INTERNAL SECRETION 509 



identical with extracts of the adrenal medulla itself. Thus it causes not only 

 a marked rise of blood-pressure but, according to the recent investigations of 

 Fulk and Macleod, in many mammals it inhibits the spontaneous contrac- 

 tions of isolated intestinal muscle and augments the tone and contractions of 

 the isolated virgin uterus. 



The Pancreas. The pancreas though engaged in the production of an 

 external secretion is yet, by reason of the specialized group of cells, the 

 islands of Langerhans, to be regarded as an organ of an internal secretion 

 as well. These islands it is generally believed are engaged in the secretion 

 of an agent which after entering the blood is carried to the muscles where it 

 activates or assists a glycolytic enzyme in promoting the oxidation of sugar; 

 or it may inhibit normally the stimulating action of adrenalin on the liver 

 cells and thus prevent an excessive output of sugar and the development of 

 hyperglycemia (see page 527). If the entire pancreas is extirpated and the 

 animal survive the operation, a glycosuria is soon established, followed by a 

 series of symptoms resembling those observed in diabetes mellitus as it occurs 

 in man, viz.: thirst, polyuria, loss of energy, decline in body-weight, etc., 

 followed by death in a few weeks. Pathologic processes that involve a large 

 portion of the pancreas likewise give rise to a similar series of phenomena. 

 As ligation of the pancreatic duct leads to a destruction of all portions of 

 the pancreas except the islands of Langerhans without the development of 

 glycosuria the inference has been made that these islands are the agents 

 engaged in the production of the internal secretion. 



The Testicles and Ovaries. The testicles and ovaries are regarded at 

 the present time as glands for the production of an internal secretion, as well 

 as for the production of the characteristic reproductive elements. The 

 testicles possess in the epithelial cells lining the peripheral portions of the 

 seminiferous tubules an apparatus for the development of spermatozoa, and 

 in the cells in the connective tissue between the tubules the so-called inter- 

 stitial cells an apparatus for the production of the internal secretion which 

 is discharged into the blood, and favorably influences the development of the 

 secondary sexual characters, as well as the body as a whole. This general 

 view is based on facts such as the following: 



The removal of the testicles early in life and before the age of puberty 

 leads to imperfect development of the vesiculae seminales and the prostate 

 gland; in addition to these defects, there is a failure of development of the 

 various and distinctly sexual characters peculiar to man and other animals as 

 well. Sexual desire is wanting and the body frequently remains in the in- 

 fantile state. In some instances, however, there is an increase in the bones of 

 the skeleton and hence the animal increases in size. It has been suggested 

 that the internal secretion controls the growth of the bones by antagonizing 

 the action of some other agent, as the secretion of the anterior lobe of the 

 pituitary, which promotes the growth of the skeleton. That these results are 

 not due to the loss of the structures producing the sperm elements, is shown 

 by the fact that ligation of the vas deferens only, while destroying these 

 structures, does not interfere with development of the sexual characters. 

 Transplantation of the testicles, in cocks and in certain of the smaller mam- 

 mals that have been castrated, has led to the development of secondary 

 sexual characters which in no apparent way differed from those of control 

 animals. In old age the testicles undergo degenerative changes characterized 



