376 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



Recently a new vitamin D with rickets-preventing power has been 

 obtained by irradiating a provitamin from sitosterol, the substance in 

 plants which corresponds to cholesterol in animals. 



Vitamin D is widespread in the animal kingdom but is abundant 

 only in the fishes. It has been suggested that it may be formed by 

 the action of the sun on the green algae, which are then swallowed by 

 the little fish and these in turn by the bigger fish. Evidence has 

 thrown some doubt on this supposition, however. Irradiation of 

 the body surface of fish appears to play no part in the formation 

 of vitamin D. Experiments performed by Bills made it appear 

 probable that a portion of the vitamin D in fish originates by syn- 

 thesis within the fish. The higher animals cannot synthesize vitamin 

 D, so they must either ingest it by eating such food as eggs, fish, 

 whole furred or feathered animals, and insolated dead vegetable 

 tissues, or receive it by exposing the body to sunlight. 



Birds, according to the findings of Hon, differ from mammals 

 in having only one gland of a sebaceous nature. This is the preen 

 gland. Preen-gland oil contains ergosterol, which the birds when 

 preening distribute over their feathers and expose to sunlight. The 

 vitamin D is then ingested either by swallowing the feathers or is 

 absorbed by the skin from the feathers. Normal birds have feath- 

 ers and skin that are antirachitic, but rickety birds or birds whose 

 preen glands have been removed have feathers and skin with little, 

 if any, antirachitic action. When the preen glands were removed, 

 the birds became susceptible to rickets, and rickety birds without 

 the preen gland received no benefit from exposure to ultraviolet ir"- 

 radiation or to sunshine. Nocturnal birds and the carnivorous ani- 

 mals that prey upon other forms of "feather and fur" possibly ob- 

 tain their vitamin D from their victims. The absence of the oil gland 

 in some birds may be explained in this manner, and it is then necessary 

 to add rabbits or small birds with their fur or feathers intact to the 

 diet of young birds in captivity. It is commonly known that horses 

 when thoroughly scrubbed with soap and water do not thrive. In 

 all these sources of the essential vitamin D it is noted that, with 

 the possible exception of fish, the origin of the vitamin is trace- 

 able to sterols that have been activated by ultraviolet light. The 

 skin absorbs the rapidly effective ultraviolet rays so strongly that 

 little, if any, radiation reaches the blood stream. 



Recent physiological experiments show that normal individuals 

 seem to have powers of compensation sufficiently great to counteract 

 any stimulation resulting from ultraviolet irradiation. 



Skin sensitivity to ultraviolet rays depends on the color of the skin 

 and hair, the age of the individual, and the time of the year when 

 exposure occurs. The pigmentation developing after irradiation 



