SECRETION. 265 



they do not return completely to the condition they were in before the first 

 pregnancy. 



Internal Secretions. 



According to the definition proposed on p. 211, the term internal secretion 

 is here used to mean a specific substance or substances formed within a gland- 

 ular organ and given off to the blood or lymph. As was said before, it is 

 difficult to make a distinction between these internal secretions and the waste 

 products of metabolism generally so far as method and place of formation 

 and elimination are concerned. Every active tissue gives off waste products 

 that are borne off in the lymph and blood, but as generallv emploved the 

 term internal secretion is not meant to include all such products, but only the 

 materials produced in distinctly glandular organs which are more or less 

 specific to those organs, and which are supposed to have a general value to 

 the body as a whole. The idea of an internal secretion seems to have been 

 suggested by Bernard, but was first seriously forced upon the attention of 

 physiologists by Brown-Sequard in the course of some work upon extracts 

 of the testis. Within the last few years the term has been frequently used, 

 especially in connection with the valuable and interesting work done upon the 

 pancreas and the so-called blood-vascular or ductless glands, the thyroids, 

 adrenals, pituitary body, and spleen. In almost all cases our knowledge of 

 the nature and importance of these internal secretions is in a formative stage ; 

 the literature, however, of the subject is already very great, and is increasing 

 rapidly, while speculations are numerous, so that constant contact with current 

 literature is necessary to keep pace with the advance in knowledge. 



Liver. — It has not been customary to speak of the liver as furnishing an 

 internal secretion, but two of the products formed within this organ are so 

 clearly known and their method of production is so typical of what is sup- 

 posed to be the mechanism of internal secretion, that it is desirable both for 

 the sake of convenience and consistency to include them under this general 

 heading. Glycogen (C 6 H 10 O 5 ) n is formed within the liver-cells from the 

 sugars and proteids brought to them in the blood of the portal vein, and in 

 many cases the presence of this glycogen can be demonstrated microscopically 

 within the cells. From time to time, however, the glycogen within the cell 

 is converted into dextrose by a process of hydration, 



C 6 H 10 O 6 + H 2 O = C 8 H 12 O 65 



and the sugar so formed is by a secretory process of some kind given off to 

 the blood to serve for the metabolism of the other tissues ..T the body, es- 

 pecially the muscles. This elimination of its stored glycogen on the part of 

 the liver may be regarded as a case of internal secret inn. | For further details 

 concerning glycogen, its properties and functions, see p. 326 and the section 

 on Chemistry.) A second substance which is formed under the influence 



of the liver-cells and is then eliminated into the bl 1 is urea. Urea constitutes 



the chief nitrogenous end-product of the metabolism of the proteid tissues ; it 



