Prevention of Rickets in Rats by Sunlight. 47 



the shafts seemed to be complete. Moreover, as the result of 

 the gross and histological examinations made on the rats fed the 

 rickets-producing diet 3,143 but exposed to sunlight it is possible 

 to say that the changes produced by sunlight in the skeleton do 

 not differ in any important respect from the changes produced 

 when the animals are kept in room light but on a diet supplemented 

 by cod-liver oil. Cod-liver oil contains something which is 

 essential for optimal cellular function. Light also contains some- 

 thing which is essential for optimal cellular function. Cod-liver 

 oil or light when made available to an organism previously de- 

 prived of either permits the organism to put into successful opera- 

 tion adaptations or defense mechanisms which otherwise would 

 have been ineffectual. Neither cod-liver oil nor light meets the 

 defects in the composition of the diet directly by supplying to 

 the body either calcium or phosphorus but meets them indirectly 

 by so raising the potential of cellular activity as to secure the 

 most efficient utilization possible of those substances available 

 in the body which are directly or indirectly concerned with ossifica- 

 tion and calcification. 



