876 PHYSIOLOGY OF DIGESTION AND SECRETION. 



reaction. It is generally believed, however, that the blood does 

 receive constantly a supply of epinephrin from the adrenal glands 

 and that this constitutes the internal secretion of the medullary 

 cells. As regards the function of the epinephrin under normal 

 conditions there is much difference of opinion. The theory that 

 at first was commonly held was a natural deduction from the 

 known physiological effect of epinephrin. Since even in small 

 doses it causes a marked rise of pressure and since there is good 

 evidence that it is constantly being secreted into the blood this 

 theory assumed that its presence and action is essential for the 

 maintenance of normal arterial tone, and, indeed, for the tonic 

 activity of all muscles innervated through the thoracic sympa- 

 thetic nerves. In accordance with this view it was found that 

 after excision of the adrenals one of the symptoms that eventually 

 develops is a gradual fall in arterial pressure to the lethal point. 

 In recent years many of the workers in this field have abandoned 

 this view. While epinephrin causes constriction in some arteries, 

 it causes dilatation in others, those of the muscles, for example, 

 and when it is injected into the circulation in amounts comparable 

 to the concentration normally existing in blood the result is a 

 fall instead of a rise in arterial pressure. * Moreover, after excision 

 of the adrenals the blood-pressure remains normal for a longer 

 time than would be expected on the view that a constant secretion 

 of epinephrin is essential to its maintenance. An alternative 

 theory that has been proposed is that the secretion of epinephrin 

 constitutes a reserve mechanism that comes into action at times 

 of stress. Under the emotional excitement at such times there is 

 a reflex excitation of the sympathetic autonomic system, including 

 the nerve supply to the adrenal glands. The increased secretion 

 of epinephrin thus produced is of benefit in that it facihtates or 

 augments the response of the sympathetic nerves, giving a more 

 rapid heart-beat, a greater flow of blood to the muscles, central 

 nervous system, and heart, and an increased output of sugar 

 from the hver.f This view has been criticised on the ground 

 that conclusive proof has not been furnished in regard to an in- 

 creased secretion as a result of emotional excitement. It would 

 seem, therefore, that in spite of the great amount of experimental 

 work done on the subject there is at present no satisfactory ex- 

 planation of the physiological significance of epinephrin. Some 

 authors, indeed, go so far as to suggest that it is merely an ex- 

 cretory product. J 



* Hoskins and McClure, "Archives of Internal Medicine," 10, 343, 1912. 

 t See Cannon, "American Journal of Psychology," 25, 256, 1914. 

 i Gley and Quinquaud, "Journal de Physiol, et de Pathol, gen.," 17, 

 807, 1918. 



