CHEMICAL ASPECTS OF LIFE HOPKINS 139 



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 functions as revealed 

 by recent research, emphasizing 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 recogni- 

 tion of this, based at first on a couple of instances, began with this 

 century, but our knowledge of their nimiber and nature is still grow- 

 ing rapidly today. 



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 struc- 

 ture of the substance — thyroxin — which is directly responsible for 

 these effects. It is a substance of no great complexity. The con- 

 stitution of adrenalin has been longer known and likewise its remark- 

 able influence in maintaining a number of important physiological 

 adjustments. Yet it 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, coordinate the phenomena of sex. 



It is the circulation of definite chemical substances produced locally 

 that determines during the growth of the individual, the proper devel- 

 opment of all secondary sexual characters. The properties of other 

 substances secure the due process of individual development from the 

 unfertilized ovum to the end of fetal life. When an ovum ripens and 

 is discharged from the ovary a substance, now known as cestrin, is 

 produced in the ovary itself, and so functions as to bring about all 

 those changes in the female body which make secure the fertilization 

 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 

 fertilized. It is clear that these two hormones do not arise simul- 

 taneously, 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 centers in the brain 

 control and coordinate the activities of lower centers, so it would seem 

 do hormones, functioning at, so to speak, a higher level in organiza- 

 tion, coordinate the activities of other hormones. It is a substance 



