DIETETIC DEFICIENCY. 295 



similar to that of the gelatine feeding experiments. Zein con- 

 tains no tryptophane in its niolecnle. although some of the other 

 maize proteins do. Mice were fed on a mixed diet, in which, 

 however, zein was the only protein j^resent. and it was fonnd that 

 yy per cent, of the mice died within 20 days. The addition of 

 tyrosine to the diet produced no effect, bttt after the addition of 

 the missing trvtophane the mice lived tnuch longer, and showed a 

 much better physical condition. 



Results such as these are easy to understand when we 

 remember that the higher organisms i)robably cannot synthesise 

 their own amino-acids to any appreciable extent, but must receive 

 them, or most of them at least, ready-made in their food ; and. 

 further, remember that the composition of anitnal tissues 

 is constant and. broadly speaking, incapable of varying 

 in response to diff'erences in the composition of the food eaten. 

 The so-called " law of the minimum " finds a simple application 

 here, and the protein demands of an animal are seen to be con- 

 ditioned not so mucli b\ the gross quantity of protein eaten as by 

 the quota of those necessary units contained in smallest amotuit 

 within the protein molecule. 



To return now to the recent experiments of Osborne and 

 Mendel. These authors are of ojMuion that amino-acids contain- 

 ing cyclic nuclei can never be synthesised de novo by the animal 

 cells as they are in plant life, and hence that proteins poor in 

 units such as tyrosine, tryptophane, and l\sine. are of inferior 

 value as foods, especially during growth where new tissue- 

 proteins are being formed. Trytophane is credited with playing 

 a imique role in preserving ma'inicnaucc. and is contrasted with 

 lysine, which is regarded as indispensal)le for growth. In one 

 series of experiments ( )sborne and Mendel added lysine to their 

 previous zein-feeding mixtures, and for the first time successfully 

 reared ( not merel\- maintained ) rats on a diet contaim'ng zein 

 as sole protein. 



\\'ith these few illustrations of dietetic adequacy or inade- 

 quacy in respect to the i)rotein moiety of a food we may pass on 

 to the question of deficiency of the second type. 



Deficiency of Subsfcuiccs Mechanically or Chonically Asso- 

 ciated ivith the Lipoids. — If a feeding composite be extracted 

 with certain organic solvents, a fraction is obtained which, for 

 ^vant of a better name, is shtmped under the term " lij)oid." This 

 fraction may contain the true fats, the li])ines — i.e.. bodies analo- 

 gotts to the fats, btit containing phosphorus or nitrogen, or both 

 {e.g., lecithin), bound up with fatty-acid groupings; phytosterol 

 or cholesterol derivatives, waxes, essences, and numerous other 

 bodies. The quantity and nattire of the li})oid fraction depends 

 on the material extracted and on the solvent used. It is not neces- 

 sary to go into the classification of this group here ; suffice it that 

 the substances concerned are numerous, and in many cases not 

 yet clearly defined, either chemically or physiologically. It will 

 be sufficient to discuss a number of cases in which the absence of 



