314 PHYSIOLOGY 



At any rate, the action of curare and of nicotine on these peripheral 

 ganglia is very similar to their action on the skeletal end-plates, 

 nicotine, however, having a relatively stronger action than curare. 

 Injection of nicotine stimulates and then paralyses the peripheral nerve- 

 cells of the visceral system ; curare in sufficiently large doses para- 

 lyses them. More instructive in relation to the presence of receptor 

 substances is the action of adrenalin. This substance, which is pro- 

 duced by the medulla of the suprarenal glands, has a specific action 

 on all tissues innervated by the sympathetic system. It causes almost 

 universal constriction of the blood-vessels, dilatation of the pupil, 

 acceleration of the heart, and inhibition of the intestinal muscles, with 

 the exception of the ileo-colic sphincter, which it causes to contract, 

 all of which effects can also be produced by stimulation of branches 

 of the sympathetic nerve. On the other hand, tissues which 

 are not innervated from the sympathetic, such as the blood-vessels 

 of the lungs, are unaffected by the drug. This fact, together with the 

 opposite effects of adrenalin on different unstriated muscles, shows 

 that its action cannot be a direct one on muscle-fibre. It presents a 

 marked contrast, for instance, to barium salts, which produce a con- 

 traction of every unstriated muscle-fibre in the body. On the other 

 hand, we cannot ascribe this action to a stimulation of the sympathetic 

 nerve-endings, since adrenalin is equally effective if applied after the 

 whole of these nerve-endings have been made to degenerate by section 

 of the post-ganglionic sympathetic nerve-trunks. Its action therefore 

 must lie at the junction between nerve and muscle, and must be on 

 some intermediate or receptor substance developed at the myoneural 

 junction, and having for its function the transference of the excitatory 

 process from the nerve fibre to the contractile substance of the muscle 

 fibre. Similar receptor substances may act as intermediaries in 

 every case of propagation of an impulse across a synapse of whatever 

 description, and may by their properties determine the peculiar 

 qualities of the synapse. We may compare them to the fulminating 

 cap which in a shell is used to transfer the process of combustion from 

 the slow-match to the bursting charge. Their existence is of especial 

 importance when we endeavour to investigate the mode of action of 

 drugs. It is probable that they will be found to play a great part in 

 determining the differential action of drugs on various tissues in the 

 bodv. 



