing chapter. Until this 
gap is filled. it is of course impossible to compare the gain of energy 
by the body with the supply of net available energy. Accordingly 
the results of the experiments upon productive feeding can at present 
be utilized only to determine what proportion of the metabolizable 
energy of the food is recovered in the gain of tissue, and the experi- 
ments cited in the following paragraphs will be considered from 
this point of view. 
Experimental Results. 
Experiments on Carnivora.—In connection with the dis- 
cussion of net availability in the preceding chapter a number of 
experiments were cited (p. 428) in which more or less gain was 
made by the animals. In addition to these Rubner * has made a 
preliminary report of investigations upon the effect of abundant 
feeding on the heat production A dog weighing 25 kgs. received 
successively isodynamic amounts of lean meat. fat, and carbo- 
hydrates (kind not stated) equivalent to 155 per cent. of its fasting 
metabolism, a two-days’ fast intervening in each case between the 
different rations. Few details are given, but presumably the 
methods were those of Rubner’s other experiments already de- 
scribed (compare p. 253). In a second experiment the effects of 
two different amounts of meat were also compared. In the follow- 
ing table the results of these experiments have been put into the 
same form as those on net availability in the preceding chapter, the 
data given being per day and head: 
* Sitzungsber. k. bayer. Akad. der Wiss., Math -phys. Classe, 15, 452. 
