6 FEEDING EXPERIMENTS WITH ISOLATED FOOD SUBSTANCES. 



degree are the further nutritive demands modified by developmental 

 changes? Are the same proteins which suffice the adult capable of 

 supplying the essential building-stones to the young? Here again a 

 wealth of new problems still confronts us. 



There are no recorded experiments on the larger animals, such 

 as are commonly used in laboratory work, which furnish any con- 

 clusive data on the broader questions involved in this research. The 

 careful experiments of Rohmann and his pupils* on dogs fed with 

 isolated proteins (edestin, vitellin, casein, myosin) extended over 

 50 days as a maximum period in one case. Such investigations, as 

 well as those of Abderhalden and his collaborators,! have yielded 

 much of interest in regard to the utilization of the food-stuffs inves- 

 tigated and have given valuable hints for future work in nutrition. 

 But the periods of observation have been far too brief to permit 

 any permanent positive conclusions regarding the adequacy of the 

 proteins fed. Minor deficiencies may fail to become conspicuous in 

 even comparatively long periods in the case of animals whose size and 

 span of life indicate a considerable store of reserve material. 



For various reasons the most successful investigations in this 

 field have been conducted on much smaller animals, especially rats 

 and mice. The preparation of suitable pure food supplies on a scale 

 sufficient for long periods and in economically procurable amounts 

 is thereby rendered possible. The necessary scientific measurements 

 and analyses can be conducted on a scale impossible for larger ani- 

 mals, and the problem of care and attention is equally simplified. 

 Experiments can be duplicated without great effort and individual 

 peculiarities eliminated by force of numbers. Furthermore, the 

 various stages of growth and maturity are completed in the smaller 

 animals within relatively short periods of time, so that the permanent 

 effects of any dietary become apparent within a range of days or 

 months that is not outside of ordinary experimental possibilities of 

 observation. As illustrations of some of these features it may be 

 recalled that the ultimate effects of complete inanition are apparent 

 in four or five days in rats or mice ; in dogs the fatal outcome may 

 be delayed for many weeks. The duration of life in the white rat is 

 about three years; about 280 days suffice to complete the entire 

 period of growth to maturity. 



To what extent, if any, the rigorous conditions, such as restraint 

 in cages, etc., set by laboratory requirements may modify the normal 

 physiological functions of the small animals is not apparent from 

 such records as we have found. It should be noted that both rats 



*Marcuse: Archiv f. d. ges. Physiologie 1896, lxiv; 1897, lxvii, p. 373. 



Steinitz: Ibid, 1898, lxxit, p. 75. 



R. Deipziger: Inaugural Dissertation, Breslau, 1899. 



E. Ehrlich: Inaugural Dissertation, Breslau, 1900. 

 tCf. papers by Abderhalden and collaborators in numerous recent volumes of the 

 Zeitschrift f. physiol. Chemie. 



