390 HUMAN PHYSIOLOGY 



onset of atrophy, so that the thymus of the ox is much 

 larger than that of the bull of the same age. Not only so, 

 but removal of the thymus in young guinea-pigs seems to be 

 followed by a more rapid growth of the testes. It is therefore 

 probable that the thymus yields an internal secretion which 

 controls the growth of the testes. 



The only other effect of its removal in young guinea-pigs 

 is a diminution in the number of leucocytes. This seems to 

 lead to a diminished power of resisting the invasion of those 

 micro-organisms e.g. staphylococci which are normally 

 combated by the leucocytes. 



7. Pancreas. That- the pancreas is the most important of 

 the digestive glands has been for long known, but a further 

 function has more recently been demonstrated. It has been 

 found that the excision of the pancreas in dogs and other 

 mammals produces a condition of diabetes an increase of 

 sugar in the blood, its appearance in the urine, an increased 

 excretion of nitrogen, and a general emaciation. In ducks 

 and geese this effect is not produced. These symptoms do 

 not occur when the duct is tied or occluded until degenera- 

 tion has developed, but they are invariable and immediate 

 when a sufficient amount of the gland is removed. They 

 are not prevented by the administration of pancreas, either 

 fresh or as extracts. In this condition, sugar is formed 

 from the proteids, since it appears after all the glycogen has 

 been removed, and its amount is proportionate to the amount 

 of nitrogen excreted. The pancreas seems therefore to form 

 something which either controls the production of sugar in 

 the liver or causes its utilisation by the muscles. Since the 

 only respect in which the pancreas differs in histological 

 character from the parotid gland removal of which has no 

 effect on the metabolism is in the presence of the islets of 

 Langerhans, it has been suggested that they are related to this 

 function of the organ. Some recent observations, however, 

 tend to show that these islets are not permanent structures, 

 but that they are formed from and revert to the ordinary 

 secreting tissue. The fact that, in ducks and geese in 

 which removal of the pancreas does not cause diabetes, 

 the injection of suprarenal extracts causes glycosuria, is 

 opposed to the view that it acts through the pancreas, 



