90 INTERNAL SECRETION 



atropine. v. Cyon believes that iodothyrin has a powerful effect 

 upon the irritability of the terminals of the vagi and of the 

 depressors, and that it also has an antagonism for atropine. v. 

 Cyon's findings have been confirmed by those of others (Boruttau, 

 Ocana, Besmertny, Kraus and Friedenthal, Coronedi), but 

 Harnack casts a doubt upon the antagonism of iodothyrin to 

 atropine and ascribes the restoration of vagal irritability to the 

 rapid passing of the effects of the atropine. Further investigation 

 showed (Fenyvessy, Isaac and van der Velden, v. Fiirth and 

 Schwarz) that the irritability of the apparatus which inhibits 

 cardiac activity is not affected by iodothyrin. v. Fiirth and 

 Schwarz also proved that the fall in blood-pressure which follows 

 the exhibition of thyroid extract is not specific. As they point 

 out, the thyroid gland, in common with many other organs 

 (suprarenals [Lohmann] ; brain [Vincent and Cramer] ; spleen, 

 thymus, lymph glands, intestinal glands, kidney, ovary, pancreas 

 [Gautrelet]), contains choline, a substance which is concerned in 

 the composition and decomposition of the lecithins and other 

 lipoid substances; and it is probable that the reduction in blood- 

 pressure which follows the exhibition of thyroid extract is due 

 to the presence of this substance. They believe, moreover, that 

 this effect of thyroid extract has no relationship to the physio- 

 logical activity of the thyroid gland. 



The direct and characteristic effect of iodothyrin upon the 

 circulatory apparatus is absent in dogs and rabbits, but is seen 

 to a very marked extent in cats. The intravenous injection of a 

 quantity of iodothyrin corresponding to 0.2 to 0.3 mg. iodine 

 is followed by a fall in pressure and by large slow vagus beats, 

 lasting for several minutes. These " Aktionspulse," as they are 

 called, result from stimulation of the vagus centre in the medulla, 

 for they disappear after resection of the vagi. The reduction in 

 pressure is due, not only to stimulation of the peripheral vessels 

 but, as oncometric experiments show, to a direct action upon the 

 heart itself. Further study of these phenomena shows, however, 

 that they are not specific to the action of iodothyrin, but are also 

 observed after the intravenous injection of iodized albuminoids. 

 v. Fiirth and Schwarz conclude that iodothyrin is a condensation 

 product of melanoid nature obtained by the action of acids from 

 the iodine albumin of the thyroid; and that its effects upon the 

 circulatory apparatus do not in any way suggest a specific activity 

 of the thyroid gland. 



In addition to the immediate effects of thyroid extract, there 

 are certain other phenomena which are produced by its continuous 

 exhibition. Hyperthyroidism has been frequently produced 

 experimentally, with a view to ascertaining its effects upon the 

 circulatory system, and a more or less marked tachycardia appears 

 to be the almost invariable result, v. Fiirth and Schwarz pro- 

 duced considerable tachycardia in dogs and cats by the sub- 



