278 



PHYSIOLOGY 



Here the nerve fibres leaving the central nervous system do not pass direct 

 to the muscle fibres, but end in arborisations round ganglion-cells, which 

 are collected to form the ganglia of the sympathetic chain or ganglia situated 

 more peripherally and nearer the reacting tissue. Relays of fibres, for the 

 most part non-medullated, arise from these ganglion- cells and pass to the 

 unstriated muscles of the blood-vessels and viscera, where they end in 

 plexuses or networks among the muscle fibres, possibly connected by short- 

 branches with the fusiform muscle fibres themselves. No structure is 



FIG. 123. 



present at the periphery exactly analogous to the end-plate, and it is possible 

 that, as Elliott suggests, the end-plate is really homologous with the whole 

 of the sympathetic ganglion with its post-ganglionic fibres passing to the 

 visceral muscles. 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. In- 

 jection of nicotine stimulates and then paralyses the peripheral nerve-cells 

 of the visceral system ; curare in sufficiently large doses paralyses them. 

 More instructive in relation to the presence of receptor substances is the 

 action of adrenalin. This substance, which is produced 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. The opposite effects of 

 adrenalin on different unstriated muscles show that its action cannot be a 

 lin-ct one on nmsclc-Jiluv. Jt presents a marked contrast, for instance, to 

 ImriuiM salts, which produce a contraction of every unstriated muscle-fibre 

 in the body. On the other hand, we cannot ascribe this action to a stimula- 

 tion of the sympathetic nerve-endings, since adrenalin is equally effective if 

 applied a I'lc i -the whole of these nerve-endings have been made to degenerate 



