xii INTERNAL SECRETION AND DUCTLESS GLANDS 219 



CHAPTER XII 



INTERNAL SECRETION AND THE DUCTLESS GLANDS 



The idea of internal secretion was first brought into 

 prominence by Brovvn-Sequard, who found that extracts of 

 the testis of mammals when injected into the blood produce 

 a marked stimulating effect upon the organism. According 

 to this investigator, the testis produces some substance which 

 passes into the circulation. Such a process is termed inter- 

 nal secretion, in contrast to the production of substances 

 which are conveyed to the outside of a gland by a duct, 

 as in the secretion of saliva or bile. In recent years the 

 subject of internal secretion has become one of the most 

 important and fruitful fields of physiological research. 



All of the cells of the body give off substances into the 

 blood, or lymph, but only in a comparatively few cases has 

 any definite physiological function of these products been 

 discovered. Two important internal secretions, sugar and 

 urea, are formed by the liver, the former arising from the 

 glycogen which is stored in the hepatic cells. The pancreas 

 produces, in addition to the pancreatic juice, an internal 

 secretion which is of even greater importance to the organ- 

 ism. Removal of the pancreas from one of the higher 

 animals results in the production of diabetes, which soon 

 terminates fatally. In this disease there is an abnormal 

 amount of sugar in the blood ; under ordinary circumstances 

 the undue production of this substance is prevented through 

 the agency of some secretion which is given off from the 

 pancreas into the general circulation. If the duct of the 



