280 THE INTERNAL SECRETIONS—1920 
terdependent; all act in a somewhat different manner, 
yet a balance normally exists between them. All are 
undoubtedly supplied with a double innervation from 
the involuntary nervous system. Stimulation of one 
member of the group automatically results in stimula- 
tion of the others. The thyroid, being most sensitive, 
responds most easily to vasodilating stimuli, and read- 
ily increases its secretion. Were it checked merely by a 
cessation of vasodilating stimuli or the registration of a 
superior vasoconstricting stimulus, it might pour too 
much secretion into the blood stream and sustain vaso- 
dilation to dangerous lengths. An excess of thyroid 
secretion in the blood stream, however, destroys the 
normal interglandular balance, and stimulates the an- 
tagonistic adrenal medulla and posterior pituitary to 
increase action. Hyperactivity of these glands in turn 
automatically inhibits thyroid activity, and their secre- 
tions thrown into the blood stream neutralize or inhibit 
the physiological action of the excess thyroid content 
therein, tend to overcome vasodilation, and reénforce 
vasoconstriction. 
Such would appear to be the logical mechanism of 
normal vasomotor control in the healthy individual. 
Normally there exists a perfect response of the glands 
of internal secretion to involuntary nervous stimuli, 
and a perfect balance of activity between such glands 
themselves. This normal response may be altered and 
the balance impaired, however, by continuous stimula- 
tion, by overexertion, by pronounced physical or men- 
tal fatigue, by disease, including focal infection, by se- 
vere nervous shock, or by the entrance into the circula- 
tory system of foreign protein matter. Rogers (7) 
suggests that the end-plates of the involuntary nerves 
are affected by fatigue, just as are those of the volun- 
tary system. As a voluntary nerve trunk may be irri- 
tated by repeated electric shocks until its functionation 
ceases because of fatigue, so presumably may the more 
