436 HANDBOOK OF PHYSIOLOGY. 



with the meat, for a time the carbon of the egesta and ingesta are equal, 

 but if the fat be increased beyond a certain point the body weight in- 

 creases from a deposition of fat; not, however, by a mere mechanical 

 deposition or filtration from the blood, but by an actual act of secretion 

 by the protoplasm whereby the fat globules are stored up within itself: 

 similarly as regards carbo-hydrates, if they are in small quantity, the 

 carbon appears in the excreta, but beyond a certain amount a consider- 

 able portion of it is retained in fat, having been by the protoplasm 

 stored up within itself in that material. The amount of proteid material 

 required to produce nitrogenous equilibrium is considerable, but it may 

 be materially diminished by the addition of carbo-hydrate or fatty food, 

 or of gelatin to the exclusively meat diet. 



It is of much interest to consider how the protoplasm acts in con- 

 verting food into energy and decomposition products, since the sub- 

 stance itself does not undergo much change in the process except a slight 

 amount of wear and tear. We may assume that it is the property of 

 protoplasm to separate from the blood the materials which it may re- 

 quire to produce secretions, in the case of the protoplasm of secreting 

 glands, or to enable it to evolve heat and energy, as in the case of the 

 protoplasm of muscle. The substances are very possibly different for 

 each process, and the decomposition products, too, may be different in 

 quality or quantity. Proteid materials appear to be specially needed, as 

 is shown by the invariable presence of urea in the urine even during 

 starvation; and as in the latter case, there has been no food from which 

 these materials could have been derived, the urea is considered to be de- 

 rived from the disintegration of the nitrogenous tissues themselves. The 

 removal of all fat from the body in a starvation period, as the first ap- 

 parent change, would lead to the supposition that fat is also a specially 

 necessary pabulum for the production of protoplasmic energy; and the 

 fact that, as mentioned above, with a diet of lean meat an enormous 

 amount appears to be required, suggests that in that case protoplasm ob- 

 tains the fat it needs from the proteid food, which process must be evi- 

 dently a source of much waste of nitrogen. The idea that proteid food 

 has two destinations in the economy, viz., to form organ or tissue proteid 

 which builds up organs and tissues, and circulating proteid, from which 

 the organs and tissues derive the materials of their secretions, or for pro- 

 ducing their energy, is a convenient one, and it is unlikely that proto- 

 plasm would go to the expense of construction simply for the sake of 

 immediate destruction. 



