FEEDING EXPERIMENTS WITH ISOLATED FOOD SUBSTANCES. 5 



indirectly. Thus the question of the inorganic constituents of the diet 

 is far from settled, and this aspect of dietetics can scarcely be said to 

 rest upon any very rational basis. We realize the need of chlorine 

 and phosphorus, of calcium and iron in certain vital functions; but 

 the physiological requirement of other elements is none the less 

 definite because it is not fully understood. On one point, namely, 

 the power of the animal organism to employ the elements phospho- 

 rus and iron in the form of either organic or inorganic compounds, we 

 have assumed that the consensus of opinion and the preponderance of 

 experimental evidence are in favor of either possibility. The debate 

 on this topic need not be reviewed here. 



Closely related to these nutritive requirements is the subject 

 of those food accessories which determine in large measure what is 

 spoken of as the palatability of any ration. They act in manifold 

 indirect ways to influence the digestion of the nutrients by their effect 

 in promoting or retarding secretion into the alimentary tract ; they 

 affect the appetite and the psychic element in digestion all of which 

 have received attention anew through Pawlow's efforts. After all 

 one is as yet not justified in insisting that these incidental features 

 of the dietary are absolutely indispensable. Quoting Tangl : 



Eine fordernde Wirkung der sog. Reiz oder Wiirzstoffe sagt Kell- 

 ner auf die Ausnutzung des Flitters ist bis jetzt bei keiner einzigen der auf 

 diesen Punkt geriehteten Untersuchungen beobaehtet worden. Dasselbe 

 gilt nicht nur fur die landwirtschaftliehen Nutztiere, sondern auch fiir den 

 Menschen.* 



The undoubted biological importance of that rather vaguely 

 defined and hetereogeneous group of compounds known as lipoids 

 has raised the question as to whether they are at all necessary in any 

 complete diet, or whether they can be synthesized by animals as 

 they are by plants. The lipoids (phosphatides and cholesterols) are 

 present as common cell derivatives in every familiar food and can 

 only be excluded by special, laborious methods of extraction. It is 

 not strange, therefore, that there is a paucity of evidence relating to 

 their absolute significance as constituents of the food-intake. Recent 

 experiments by Steppf on the indispensability of the ether-soluble 

 constituents of the food for the life of mice are far from conclusive. 



Lastly, it seems worth while to point out that the nutritive 

 conditions which pertain during the period of growth are in many 

 respects perhaps far more than is realized different from those 

 existing at a later period. During the active constructive phase new 

 material must be supplied which differs both in quality and amount 

 from the quota furnished as a maintenance ration. We realize well 

 enough that the growing skeleton requires calcium; but to what 



*Tangl: Oppenheimer's Handbuch der Biochemie, 1910, in (2), p. 55. 

 jStepp: Biochemische Zeitschrift, 1909, xxn, p. 452. 



