THE NEURO-MUSCULAR JUNCTION 313 



necessary to excite. In a normal sartorius he finds three substances, 

 each distinguished by its own 'excitation time.' In the pelvic 

 nerve-free end of the sartorius there is only one substance, with an 

 excitation time of -017 sec. This may be regarded as the muscle 

 substance proper. In the sciatic nerve-trunk there is a second sub- 

 stance with a much steeper characteristic and with an excitation 

 time of -003 sec. On experimenting on the middle region of the 

 sartorius we find not only these two substances but a third substance, 

 which Lucas calls the substance ft, with an extremely rapid excita- 

 tory process. Its excitation time is 'OOC05 sec. The presence of 

 these three substances in the middle part of the toad's sartorius is 

 shown in the diagrams (Fig. 124), which represent the relation of 



FIG. 124. 



strength to duration of the currents necessary to evoke a contrac- 

 tion. In this curve a represents the muscle material, y -the nerve 

 material, and /3 the curve of the intermediary substance. 



Similar conditions are found in the visceral neuro- muscular system. 

 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 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. 



