SECRETIONS 



97 



There are a number of glands which do not have ducts by which 

 to discharge their secretions, but which pass their products into 

 the blood stream. Such glands are known as the ductless glands, 

 glands of internal secretion, or endocrine glands; and their secre- 

 tions are called internal secretions, or endccrines, tjecause they are 

 never passed out on surfaces such as the digestive canal or skin 

 (Fig. 54). A well-known example of the way in which endo- 

 crines function in coordinating bodily processes is the control 



Alvioli 



Fig. 53. — Diagram of a gland with a duct which passe.s to some surface where 



the secretion is di.schargcd. 



(From Hough and Sedgwick, "Human Mechanism," copyright, 1918, by Gion and Co., 

 reprinted by permission.) 



of the flow of pancreatic juice. This fluid is not passed into the 

 small intestine continually but only when food is present to be 

 digested. It was for a long time supposed that this was due to 

 nervous control. Experimentation revealed the fact that the 

 nerves leading to the pancreas could be cut without affecting the 

 control of flow of the pancreatic juice. It was then discovered 

 that if the blood vessels were tied so that blood did not flow into 

 the pancreas the digestive juice was not discharged when food 

 entered the intestine. Further study revealed that when the 



