39 THE PRESIDENTIAL ADDRESS. 
Experiments on rats have also shown that in animals fed on a rickets- 
producing diet, rickets does not occur if the rats are exposed regularly to 
sunlight or to the rays of the mercury lamp, or other form of artificial 
ultra-violet radiation ; whereas if they are kept in the dark, rickets does 
develop. If, on the other hand, the diet is complete in all respects, includ- 
ing abundance of fat-soluble vitamins, the animals do not develop the 
disease, even if kept constantly in the dark. 
How this is brought about is not known. At one time it was thought 
that the action of the ultra-violet rays on the tissues might enable the 
animal to synthesise fat-soluble vitamins, as it does in the tissues of plants, 
but recent evidence brought forward by Miss Margaret Hume in Vienna, 
and by Goldblatt and Soames at the Lister Institute, suggests that light 
can neither create nor act as a substitute for the vitamin. It seems rather 
to act as a stimulant, enabling the animal to make full and economical use 
of its store of fat-soluble vitamins, and when the store is used up growth 
ceases in spite of the continued action of the rays. 
An important and practical point in regard to the connection between 
diet and sunlight and the formation of the anti-rachitic vitamin is the 
relation to cow’s milk. Recent work carried out by Dr. Ethel Luce at 
the Lister Institute has shown that milk obtained from a cow on pasture 
in summer contains a sufficiency of the growth-promoting and anti- 
rachitic fat-soluble vitamins. In winter, on the other hand, if the cow is 
stall-fed and kept in a dark stable, the milk may become deficient in these 
respects and young animals fed on it may become rachitic. This work 
shows that the seasonal variation in quality of the cow’s milk may be an 
additional factor in the seasonal incidence of infants reared upon it. It 
also disposes of the idea, very current in some quarters, that cow’s milk 
possesses low and negligible anti-rachitic properties and that the anti- 
rachitic properties of cod-liver oil are specific and peculiar to that 
substance. 
Enough has been said to show that rickets may be regarded as a disease 
of sunless houses combined with a diet deficient in the anti-rachitic vitamin, 
and the means of prevention are sufficiently obvious, if not always easy 
and simple to carry out. 
Doubtless in the future this new knowledge in regard to the accessory 
food factors in diet will be used to a greater extent than it has been up to 
the present, in which case it is not too much to expect that the city children 
of some future generation will have better-grown bodies and stronger, 
healthier teeth than their predecessors of the pre-vitamin age. 
