ASSIMILATION 79 



requisite amino acids. The necessity of including water in a diet 

 is altogether obvious in view of what has been said of its impor- 

 tance as a constituent of protoplasm. It is also clear that mineral 

 salts are necessary to maintain the supply of chemical elements 

 used in metabolism. These inorganic salts are present to a greater 

 or lesser extent in the water we drink, but the best single source 

 of these materials is milk, which contains potassium, calcium, 

 phosphorus, chlorine, sodium, and magnesium. 



Our knowledge of the importance of vitamins as dietary neces- 

 sities has developed rapidly since 1910. Vitamins are organic 



Fig. 44. — The rations of these two rats from weaning time were exactly ahke 

 except in the character of the fats which they contained. The one on the 

 left was given 5 per cent of sunflower seed oil. The one on the right was 



given 1.5 per cent of butter fat. 



Butter fat, egg yolk and the leaves of plants contain a dietary essential, the chemical 

 nature of which is still unknown, which is necessary for growth or the maintenance of health. 

 This substance is known as fat-soluble A, and is not found in any fats or oils of vegetable 

 origin. The animals were the same age when photographed. (From McCoUum, "Newer 

 Knowledge of Nutrition," copyright, 1922, by the Macmillan Co., reprinted by permission.) 



compounds, of unknown composition, which must be present 

 as such in foods, since it is apparently impossible for the proto- 

 plasm to build them up. Three different vitamins, known 

 respectively as fat-soluble A, water-soluble B, and water-soluble C, 

 have been found to be essential in human nutrition, and it is pos- 

 sible that others will be identified as knowledge progresses. Fat- 

 soluble A is present in milk and butter but not in butter substitutes 

 like oleomargarine; it is also present in egg yolk, in cod-liver oil, 

 and in leaf}' vegetables such as spinach and lettuce (Fig. 44). 

 Water-soluble B occurs in oranges and other citrus fruits, prunes, 

 milk, egg yolk, whole wheat, yeast, and thin-leaved vegetables. 

 Water-soluble C is found in citrus fruits and in many fresh leafy 

 vegetables, in young carrots, and in tomatoes Heating ref^uces 



