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In order to comprehend the point of attack of adrenalin, the specific 

 secretion of the medullary part of the suprarenal glands, we shall do well 

 to go back to the mode of development of these organs. It was shown 

 by Balfour that the suprarenals have in the foetus a two-fold origin, the 

 cortex being derived from the mesoblastic tissue, known as the inter- 

 mediate cell-mass, while the medulla is formed by a direct outgrowth 

 from the sympathetic system, and consists at first of an aggregation of 

 neuroblasts. In some animals e.g., teleostean fishes the two parts of 

 the gland thus formed remain separate throughout life, but in the higher 

 vertebrates the sympathetic outgrowth becomes surrounded by the cortex, 

 and the cells rapidly lose all traces of resemblance to a nerve cell. But 

 the medulla is genetically part of the sympathetic system, and its specific 

 secretion, adrenalin, has an action which is apparently confined to the 

 sympathetic system. In whatever part of the body we test the effects of 

 adrenalin, we find that they are identical with the results of stimulating the 

 sympathetic nerve fibres which run to that part. Thus, in all the blood- 

 vessels of the body, adrenalin causes constriction ; the contraction of the 

 heart muscle is augmented, the pupil is dilated, while the intestinal 

 muscle, with the single exception of the small ring of muscle forming the 

 ileo-colic sphincter, is relaxed. The action of the sympathetic on the 

 bladder differs, as shown by Elliott, markedly in various animals ; but, 

 whatever its effect, a similar one will be produced in the same animal by 

 the injection of adrenalin. I have already mentioned that excision of 

 the suprarenal bodies causes a profound fall of blood pressure, which 

 continues until the death of the animal, and it has been stated that, when 

 this fall is well established, it is impossible to raise the blood pressure by 

 stimulation of the splanchnic nerve, or indeed to produce any effect at 

 all on stimulation of the sympathetic nerve. Thus, not only does adrenalin 

 excite the whole sympathetic system in its ultimate terminations, but its 

 presence in the body as a specific secretion of the suprarenal bodies 

 seems to be a necessary condition for the normal functioning, by ordinary 

 reflex means, of the whole sympathetic system. We are dealing here 

 with a problem which, betraying, as it does, an intimate relationship 

 between nerve excitation and excitation by chemical means, promises by 

 its solution to throw a most interesting light on the nature of the nerve 

 process and of excitatory processes in general. 



Our knowledge of certain other members of this group of chemical 

 reactions is so shadowy that a mere mention of them will suffice. As an 

 antithesis to the vaso-constrictor action of adrenalin, we find that every 

 organ, when active, is supplied with more blood in consequence of a vaso- 

 dilatation of the vessels which supply it. In certain instances Bayliss 

 and I have found that boiled extracts of organs, when injected into the 

 circulation, may evoke vaso-dilatation of the same organs of the animal 

 under investigation, and we have suggested that the normal vaso-dilata- 

 tion accompanying activity is brought about in consequence of the 

 specific sensibility of the arterial walls to the metabolites of the organ 

 which they supply. Too much stress, however, cannot be laid upon 



