276 THE INTERNAL SECRETIONS—1920 
contraction of blood vessels (peripheral vasoconstric- 
tion), and a resultant increased blood pressure. 
The simplest and most common examples of general 
vasomotor nervous control are those of blushing and 
paling. These phenomena are unquestionably the re- 
sult of nervous stimulation, via the involuntary sys- 
tem, of the nonstriated muscle fibres existing in the 
peripheral blood vessels. A mild stimulus, such as a 
sense of embarrassment, produces a dilation of these 
vessels, with a resultant flushing of the skin or blush- 
ing. A sharper stimulus, such as fright, produces vaso- 
constriction, or paling instead. The route taken by 
each type of stimulus of necessity must be via the 
involuntary nervous system, as the result follows too 
quickly after the cause for an intermediary to exist. 
It is definitely known, also, that the secretion of 
the thyroid gland is capable of producing vasodilation 
when taken by mouth, or administered hypodermically, 
intramuscularly, or intravenously. Its action is via 
the blood stream and hence must be exerted directly 
upon the end plates of the dilating fibres of the invol- 
untary system. Hyperthyroid cases exhibit vasodila- 
tion as one of their most strongly marked clinical 
manifestations. 
Here, then, are two outstanding causes of general 
vasodilation: (a) stimulation of vasodilating fibres 
through the central nervous system direct, and (b) 
the thyroid secretion via the blood stream. The ac- 
tion of both is exerted on the nerve end-plates of the 
nonstriated musculature of the blood vessel walls. 
Stimulation of the involuntary nervous system causes 
vasoconstriction as well; but vasoconstriction is a 
property long since proven to belong also to the in- 
ternal secretion of the adrenal medulla and of the pos- 
terior lobe of the hypophysis or pituitary body. There 
would appear to exist between the adrenal medulla, 
the posterior pituitary lobe, and the involuntary nerv- 
