12 THE PRESIDENTIAL ADDRESS 
these two hormones do not arise simultaneously, for they must act 
in alternation, and it becomes of great interest to know how such 
succession is secured. The facts here are among the most striking. 
Just as higher nerve centres in the brain control and co-ordinate the 
activities of lower centres, so it would seem do hormones, functioning 
at, so to speak, a higher level in organisation, co-ordinate the activities 
of other hormones. It is a substance produced in the anterior portion 
of the pituitary gland situated at the base of the brain, which by 
circulating to the ovary controls the succession of its hormonal 
activities. "The cases I have mentioned are far from exhausting the 
numerous hormonal influences now recognised. 
For full appreciation of the extent to which chemical substances 
control and co-ordinate events in the animal body by virtue of 
their specific molecular structure, it is well not to separate too widely 
in thought the functions of hormones from those of vitamins. 
Together they form a large group of substances of which every 
one exerts upon physiological events its own indispensable chemical 
influence. 
Hormones are produced in the body itself, while vitamins must be 
supplied in the diet. Such a distinction is, in general, justified. 
We meet occasionally, however, an animal species able to dispense 
with an external supply of this or that vitamin. Evidence shows, 
however, that individuals of that species, unlike most animals, can 
in the course of their metabolism synthesise for themselves the 
vitamin in question. The vitamin then becomes a hormone. In 
practice the distinction may be of great importance, but for an 
understanding of metabolism the functions of these substances are 
of more significance than their origin. 
The present activity of research in the field of vitamins is prodigious. 
The output of published papers dealing with original investigations 
in the field has reached nearly a thousand in a single year. Each 
of the vitamins at present known is receiving the attention of 
numerous observers in respect both of its chemical and biological 
properties, and though many publications deal, of course, with matters 
of detail, the accumulation of significant facts is growing fast. 
It is clear that I can cover but little ground in any reference to 
this wide field of knowledge. Some aspects of its development have 
been interesting enough. The familiar circumstance that attention 
was drawn to the existence of one vitamin (B, so called) because 
populations in the East took to eating milled rice instead of the 
whole grain; the gradual growth of evidence which links the 
physiological activities of another vitamin (D) with the influence of 
solar radiation on the body, and has shown that they are thus 
related, because rays of definite wave-length convert an inactive 
precurser into the active vitamin, alike when acting on foodstuffs 
