80 THE VERTEBRATE ANIMAL: METABOLISM 



the effectiveness of vitamin C, but not of A and B. The complete 

 absence of these dietary factors produces specific abnormal con- 

 ditions (Fig. 45). For example, scurvy results from the absence 

 of water-soluble C; beri-beri, from the elimination of water-sol- 

 uble B ; and xerophthalmia, resulting in blindness, from deficiency 

 in fat-soluble A. 



In the light of modern investigations on the nutritional require- 

 ments of animals, it becomes evident that in order to obtain a 

 proper balancing of the diet one must do more than obtain a cer- 



FiG. 45. — Tadpoles of Rana pipiens showing general effect of absence of 



vitamins in diet. 



The tadpoles of each pair are the same age and the period intervals are one week. The 

 larger member of each pair had received a complete diet. The smaller one had no vitamins 

 in its diet which was otherwise complete. (From Emmet and Allen, Jour. Biol. Chem., 

 1919.) 



tain ratio between carbohydrates, fats (lipoids), and proteins. It 

 is essential that the proteins eaten contain the amino acids needed 

 by the protoplasm, that the inorganic elements be present in 

 proper concentrations, and that vitamins be furnished in abun- 

 dance. The abnormal functioning produced by inadequacy of 

 food with respect to single vitamins, and the conditions, such as 

 rickets and pellagra, that are recognized as due to more complex 

 deficiencies, are very serious factors in human welfare (Fig. 46). 

 On the whole, however, it is the very numerous cases of borderline 

 malnutrition, predisposing to various diseased conditions and 

 general lack of physical and mental efficiency, that are of more 

 vital importance to society as a whole. 



It is characteristic of the nutrition of animals that they cannot 

 build their foods from the constituent chemical elements, but must 

 take compounds produced by the protoplasm of other animals or of 

 plants. The protoplasm and the products of metabolism of one 



