234 INTERNAL SECRETION 



or are morphologically differentiated, is a question to which these 

 observations of Dale's do not supply an answer. The results 

 included in Group 3 render the former supposition the more 

 probable; and these results are, moreover, evidence in favour of 

 Langley's theory of the existence of a " receptive substance." 



The results described above, together with extensive investi- 

 gation into the action of nicotine and of curare, suggested to 

 Langley that the cells possess two component parts: (i) a sub- 

 stance which performs the specific function of the cell, contraction 

 and secretion ; (2) receptive substances, which react to chemical 

 influences and nervous stimuli and which are able, in their turn, 

 to affect the metabolism of the cells. According to this view, 

 toxins such as nicotine, curare, atropine, pilocarpine, adrenalin, 

 and strychnine, which were believed to exercise an influence upon 

 the nerve terminals, do not act upon the terminals but upon a 

 constituent of the reacting cell. This theory supplies a final 

 answer to the question as to whether adrenalin acts directly upon 

 the muscle itself, as Langley at first supposed, or whether it acts 

 upon the sympathetic nerve endings. The myoneural junction 

 forms a portion of the receptive substance, and is situated in 

 the neighbourhood of the nerve terminal. The cells may contain 

 either stimulatory or inhibitory receptive substances, or they may 

 contain both ; the effect of nervous stimulation will depend upon 

 the extent to which both receptive substances are influenced by 

 the nervous impulse. 



According to Langley, not adrenalin only but other products 

 of secretion, such as thyroidin and the internal secretion of the 

 reproductive glands, react upon receptive substances in the cells- 

 which are not necessarily connected with nerve fibres. 



THE BY-EFFECTS OF ADRENALIN. 



In addition to the physiological effects already described,, 

 adrenalin exercises certain other influences, partly physiological 

 and partly toxic in character, which it is convenient to describe 

 as "by-effects." 



Such a physiological by-effect is the stimulation by adrenalin 

 of the vasomotor and vagus centres in the medulla oblongata, to 

 which reference has already been made. From what we now 

 know of the action of adrenalin, those phenomena which appear 

 after intravenous injection as the outcome of the stimulation of 

 certain nervous centres, are due, not to the direct action of the 

 substance itself, but to changes which it brings about in the 

 circulatory conditions. In regard to its action upon the vagus 

 and the consequent slowing of the pulse, there is no doubt that 

 the result is due to stimulation of the vagal centre in consequence 

 of the increased blood-pressure (Biedl and Reiner). The fact 

 that the slowing of the pulse is immediately relieved by resection 



