278 THE INTERNAL SECRETIONS 1920 



internal organs and glands, is subject to vasodilation 

 under exactly the same conditions as the peripheral 

 circulatory system. Under vasodilative stimulation it 

 increases its discharge of secretion into the blood 

 stream. Vasodilation following stimulation of the in- 

 voluntary nervous system for a time most certainly is 

 continued and maintained, or at least reenforced, by 

 the stimulating effect of normal amounts of thyroid 

 secretion circulating in the blood; and this effect is 

 most probably exerted upon vasodilator nerve end- 

 plates direct. It appears, therefore, that the thyroid 

 is a selective adjuvant to the vasodilator mechanism 

 of the involuntary nervous system, capable of produc- 

 ing the same results as the nervous impulse, more 

 slowly developed, but of greater potentiality for con- 

 tinuous effect. In the production of this result may 

 be included also the secretion of the anterior of the 

 pituitary lobe, a vasodilating agent, but less potent 

 than the thyroid. 



The adrenal medulla and the posterior hypophysis 

 function after a similar manner, but with different re- 

 sults. Both extracts of the adrenal medulla and of the 

 posterior lobe of the pituitary body have power to cause 

 contractions of nonstriated muscle fibres, but the pitui- 

 tary principle appears to have a more pronounced con- 

 tractile effect upon the musculature of the internal or- 

 gans, while adrenalin possesses a selective contractile 

 effect upon the muscle fibres of the circulatory system. 

 Approximately three times as much pituitrin as adre- 

 nalin, administered intramuscularly or intravenously, 

 is required to produce equivalent results in connection 

 with the control of hemorrhage and the raising of 

 blood pressure. Applied locally, adrenalin is a far more 

 effective hemostatic or local constricting agent than 

 pituitrin. This would indicate with little question the 

 selective action or chromaffin substance secretion upon 

 blood vessels. 



