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2 The Endocrine Organs 



parathyroids, adrenals, and pituitary, and even the pineal gland, that we 

 may, at least provisionally, exclude them from the class of bodies which 

 secrete active chemical agents into the blood for the purpose of influencing 

 other organs. It is to this latter class that I intend to restrict my remarks, 

 • and it is to them and them alone that the term endocrine will be applied 

 in these pages. 



It follows from what has just been said that by the expression endo- 

 crine organ we imply an organ which is known to form some specific 

 chemical substance within its cells, and to pass this directly or indirect^ 

 into the blood stream. 1 The substance thus formed is the active material 

 of the secretion, just as ptyalin is the active agent of the salivary secretion. 

 But while in the case of such glands as the salivary their secretion is con- 

 ducted by a duct to the exterior, in the case of the endocrine glands the 

 secreted material remains within the body and circulates with the blood ; 

 hence the term internal applied to their secretions. 



The expression internal secretion was originally used in a sense somewhat 

 different from that in which it is now applied, having been employed by Claude 

 Bernard to describe the grape sugar which, as he showed, is passed from the liver 

 cells into the blood. It has also been used to designate all materials which are 

 contributed to the blood by the tissues. In this sense the carbon dioxide and other 

 products of metabolism which are taken up by the blood in its passage through the 

 capillaries, or are received by it through the medium of the lymph stream, are 

 internal secretions, and every tissue is an internally secreting structure. It is, 

 however, convenient to restrict the term to secretions containing specific organic 

 substances such as the active chemical agents which are produced by certain 

 ductless glands, and this is the sense in which the expression will be employed 

 in this book (see also pp. 5 and 6). 



It is well known that the production of specific chemical agents which 

 are passed into the blood and carried by it to influence distant structures is 

 not confined to the ductless glands — that an active internal secretion may 

 be produced by other organs than these. A notable example is met with 

 in the pancreas, the more obvious function of which is the production and 

 excretion into the intestine of pancreatic juice ; which, by virtue of the 

 ferments it contains, is the most active agent in the digestion of foodstuffs 

 within the alimentary canal. It was shown, however, by v. Mering and 

 Minkowski in 1889, that the pancreas possesses an internal secretory function 

 which is of even greater importance in the economy than its long-recognised 

 external secretory activity. For by totally removing the pancreas in 

 animals, these observers proved that the presence of the gland, and of some 

 substance yielded by it to the blood, is essential to the proper utilisation of 

 carbohydrate material in the tissues, so that when the pancreas is totally 

 removed grape sugar is no longer stored in the liver, to be split up little by 



1 It must be clearly understood that although this passage of a specific substance into 

 the blood is an essential part of the doctrine of internal secretion, the definite proof of 

 such substances in the blood has only been furnished in very few cases. 



