B. A. HOUSSAY 



REGULATION OF EACH GLAND 



Each endocrine gland has its own regulating mechanisms for 

 the secretion of its hormones. As judged by experiments on extirpation 

 and restitution, there must be a basal secretion, generally uninterrupted. 

 In certain instances this has been well demonstrated (adrenalin, insulin). 

 This secretion is submitted to regulating factors, the principal ones being 

 humoral, and in some cases also nervous. Thus the basal amount of 

 insulin secreted by the islets of Langerhans depends on the blood sugar 

 level; and, reciprocally, the blood sugar level depends on the amount 

 of insulin secreted. The basal secretion of insulin increases when 

 glycemia increases; conversely, it decreases when the blood sugar level 

 is lowered. The parathyroid secretion increases when calcemia de- 

 creases. In both cases the secretion of either insulin or parathormone 

 tends to restore the altered equilibrium of the internal medium. A 

 similar regulation of the secretion of the thyroid hormone is probable, 

 udging by the constancy of basal metabolism. 



The regulation of mechanisms of hormone secretion are very pre- 

 cise and well designed to attain their objective. Thus, one-seventh of 

 the pancreas is adequate to maintain the basal glycemia at the normal 

 level; also, the basal glycemia continues at a normal level in an animal 

 even if four pancreases are grafted by vascular anastomosis. These 

 facts show that a normal secretion of insulin is maintained either with 

 one-seventh of the pancreas or with five pancreases — this because gly- 

 cemia governs insulin secretion. Only in abnormal cases (diabetes or 

 hyperinsulinism) is regulation of insulin secretion deficient or excessive 

 so that the gland works at a new level. In some cases of hyperplasia, 

 adenomata, or cancers, the endocrine glands have been known to pro- 

 duce hormonal hypersecretion. 



Although the reduction in mass of an endocrine organ may 

 not alter its ability to function under basal conditions, it may lead to its 

 insufficiency in cases of emergency. The pancreas of the dog reduced 

 to one-fifth of its mass is enough to maintain normal glycemia, but if 

 grafted to a diabetic dog it does not replace a normal pancreas in cor- 

 recting the existing hyperglycemia. The resistance of the surgically 

 reduced pancreas is diminished against the action of injurious agents 

 such as extracts of the anterohypophysis and thyroid and, in conse- 

 quence, diabetes develops readily. When the pancreas is normal, these 



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