360 THE HUMAN BODY. 



ing preceded by muscular weakness, dilatation of the arteries, 

 mental feebleness and general prostration. The exact role 

 played in the organism by these small but essential organs is 

 still unknown, but they form substances which have a pro- 

 found effect on the nerves of the heart and blood-vessels. 

 A very minute portion of the watery or alcoholic extract of a 

 suprarenal capsule when injected into avein of an animal causes 

 a very slow heart-beat, or even complete inhibition of the 

 auricles. If the cardio-inhibitory nerves have first been cut, 

 on the other hand, the injection causes a great increase in 

 the rate of heart- beat and a great increase of its force, espe- 

 cially that of the auricles. The small arteries become greatly 

 contracted, and this combined with the powerful heart-beats 

 leads to a very great increase of arterial pressure. The arterial 

 constriction is not due to stimulation of the vaso-con stricter 

 centre, but to a direct action on the muscular coats of the 

 arteries: it is very transient. The skeletal muscles are also 

 affected, the period of a simple muscular contraction being 

 greatly prolonged, and this effect lasts much longer than the 

 changes produced in the organs of circulation. The active 

 material exists only in the medulla of the adrenal, is efficient 

 in extremely minute doses, is dialyzable, and its efficacy is not 

 impaired by short boiling. 



It would appear then that the suprarenals are constantly 

 forming and passing into the blood minute quantities of a 

 substance which is of great importance for the maintenance 

 of the " tone " of the muscles, especially of the cardiac and 

 arterial muscles. Whether in addition they also remove 

 noxious substances from the blood, the accumulation of which 

 after their removal is one cause of the death which results, is 

 still undecided. The blood of such animals acts as a poison 

 to other animals, and this has been supposed to be due to the 

 presence in it of a specific poison which the adrenals normally 

 pick up and destroy: but it is clear that the blood of an ani- 

 mal dying from extensive malnutrition produced in any way 

 would be quite abnormal, ana might well be poisonous to other 

 animals. The same remark may be made as to the poisonous 

 character of the blood of animals dying as a result of removal 

 of the thyroid: there is no satisfactory evidence that it is due 

 to the accumulation of any one special toxic substance which 

 it is a function of the thyroid to remove: still, it maybe. 

 The symptoms produced by its injection are quite different 

 from those produced by injection of thyroid extract 



