RADIATION AND VITAMINS 335 



'feather and fur,' may derive their vitamin D supply mainly from their 

 victims, yet the source of the vitamin in the prey would appear to lie 

 in the oil secretion. This may explain the absence of oil gland in some 

 species of birds . . . and the necessity of adding rabbits or small birds 

 with the fur or feather intact, to the diet of young carnivora in captivity 

 to ensure their successful development. . . . Further it is commonly 

 known that if herbivora, e.g., horses, are scrubbed thoroughly with soap 

 and water, they do not thrive. It may thus be inferred that the sebaceous 

 secretion, which is the homologue of the secretion of the preen gland of 

 birds . . . , is an important source of vitamin D in the mammal." 



Hou (40) found that rickety chickens with preen glands removed 

 could be cured by irradiating the feet, even though irradiation of the 

 body or of the head was ineffective. In this case the substance activated 

 was obviously neither preen gland oil nor circulating blood, but some- 

 thing in the tissues of the feet which was directly absorbed. 



Evidence enough has been presented to show that the higher animals 

 obtain vitamin D in at least three ways, the relative importance of which 

 must vary with habits, requirements, and opportunities: (a) By eating 

 such foods as eggs, fish, whole furred or feathered animals, and insolated 

 dead vegetable tissues; (6) by ingesting insolated sebaceous matter in 

 the process of neatening the body — licking and preening; and (c) by 

 directly absorbing the products of insolation formed on or in the skin. 

 It is to be noted that in all of these sources, except possibly fish, the 

 ultimate origin of the vitamin is traceable to sterols activated by light. 



REFERENCES 



1. Angus, T. C, F. A. Askew, R. B. Bourdillon, H. M. Bruce, R. K. Callow, 

 C. FiscHMANN, J. St. L. Philpot, and T. A. Webster. A crystalline antira- 

 chitic substance. Proc. Roy. Soc. London B 108 : 340-359. 1931. 



2. Askew, F. A., R. B. Bourdillon, H. M. Bruce, R. K. Callow, J. St. L. 

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3. Askew, F. A., R. B. Bourdillon, H. M. Bruce, R. G. C. Jenkins, and T. A. 

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