PHYSIOLOGICAL CHEMISTRY 359 



will crystallise out (Fig. 235). The zinc salt is filtered off, washed several times 

 with spirit, dissolved in water, 1 and the zinc separated by passing a stream of 

 sulphuretted hydrogen through the solution. The zinc-free filtrate is then freed 

 of water by evaporation, when the lactic acid is obtained as a syrup. 



CHAPTER XL 

 DIETETICS, FOOD, METABOLISM. 



FOOD is taken into the body for two purposes (1) to replace tissue 

 waste j (2) to supply energy for work and for heat. 



It is necessary that such food should be taken in a proper amount, 

 in a digestible form, and adapted to the climatic circumstances and 

 occupation of the eater. It is also essential that the food should be 

 as varied as possible, for experience shows that an unvaried diet is 

 not eaten with relish and may impair the general health and resistance 

 to disease. The appetite of a healthy man is the best guide for the 

 selection of his food ; it is the expression of physiological needs which 

 have not been investigated sufficiently to justify dogmatic statements 

 as to what a man should and should not eat. 



We may, therefore, divide the proximate principles 2 of food into 

 two groups, viz. : 



Tissue formers Protein, inorganic salts, water. 



Combustion material Protein, fats, carbohydrates. 



It will be observed that, of the organic food stuffs, protein may 

 act in either capacity, and hence, that a diet containing protein and 

 salts alone can serve as an efficient food. That this is so is proved 

 by the fact that the Indians of the Pampas live entirely on flesh. 

 Without protein it is impossible to maintain life ; but if more than 

 is necessary for the repair of the broken-down tissues be supplied, 

 the excess serves as a fuel. 



The most serviceable combustion material seems, however, to be 

 carbohydrate ; next comes protein, and the least available is fat, this 

 latter being pre-eminently the form in which the excess of food over 



a The watery solution should be evaporated until the crystals of zinc sarco- 

 lactate begin to appearf^fhis being ascertained by examining a drop under the 

 microscope. 



2 These nutritive constituents are called the proximate principles of food, 

 because, consisting as they do of carbon, hydrogen, oxygen, and nitrogen 

 combined into highly complex bodies, they are really elementary constituents 

 of the organism. 



