740 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES 



from those present throughout the muscle fibre. Both the slow 

 local tonic contraction an4 the quick, brief conducted contractions 

 or twitches set up in a muscle fibre by nicotine, but especially the 

 latter, are much more easily elicited in that part of it which lies 

 under the nerve-ending than elsewhere. Indeed, the position of 

 the nerve-endings in the superficial fibres of a muscle can be ascer- 

 tained by observing the points which respond most readily to nico- 

 tine. Nicotine and curara, etc., are supposed to combine with the 

 receptive substance, which is then in both cases rendered incapable 

 of being affected by nerve impulses. In the case of nicotine an 

 additional action results from the combination with the receptive 

 substance -viz., the change in the contractile substance which leads 

 to contraction. Curara paralyzes the transmission of the excitation 

 from the motor nerves to smooth muscle the muscles of the 



Fig. 246. Tonic Contraction of Muscle during Passage of Constant Current. Two 

 sartorius muscles of frog connected by pelvic attachments. Current from 12 

 small Daniell cells in series passed through their whole length. Current closed 

 at m, opened at ft. Time trace, two-second intervals. 



bronchi, for instance with much greater difficulty than to ordinary 

 skeletal muscle, and the same is true of the inhibitory nerves of 

 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 stimula- 

 tion, for the nerve-fibres within the muscle are also excited. Induced 

 currents stimulate nerve more readily than muscle. Voltaic currents 

 may excite a muscle whose nerves have degenerated, while induced 

 currents are entirely without effect. 



For direct stimulation, a curarized frog's sartorius or semi-mem- 

 branosus is generally used on account of their 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 em- 

 ployed, as it is easy to isolate the muscle without hurting its nerve. 



