INTRODUCTION. 57 



by M. S. Fine,* while they do not completely do away with this 

 possibility, make it more evident than before that incomplete diges- 

 tion is, in the case of plant products, for the most part associated 

 with the peculiar vegetable tissues therein contained, rather than a 

 specific resistance of the isolated nutrients. 



The need of "roughage" to facilitate the normal evacuation of 

 the gut has also been debated. We have, as a general procedure, 

 added the indigestible polysaccharide carbohydrate agar-agar to food- 

 pastes in order to approximate more nearly the conditions which 

 prevail where cellulose enters into the mixed dietary. It can not be 

 maintained, however, that this is necessary for satisfactory nutrition ; 

 for we have maintained animals over a year on foods (cf. Chart 

 XXIX) devoid of indigestible principles, if perhaps an exception be 

 made of some of the inorganic ingredients. It is well known that 

 inorganic salts, notably bone ash, may exert the same influence as 

 cellulose in giving bulk to the faeces ; and they are often so employed 

 in the technique of metabolism experiments at the present time.f 



Aside from the proteins, in which our experimental interest has 

 been primarily centered, our attention has been drawn more and 

 more to those components of the diet which are not sources of energy, 

 yet fundamentally indispensable namely, the inorganic compounds. 

 It is possible that further investigation will compel the inclusion of 

 some of the more vaguely defined and unknown members of the 

 groups spoken of as extractives, lipoids, etc., in this category. Every 

 attempt made by us to approach the solution of the problem of 

 inorganic salts in the dietary has brought fresh surprises. 



When Forsterj fed dogs and pigeons on salt-free foods he made 

 the interesting observation that the animals speedily died more 

 rapidly even than when all food was withheld. He concluded: 



Der im Uebrigen in Stickstoffgleichgewicht sich befindende thierische 

 Organismus bedarf zu seiner Erhaltung der Zufuhr gewissen Salze; sinkt 

 die Zufuhr unter einer gewisse Grenze oder wird sie ganzlich aufgehoben, 

 so gibt der Korper Salze ab und geht daran zu Grunde. 



The classic experiments of Lunin on mice led to a somewhat 

 different interpretation of the need of salts. He showed that the 

 animals survived longer on a diet containing an addition of sodium 

 carbonate to the ash-free food than when sodium chloride was added. 

 In the latter case the duration of life corresponded approximately 

 with that observed on a salt-free dietary. From these facts it was 

 argued that the foremost value of the sodium lies in its capacity to 

 neutralize the acids (sulphuric, phosphoric) formed in the metabolism 



*M. S. Fine: Dissertation, Yale University, 191 1 (unpublished). Cf. Mendel and 

 Fine: Journal of Biological Chemistry, 191 1, vols. X and XI. 



fCf. Lothrop: American Journal of Physiology, 1909, xxiv, p. 297. 

 JForster: Zeitschrift fur Biologic, 1873, ix, pp. 297-380. 

 Lunin: Zeitschrift fiir physiologische Chemie, 1881, v, p. 31. 



