200 THE HUMAN BODY 



diseased condition results, death taking place in warm-blooded 

 animals within two or three days, and being preceded by muscu- 

 lar weakness, dilation of the arteries, mental feebleness and general 

 prostration. 



These symptoms show that the hormone is essential to life, al- 

 though they do not afford any very positive evidence as to the 

 manner of its working. Careful studies have shown that adrenin 

 is present in the blood under ordinary circumstances in almost 

 inconceivably minute amounts. A striking feature of this, and of 

 hormones in general, is their remarkable potency as chemical stim- 

 ulants. Our detailed knowledge of the functioning of adrenin 

 has been gained chiefly by observing the results of its introduction 

 into the blood in larger than normal amounts. The Body responds 

 to these enlarged doses by a considerable number of very definite 

 reactions which, when first observed, seemed to be quite unrelated, 

 but are now recognized as combining to bring about a particular 

 bodily condition, and one which, as we shall see, is sometimes of 

 great importance to the organism. Not all these effects of adrenin 

 can be described in this place, some will have to be deferred to 

 later chapters; but enough can be presented to make clear the 

 significance of its action. 



One of the properties of adrenin is to stimulate chemically the 

 terminations of the thbracico-lumbar autonomic system. It is 

 thus able to bring about the same bodily reactions as are called 

 forth through thoracico-lumbar autonomic activity. Dilation of 

 the pupil, acceleration of the heart, constriction of the blood-ves- 

 sels, with consequent heightened blood pressure, all are brought 

 about by the injection of adrenin into the Body. These manifesta- 

 tions, as we saw above, are part of what we have described as the 

 emergency reaction of the Body, and the ability of adrenin to bring 

 them about reveals its function as the emergency hormone. The 

 emergency reaction is so vital in time of stress that the Body does 

 not depend wholly on the nervous system to evoke it. The action 

 of the thoracico-lumbar autonomies is reinforced by the chemical 

 stimulation of adrenin. This adrenin action depends, as we have 

 seen, on the presence in the blood of larger than normal amounts of 

 the hormone. The adrenal bodies are under the control of nerves 

 which form part of the thoracico-lumbar system. Whenever, in a 

 time of excitement, there is an outrush of impulses over this sy&- 



