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. 
