552 



A MANUAL OF PHYSIOLOGY 



their terminations, are similarly affected by curara. We 

 must carefully remember that the 'nerve-endings' which 

 are paralyzed by curara do not necessarily, nor even pro- 

 bably, coincide exactly with the 'nerve-endings' of histology. 

 Still, it is significant that the histological differences between 

 the nerve-terminations in striped and smooth muscle should 

 correspond to a physiological difference in the action of 

 curara on them. This drug paralyzes the nerve-endings 

 in smooth muscle the muscles of the bronchi, for instance 

 with much greater difficulty than those in ordinary skeletal 

 muscle, and the same is true of the vagus-endings in the 

 heart. 



The action of curara gives us the means of stimulating 

 muscle directly : when electrical currents are sent through 

 a non-curarized muscle, there is in general a mixture of 

 direct and indirect stimulation, for the nerve-fibres within 

 the muscle are also excited. Induced currents stimulate 



nerve more readily 

 than muscle. Vol- 

 taic currents may 

 excite a muscle 

 whose nerves have 

 degenerated, while 

 induced currents 

 are entirely with- 

 out effect. 



For direct 

 stimulation, a 

 curarized frog's 

 sartorius or semi- 

 FIG. 162. TONIC CONTRACTION DURING AND membranosus is 



AFTER X 1 LOAV* % 



Curve from frog's gastrocnemius. At M constant generally USed OH 

 current closed, at B broken. Contracture continues after Q rrrmnf nf th^ir 

 opening of current. Time trace, two-second intervals. 



long parallel 



fibres ; for indirect excitation, a muscle-nerve preparation, 

 composed of a frog's gastrocnemius with the sciatic nerve 

 attached to it, is commonly employed, as it is easy to isolate 

 the muscle without hurting its nerve. 



Stimulation by the Voltaic Current. While the current con- 



