CHAPTER XLVI. 



SECRETION OF THE DUCTLESS GLANDS-INTERNAL 



SECRETION. 



The term "internal secretion" is used to designate those secre- 

 tions of glandular tissues which, instead of being carried off to the 

 exterior by a duct, are eliminated in the blood or lymph. The idea 

 that secretory products may be given off in this way has long been 

 held in reference to the ductless glands, such as the thyroid, pitui- 

 tary body, etc., the absence of a duct suggesting naturally such a 

 possibility. The term, however, seems to have been employed 

 first by Claude Bernard, who emphasized the distinction between 

 the ordinary secretions, or external secretions, and this group of 

 internal secretions. Modern interest in the latter is due largely to 

 work done by Brown-Sequard (1889) upon testicular extracts, work 

 which itself was of doubtful value. This author was led to amplify 

 the conception of an internal secretion by the assumption that all 

 tissues give off a something to the blood which is characteristic, 

 and is of importance in general nutrition. This idea led in turn to 

 a revival of some old notions regarding the treatment of diseases 

 of the different organs by extracts of the corresponding tissue, 

 a therapeutical method usually designated as opotherapy. Brown- 

 Sequard's extension of the idea of internal secretion has not been 

 justified by subsequent work, and to-day we must limit the term 

 to definitely glandular tissues. Experience has shown, however, 

 that not only the ductless glands, but some at least of the typical 

 glands provided with ducts may give rise to internal secretions, 

 the pancreas, for example. In some of the ductless glands, on the 

 contrary, the existence or non-existence of an internal secretion 

 is still an open question. The work done since 1889 has, however, 

 demonstrated fully that some of the ductless glands play a role 

 of the very greatest importance in general nutrition, and this 

 knowledge has proved useful in widening our conception of the 

 nutritional relations in the organism and besides has found a valuable 

 application in practical medicine. The conception that certain 

 glandular organs may give rise to chemical products which on 

 entering the circulation influence the activity of one or more other 

 organs has recently found a fruitful application in the study of the 

 digestive secretions. The gastric and pancreatic secretins may 



793 



