ing to arouse or stir up. Later an American physiologist showed that secretin 

 consists of at least two different substances, one of them acting on the 

 pancreas, the other on the gall bladder. 



What Ductless Glands Are There? 



Modern Humors No scientific physician today takes the four humors 

 seriously. For one thing, the number is much too small. Instead of going 

 back to Hippocrates, we now watch a growing list of hormones, specific 

 chemical substances that influence metabolism. The fluids produced by the 

 ductless glands are also called internal secretions, or endocrines. Each of 

 these hormones is distinct chemically, and it is distinct in the effects which 

 it produces in the body; but all have certain features in common: (1) they 

 change the rate of metabolism in various cells or tissues; (2) they originate 

 in specific tissues; (3) they are rapidly distributed by means of the blood; 

 (4) they produce effects, although present in amazingly small quantities. 



Hormones do not supply fuel; yet they may determine the rate at which 

 energy is released, or whether oxidation takes place at all. Hormones do 

 not supply building material for protoplasm, but they may influence the 

 rate of assimilation, the growth of cells, and the growth of tissue. Some of 

 them arouse chemical actions but others may repress or retard them. An 

 extremely small quantity of a particular hormone may at any moment 

 determine the issue between life and death. 



The Endocrine System^ At present at least eight or nine distinct struc- 

 tures have been sufficiently studied to be classed as hormone-producers. Be- 

 sides the ductless glands shown in the diagram on the opposite page, the re- 

 productive organs, or gonads (ovaries and testes), also produce hormones in 

 addition to the reproductive cells (see page 379). Scattered throughout the 

 body are small groups of cells that behave like some of the endocrine tissues. 

 Although every one of the endocrines is distinct, each may reinforce or 

 counteract one or several of the others. Taken together, therefore, they 

 behave like a unified system; and as each hormone influences the various 

 organs and tissues and processes in a distinct way, the endocrines play an 

 important role in unifying the several parts of the entire organism. 



One interesting feature of the endocrines is their great similarity in all 

 mammals and probably in all vertebrates. This has made it easier to carry on 

 research by trying out our problems and hypotheses on various smaller ani- 

 mals, and also to make use of new discoveries. If a human being is deficient 

 in the pancreatic hormone, for example, the shortage can be made up by 

 using extracts from the pancreas of a sheep or an ox or a pig. 



iSee No. 1, p. 320. 

 304 



