CHAPTER XIX. 

 THE SECRETORY TISSUES AND ORGANS. 



Definitions. .In its broad etymological meaning a secre- 

 tion is any substance separated or derived from the blood, so 

 that in a certain sense all the solid tissues of the Body, built 

 up from materials supplied by the blood, are secretions. In 

 practice the name has a more limited application and is given 

 to two classes of substances, distinguished as true or external 

 secretions and internal secretions. 



Internal secretions are the results of the vital activities of 

 various organs, their by-products, passed out directly into 

 the lymph and blood; and in many cases are simple wastes, 

 sent to the blood-stream for conveyance to other organs which 

 get rid of them: such, for example, is the carbon dioxide 

 formed in every part of the Body. In other cases the by- 

 products of certain organs, after absorption into the blood, 

 have to be further changed in a second organ before elimina- 

 tion, and are probably of use to this second a part of its 

 pabulum: as an instance we may take leucin (amido-caproic 

 acid), which is formed in many organs and, given by them, to 

 the blood, is carried to the liver, the cells of which convert it 

 (or at least a great part of it) into urea,' to. be subsequently 

 eliminated by the kidneys. A third very important class of 

 internal secretions consists of substances form-ed only in one 

 organ or one pair of organs and yielded by them to the blood 

 which flows through them, the presence of which substances 

 in the blood is essential to the healthy nutrition and the con- 

 tinuance of the life of the Body : in such cases removal or 

 extensive disease of the producing organ results in death. 

 Examples are to be found in substances which the thyroid 

 body and suprarenal capsules produce; they will be consid- 

 ered more fully in Ch&pter XXIII. 



Excluding such things as cast hairs and epidermic scales, 

 the true or external secretions may be denned as gases or 

 liquids, often of very complex composition, passed out on 



