CH. XI] ORGANS OF INTERNAL SECRETION 141 



other hormones of the body may have arisen in cvokitionary 

 development as waste products, and that the organs and tissues 

 on which they now act may have only gradually learnt to respond 

 to their presence in the progress of phylogeny (i.e. the develop- 

 ment of the race). However this may be the functional corre- 

 lation existing between certain often distantly situated organs 

 has become very perfect as the examples already given and 

 described below sufficiently demonstrate. 



The Pancreas. It has been shown by von Mering and 

 Minko\\sky that extii-pation of the pancreas is followed by the 

 appearance of sugar in the urine even though carbohydrate is 

 excluded from the diet. At the same time the quantity of urine 

 excreted tends to increase, the percentage of urea is greater, 

 acetone makes its appearance, there is an abnormal hunger and 

 thirst, and these symptoms are associated or followed by emacia- 

 tion endmg in death. Retention of one-fourth or one-fifth of 

 the total normal amount of gland tissue is sufficient to prevent 

 glycosuria. Moreover, the connection of the pancreas with the 

 duodenum may be cut off, and yet there is no glycosuria. It 

 has been stated also that grafting of pancreatic tissue under the 

 skin may stop these symj)toms from appearing after the removal 

 of the whole gland from the normal position. Ligature of the 

 pancreatic duct does not cause glycosuria, though the gland 

 exceptmg for the islets of Langerhans undergoes atroph3\ It 

 has been concluded therefore that the pancreas elaborates an 

 internal secretion, probably in the islets of Langerhans, which 

 in some way regulates the gh^cogenic function of the liver, and 

 that without this secretion the liver is no longer able to store up 

 glycogen but discharges it as sugar into the blood whence it is 

 excreted by the kidneys. The pancreas therefore, is an example 

 of an externally secreting gland (i.e. a gland provided with a duct) 

 which is at the same time an organ of internal secretion, and this 

 secretion is essential for the maintenance of the life of the animal. 



The Thyroid. The thjToid proper is represented by two oval 

 bodies Ij'ing one on each side of the trachea at its junction with 

 the larynx. It is composed of vesicles filled with a colloid sub- 

 stance and bounded by a cubical epithelium, the vesicles being 

 separated by connective tissue containing vessels. 



Schiff found that extirpation of the thyroid in dogs led to 

 death m from one to four days Previous to death the animals 



