4 io VETERINARY PHYSIOLOGY 



of the body, till about the age of puberty. After about 

 twenty-four years of age it atrophies and is replaced by a mass 

 of fatty tissue. The Hassall's corpuscles seem to atrophy 

 earlier than the lymph-like tissue. 



Castration in cattle and guinea-pigs markedly retards the 

 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 male genital organs. 



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

 is a transient 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. 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, ah increased 

 excretion of nitrogen, and a general emaciation. These 

 symptoms do not occur when the duct is tied or occluded 

 until degeneration 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. The sugar is formed from the 

 proteins, since it appears in amounts proportionate to the 

 amount of nitrogen excreted, after all the glycogen has been 

 removed. The pancreas seems therefore to form a hormone 

 which either controls the production of sugar in the liver or 

 causes its utilisation by the muscles. 



It has been suggested that adrenalin causes diabetes by 

 checking the production of this hormone in the pancreas. But 

 the facts that, in ducks and geese, in which removal of the 

 pancreas does not cause diabetes, the injection of suprarenal 

 extracts causes glycosuria, and that its administration increases 

 the output of sugar in dogs without a pancreas, are opposed to 

 the view that it acts through the pancreas, and suggests that 

 it must act directly on the liver or other tissues. 



Since the only respect in which the pancreas differs in 

 histological character from the parotid gland removal of which 



