NUTRITION. 367 



be made. The factors which center in the control of growth are now 

 being tested in other laboratories — in part as the outcome of our exper- 

 ience in this field — with reference to the control of abnormal growths. 



Attention has been devoted to the proportions of "protein-free milk" 

 that are necessary or desirable in rations for various purposes. New 

 experiments in which artificial mixtures made in imitation of the 

 natural product from milk have been employed have furnished added 

 evidence of the presence of something in the natural "protein-free 

 milk" which we have as yet been unable to identify. The detection 

 of these essential ingredients, if it become possible, may have an 

 important bearing upon the so-called vitamine problem. It is occa- 

 sionally possible to raise animals to a considerable size on a diet of 

 purified protein, starch, sugars, purified natural fats, and artificial 

 mixtures of inorganic compounds. In some cases animals have been 

 maintained for so long a time as to lead one to doubt whether any 

 undiscovered factors in the diet are really necessary; but success of 

 this sort has, after all, been a rare exception. Whenever failure has 

 resulted, the addition of the natural milk product has invariably 

 restored the declining animal. The efficiency of our "protein-free 

 milk" is apparently not impaired by boiling during the process of its 

 preparation. The ash of the product, on the other hand, is in no way 

 comparable with "protein-free milk" as a substitute to furnish the 

 inorganic ingredients of the dietary. 



From experiments with the by-products of the manufacture of milk 

 sugar, as a substitute for our "protein-free milk," it seems not unlikely 

 that large amounts of products peculiarly valuable in the nutrition of 

 domestic animals, particularly during the periods of growth, are being 

 continually wasted in this country. 



Sufficient experience has been gained to show that our methods of 

 feeding will permit us to approach the problem of the influence of 

 heating on the nutritive value of milk more satisfactorily than has been 

 the case in previous experiments. Brief periods of heating, such as 

 are employed in ordinary pasteurization or sterilization, are without 

 apparent detrimental effect on the growth of rats. Heating at 100° for 

 more than an hour appears to deteriorate the milk. At any rate, in the 

 limited number of our experiments, these heated milks have appeared 

 inferior to unheated milk when they formed the chief basis of the diet. 



The unique virtue of butter-fat in restoring animals that had failed 

 on our earlier dietaries, which was briefly alluded to in the last report, 

 has been subjected to continued study. Animals which have received 

 milk fat continuously from the very early periods of growth have uni- 

 formly failed to show any nutritive decline. The significance of the fat 

 component in the milk, in furnishing something essential for prolonged 

 adequate nutrition, is further substantiated by comparisons which we 

 have' instituted between the use of foods prepared from desiccated 



