PART II. 



DIETARIES AND RATIONS. 



A Mixed Diet. Food in order to fulfil the various offices de- 

 scribed in the preceding pages must not contain one set of nutrients 

 only, but all. Moreover it must contain the several nutrients in 

 a certain ratio or proportion as well as to a certain amount. 

 Absolute exactness of ratio or of amount is not required, as a 

 small excess of one or more nutrients may remain unused, or may, 

 in some cases, do the work of others. Thus an excess of water 

 or of common salt is excreted without doing harm, and, within 

 certain limits, albuminoids may do the work of oil or starch. 

 Indeed, the human body may be sustained for a time upon 

 water and albuminoids alone, although in this instance, the 

 extra strain thrown upon those secretions and viscera which have 

 to digest and to eliminate the excess of nitrogenous matters, could 

 not be maintained for long without resulting in disease. From 

 these and other considerations we may learn the necessity of a 

 mixed diet by means of which each secretion and each organ of 

 the body shall be properly utilised and perform its proper share in 

 the work of nutrition. There must be starch, to be dissolved 

 and saccharified by the ptyalin of the saliva and by the diastatic 

 ferment of the pancreatic secretion ; there must be oil to be 

 emulsified and saponified and decomposed by the bile and the pan- 

 creatic juice ; there must be albuminoids to be peptonised by the 

 pepsin of the gastric juice, and by the trypsin of the pancreas. 

 And the mixed food as a whole must be palatable and digestible, 

 and it must occupy a sufficient but not excessive space. 



