334 BIOLOGICAL EFFECTS OF RADIATION 



It would seem that if fish obtain vitamin D from plankton, they must 

 salvage it with extraordinary economy. 



Irradiation of the body surface appears to play no part in the forma- 

 tion of vitamin D in fish. Bills (8) observed that catfish are very sensi- 

 tive to ultra-violet irradiation, yet do not elaborate more vitamin D as a 

 result of it. Schmidt-Nielsen and Schmidt-Nielsen (76) found that the 

 basking shark, Cetorhinus maximus, has comparatively little vitamin D 

 in its liver oil, in spite of the fact that it basks for hours at the surface 

 of the water, and feeds on plankton. Piscicultural experiments by 

 Bills (8) made it appear probable that at least a portion of the vitamin D 

 of fish originates by sjmthesis within the fish. 



The higher animals lack the power to synthesize vitamin D, so their 



requirements must be met by ingesting it, or particularly by exposing the 



body surface to sunlight. All animal fats contain sterols, those of the 



skin being particularly rich in provitamin. Hess and Weinstock (33) 



demonstrated that irradiated skin is antirachitic, and that irradiated 



sterol exerts its action when administered subcutaneously. Rekling (66) 



found that irradiation did not protect rats from rickets when they were 



prevented from hcking their fur. Hou and Tso (41) found that the skin 



of normal rabbits was slightly antirachitic, the dorsal skin more so than 



the ventral, but that the skin of rickety rabbits or of normal rabbits 



reared indoors was without protective action. Hou (39) noted that the 



effectiveness of ultra-violet irradiation for curing rickets in rabbits was 



almost lost when the skin had been previously washed with ether. 



Rowan (73) observed that birds of prey on a meat diet developed rickets, 



and that the addition of feathers to the diet supplied protection. He 



suggested that the preen gland is concerned with the formation of vitamin 



D. Hou (36, 37, 38, 40) made an elaborate study of the formation of 



vitamin D in birds. In brief, his findings were as follows: Birds differ 



from mammals in having only one gland of a sebaceous nature. This 



is the glandula uropygialis, or preen gland. Preen gland oil contains 



"ergosterol," w^hich birds, by preening, distribute over their feathers 



and effectively expose to sunlight. The vitamin D is either ingested by 



swallowing the feathers, or absorbed by the skin from the feathers. The 



feathers and skin of normal birds were shown to be antirachitic, but in 



rickety birds, or birds whose preen glands had been removed, the feathers 



and skin had little, if any, antirachitic action. Removal of the preen 



gland made the birds susceptible to rickets, and rickety birds without 



the gland were not benefited by exposure to ultra-violet radiation or 



sunshine. 



Thus Hou (37) was led to see that "... vitamin D or its precursor 

 is principally derived from the oil secretion rather than from the diet. 

 This may be more or less true for all birds. For although nocturnal 

 birds, and the carnivorous animals which prey upon other forms of 



