REGULATORY SUBSTANCES 509 



are best regarded as types of reactions rather than as actual 

 chemical entities. 



Present-day research on the enzymes is concerned primarily 

 with establishing their chemical nature, their mode of action, and 

 their specificity. Their mode of action has been suggested (page 

 502), though it is problematical. Linderstr0m-Lang has demon- 

 strated a regulatory influence of dipeptidase. He finds that it is 

 most concentrated in the meristem (the center of most active cell 

 division) of the root and rapidly falls off away from the meri- 

 stematic region. Dipeptidase is, therefore, an activator of mito- 

 sis and thus through its presence or absence becomes a regulator 

 of cell division, which is the basis of (proliferative) growth. 



HORMONES 



Endocrinology. — The name hormone was first applied to the 

 secretions of glands. The presence of substances that control 

 growth and regulate body functions had previously been demon- 

 strated by the botanist Sachs. All forms of cellular secretions 

 become hormones in so far as they function as regulatory sub- 

 stances. Secretin, from the mucosa of the duodenum, was the 

 first hormone to be so named. Insulin, from the pancreas, is one 

 of the best known hormones because of its function in maintain- 

 ing the normal metabolism of carbohydrates in the body; arti- 

 ficially injected into the body it becomes of great medical value in 

 the treatment of diabetes. 



Endocrinology is the science of the function of the endocrine 

 glands — the ductless, or internally secreting, glands — it consti- 

 tutes one of the most important fields of study in modern medical 

 research. One might almost say that what we are is determined 

 by our chromosomes and what we do, by our hormones. Our 

 bodies contain at least 12 recognized endocrine glands; they 

 include the pineal, pituitary, thyroid, parathyroid, adrenal, 

 ovarian, and testicular glands. Each elaborates a specific active 

 principle, or hormone. Hormones control definite chemical 

 reactions in the body, functioning as catalysts. 



One after another, these formerly mysterious substances are 

 being isolated, crystallized, and synthetically produced in the 

 laboratory, even on a commercial scale. The capacity of the 

 thyroid gland to maintain regulated growth in the body and to 



