394 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 193 2 



reach the cell nucleus. Small doses of light appear to have a stimu- 

 lating action, while large doses produce deferred physiologic changes, 

 and still larger doses may destroy unicellular organisms. Amebas 

 that are exposed to ultra-violet rays become strongly phagocyted by 

 other nonirradiated amebas.^** 



Melanophores in the scales of fish develop an increased irritability 

 with a small amount of ultra-violet radiation, while with larger doses 

 they develop a decreased irritability and eventually die. The con- 

 tractions are similar to those of smooth muscle. It has been shown 

 that an increase in tonus of involuntary muscle has taken place after 

 irradiation with ultra-violet; likewise skeletal muscle shows an in- 

 crease on such irradiation, these results running parallel to those 

 obtained with melanophores. The contraction of muscles under 

 ultra-violet appears to be due to changes in the muscle cell and not to 

 nerve stimulation. 



Cholesterol, a substance found in comparatively large quantities in 

 the skin, is activated chemically by ultra-violet irradiation. This has 

 been shown by isolating cholesterol and rendering it antirachitic by 

 irradiation with ultra-violet rays. It becomes endowed with anti- 

 rachitic power in an unknown way through the action of the ultra- 

 violet energy. It has been recently shown that pure cholesterol can 

 not be rendered antirachitic by the ultra-violet ray. The substance 

 which is made antirachitic appears to be ergosterol, or an allied sub- 

 stance which is found in ordinary cholesterol as an impurity. Like- 

 wise, phytosterol of plants is so activated. Many food products, such 

 as fats, oils, milk, vegetables and lettuce, may also become endowed 

 with antirachitic power through exposure to sources of ultra-violet 

 energy.-^ Ultra-violet irradiation of the pregnant mother renders 

 her milk antirachitic.^^ How this takes place is still a matter of 

 speculation; knowledge of the exact nature of radiation itself is so 

 limited that its effects on tissues or food products becomes doubly 

 difficult to understand. The greenness of many edible plants depends 

 on several environmental factors, and sunlight in particular."^ Vita- 

 min A appears to be associated in some way with the greenness; that 

 is, with the relative development of chlorophyll in the plant. Dry 

 seeds and etiolated plants are as a rule poor sources of vitamin A. 

 Mushrooms that thrive in darkness contain little of vitamin A. 



^ Mayer, Edgar, Clinical Application of Sunlight and Artificial Radiation, Baltimore, 

 Williams & Wilkins Co., 1929. 



" Sonne, Carl, Arch. Physical Therapy, vol. 10, p. 4, January, 1929. Hess, A. F., 

 Antirachitic Activity of Irradiated Cholesterol, Ergosterol and Allied Substances, Journ. 

 Amer. Med. Assoc, vol. 89, p. 337, July 30, 1927. Steenbock, Harry, and Black, A., Journ. 

 Biol. Chem., vol. 61, p. 405, September, 1924. 



=»Hess, A. F., Weinstock, Mildred, and Sherman, D., Journ. Biol. Chenr., vol. 66, p. 145, 

 November, 1925. 



="> Sherman, H. C, and Smith, S. L., The Vitamins, ed. 2, Now York, Chemical Catalog 

 Co., 1931. 



