CHAPTER XXX 



INTERNAL SECRETIONS AND THE DUCT- 

 LESS GLANDS 



We have already given several examples of ordinary 

 secretion in which a gland pours out certain substances to 

 the outside through a duct. While salts, urea and other 

 materials are taken as such out of the blood and passed 

 through the glands unchanged, in many other cases the 

 substances that are discharged are manufactured by the 

 glands themselves. Such substances are found in the 

 saliva, gastric juice and bile. Hydrochloric acid and 

 pepsin do not occur as such in the blood, but are made in 

 the cells of the gastric glands. Secretion in these cases, 

 therefore, does not consist merely in filtering out materials 

 that are present in the body, but in the formation and 

 discharge of new compounds. There are many cases in 

 which the compounds formed by an organ are not dis- 

 charged to the outside but are given off into the blood. 

 This process is known as internal secretion. We have 

 already met with one example of this in the urea which is 

 formed in the liver out of various products of protein 

 metabolism and given off into the blood to be eliminated 

 from the body by the kidneys. This substance, there- 

 fore, is an internal secretion of the liver and an external 

 secretion of a quite different organ. 



Many organs which produce internal secretions have no 

 outlet and hence are known as ductless glands. The func- 

 tion of most of the ductless glands was for a long time un- 

 known, but it is now well established that some of these 



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