THE PRESIDENTIAL ADDRESS II 
From substances produced temporarily and locally and by 
virtue of their chemical properties translating for the tissues the 
messages of nerves, we may pass logically to consideration of those 
active substances which carry chemical messages from organ to 
organ. Such in the animal body are produced continuously in 
specialised organs, and each has its special seat or seats of action 
where it finds chemical structures adjusted in some sense or other 
to its own. 
I shall be here on familiar ground, for that such agencies exist, 
and bear the name of hormones, is common knowledge. I propose 
only to indicate how many and diverse are their fuctions as revealed 
by recent research, emphasising the fact that each one is a definite 
and relatively simple substance with properties that are primarily 
chemical and in a derivate sense physiological. Our clear recognition 
of this, based at first on a couple of instances, began with this century, 
but our knowledge of their number and nature is still growing rapidly 
to-day. 
We have long known, of course, how essential and profound is 
the influence of the thyroid gland in maintaining harmonious growth 
in the body, and in controlling the rate of its metabolism. Three 
years ago a brilliant investigation revealed the exact molecular 
structure of the substance—thyroxin—which is directly responsible 
for these effects. It is a substance of no great complexity. The 
constitution of adrenalin has been longer known and likewise its 
remarkable influence in maintaining a number of important physio- 
logical adjustments. Yet is is again a relatively simple substance. 
I will merely remind you of secretin, the first of these substances 
to receive the name of hormone, and of insulin, now so familiar 
because of its importance in the metabolism of carbohydrates and its 
consequent value in the treatment of diabetes. ‘The most recent 
growth of knowledge in this field has dealt with hormones which, 
in most remarkable relations, co-ordinate the phenomena of sex. 
It is the circulation of definite chemical substances produced 
locally that determines during the growth of the individual, the 
proper development of all the secondary sexual characters. The 
properties of other substances secure the due progress of individual 
development from the unfertilised ovum to the end of feetal life. 
When an ovum ripens and is discharged from the ovary a substance, 
now known as estrin, is produced in the ovary itself, and so functions 
as to bring about all those changes in the female body which make 
secure the fertilisation of the ovum. On the discharge of the ovum 
new tissue, constituting the so-called corpus luteum, arises in its 
place. This then produces a special hormone which in its turn 
evokes all those changes in tissues and organs that secure a right 
destiny for the ovum after it has been fertilised. It is clear that 
