68 GLANDS; MUCOUS AND SEROUS MEMBRANES. 



of the secreting portion of glands is usually different from 

 that of the ducts. 



Secretory glands are of two kinds : closed or ductless glands ; 

 and the ordinary open glands with ducts. 



The secretory ductless glands are the thyroid, parathyroids, 

 adrenals, pituitary body, and pineal body. They have no 

 outlets nor ducts, and hence their secretions are not discharged 

 elsewhere except by reabsorption into the blood or lymph cir- 

 culation. The lymphoid and blood-glands are also " duct- 

 less." 



The open glands are the ordinary variety, as the kidney, 

 salivary glands, pancreas, with outlet-ducts through which 

 their secretions are discharged. 



In addition to these secreting bodies, certain endothelial 

 structures may be mentioned, as bursse, synovial sacs, etc., 

 which contain glairy lubricating fluids produced by the 

 endothelium also by a true secretory process. 



Internal and external secretions : The purpose and mechan- 

 ism of the ductless glands, closed as they are, have long been 

 obscure, but recent theories afford some explanation of the 

 subject. According to these theories, the secretions elaborated 

 by glands are of two kinds, internal and external. 



The external secretions are those of the ordinary kind, dis- 

 charged through the ducts of the glands and conveyed to the 

 places where their functions are carried out, as the tears, the 

 bile ; their action is easily comprehensible. 



The internal secretion, however, is not discharged by gland- 

 ducts, but is absorbed into the circulation and there serves 

 purposes useful in the animal economy. This is the only 

 kind of secretory activity which the ductless glands manifest. 

 The secretion of the thyroid, for instance, when in the blood, 

 increases oxidation, promoting metabolism, and increasing the 

 rapidity of cell-life ; stimulates cerebral activity, makes the 

 heart action more rapid, causes palpitation, and dilates the 

 bloodvessels. That of the suprarenals is a powerful vaso- 

 motor constrictor and probably serves to maintain vascular 

 tone. 



The effects of these internal secretions are most markedly 

 manifested when they are absent from the blood, as when the 

 glands producing them are removed or impaired by disease ; 



