INTERDEPENDENCE OF ORGANS 65 



and this fact suggested that the disease is due to the failure 

 of the thyroid to perform its normal functions. 



The subject was further cleared up by the discovery that 

 after the removal of the thyroid in a lower animal the disease 

 in question could be prevented by feeding the animal thyroids 

 or even by giving to it a certain substance extracted from 

 them. Evidently the thyroid manufactures and discharges 

 into the blood a peculiar substance necessary to the healthy 

 life of the cells of the body ; and when the gland fails to 

 manufacture this substance it can still be supplied artifi- 

 cially by introducing it into the blood by absorption from 

 the alimentary canal. 



3. Internal secretions. In our study of secretion in Chap- 

 ter IV (p. 43) we dealt only with glands which discharge 

 their principal products through a duct into some part of the 

 alimentary canal; such glands are the salivary glands, the 

 pancreas, and the liver. Other glands send ducts to the surface 

 of the body for example, the sweat glands, which discharge 

 perspiration upon the skin ; and the lachrymal glands, which 

 discharge the tears on the eyeball. In the case of the thyroid, 

 on the other hand, we have an example of an organ which, 

 like those just mentioned, manufactures a special substance 

 from the blood, but, having no duct, contributes the products 

 of its manufacture to the blood, for the use of other cells. 

 This process is spoken of as internal secretion, to distinguish 

 it from ordinary secretion, in which case something is dis- 

 charged on a free surface like the skin or into the alimentary 

 canal, the nasal cavity, or the air passages. 



4. The adrenal glands. Lying immediately above the kid- 

 neys are two small glandular organs, the adrenals, which, like 

 the thyroid, were formerly considered of minor importance. 

 It has been shown, however, that these also contribute to 

 the blood a most important internal secretion known as 

 adrenaline. This substance is manufactured by the gland cells 

 and, during their periods of inactivity, is stored within the 



