J42 LIFE : OUTLINES OF GENERAL BIOLOGY 



accessory food-factors, of unknown chemical composition, of which 

 small amounts must be present in the food if health is to be pre- 

 served, and esf)ecially if growth is to take place. These substances 

 are known chiefly in a curious negative way, by the disorders which 

 appear in the body if they are excluded from the diet. 



For clearness we may sum up the various vitamins in tabular 

 form — 



Vitamin A: in animal fats, green vegetables, etc.; promoting 

 normal growth. 



ViT.AMiN B: in outer parts of cereals and in yeast: anti-neuritic, 

 preventing Beri-bcri and some nervous disorders. 



Vitamin C: in oranges, lemons, sprouting peas, etc.; anti- 

 scorbutic, preventing scurvy. 



Vitamin D: allied to A, fat-soluble, in animal fats like cod-liver 

 oil and in green plants; anti-rachitic, promoting the 

 absorption and utilisation of calcium and phosphorus, 

 preventing rickets. 



Vitamin E: from the wheat-oil of sprouting grain, from lettuce, 

 etc.; promoting successful reproduction in female 

 rats; less securely known than the others. 



There must remain some unsatisfactoriness in regard to vitamins 

 until they are convincingly isolated and analysed. Some critics 

 maintain that they are not special chemical entities, but rather 

 physical attributes of certain constituents of the food. In this 

 connection it should be noted that food which induces certain 

 deficiency diseases in rats, may become satisfactory if irradiated 

 with ultra-violet rays — a fact which may tell against the usual view 

 that vitamins are specific chemical substances. The question often 

 asked, How our ancestors flourished in ignorance of vitamins, 

 must be answered in two ways: first, that they did not always 

 flourish, as the terrible tales of scurvy show; and, second, that 

 most mixed meals of unsophisticated food include an abundant 

 supply of vitamins. 



3. Digestion and Absorption. — The process of digestion is a 

 process of simplification of the complex organic comix)unds of the 

 food, and this simplification has two aspects. In the first place, when 

 a protein from the food is to be used as a source of protein for the 

 body, it is evident that it must first be reduced to their "highest 

 common factor", to use an arithmetical analogy; and this highest 

 common factor is naturally a mixture of the amino-acids of which 

 all proteins are com]x)sed. In the second place, proteins, fats and 

 complex carbohydrates cannot pass into the cells lining the intestine 

 and so into the body, unless they are first greatly simplified in 

 structure ; the cells appear to be impermeable to the larger molecules. 



