CHAPTER XIII 

 INTERNAL SECRETION 



Internal secretion is the elaboration by an organ of 

 a specific substance, which passes into the blood-stream 

 and exerts a stimulating or inhibiting effect upon some 

 function of the body. In some cases the formation of an 

 internal secretion is not the sole function of the organ. 

 The ovary and testes, for instance, in addition to forming 

 the morphological elements of reproduction, pour into the 

 blood substances upon the presence of which depend the 

 development of secondary sexual characteristics. The 

 duodenal epithehum, besides secreting the gastric juice 

 externally, secretes secretin internally. The pancreas not 

 only forms the pancreatic juice, but also secretes into the 

 blood, probably from the Islets of Langerhans, a substance 

 which regulates carbohydrate metabolism. 



In some organs the formation of an internal secretion 

 constitutes their only function. Of these there are three 

 of outstanding importance — the thyroid and parathyroid 

 apparatus, the suprarenal glands, and the pituitary body. 

 It is with these that we are mainly concerned in the present 

 chapter. 



The substances secreted are known as hormones. They 

 are not enzymes, for they are of much simpler constitution ; 

 they are dialy sable and are not destroyed by heat. Some 

 have been isolated, and one — ^adrenahn — can be prepared 

 synthetically. 



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