350 - Multicellular Animals, Especially Man 



Fig. 18-2. Upper photo- 

 graph: Drastic scurvy in a 

 guinea pig. Lower photo- 

 graph: Restoration with pure 

 ascorbic acid (crystalline vita- 

 min C.) (Courtesy E. R. Squibb 

 and Sons.) 



cent work indicates that the fragility of the 

 capillaries may be due partly to a deficiency 

 of an unidentified factor, which is closely 

 associated with ascorbic acid in many 

 foods. 



The minimum daily requirement of each 

 vitamin is very small. Ten milligrams (.01 g) 

 of ascorbic acid, and considerably less of 

 most of the other vitamins, are entirely ade- 

 quate in preventing a deficiency. Conse- 

 quently it is plain that the vitamins are not 

 important as sources of energy for the body. 

 Rather the vitamins represent important 

 structural components in the protoplasm. 



Facts about Some Other Vitamins. Vita- 

 min research has advanced very rapidly in 

 recent years, and only a brief description of 

 the firmly established results can be presented 

 in this account. 



Vitamin A is a colorless compound 

 (C 20 H 30 O), but the body can derive vita- 

 min A from carotene (C 4(l H r ,, ; ), an orange 

 pigment present in many green and yellow 



vegetables. Accordingly, carotene may be 

 specified as a provitamin. Mild deficiencies 

 of vitamin A result in night blindness (p. 

 423), whereas drastic deficiency leads to a 

 serious eye defect, called xerophthalmia, in 

 which the cornea becomes very dry and ul- 

 cerated (Fig. 18-3 and Table 18-3). 



Recently it has been shown that vitamin 

 A plays an important role in vision. It is a 

 component needed by our retinal rod cells 

 for the synthesis of rhodopsin, the photo- 

 sensitive pigment concerned with black- 

 white vision (p. 423). Night blindness — that 

 is, an inability to see properly in dim light — 

 results, therefore, when the pigment content 

 of rods of the retina becomes depleted. 



Vitamin A is one of the group of the fat- 

 soluble vitamins (A, D, and E), which are 

 stored to some extent in the liver; and this 

 accounts for the fact that liver oils, derived 

 from fish such as the cod, halibut, and shark, 

 are excellent natural sources of these vita- 

 mins. 



