MUSCLE 635 



are in either case prevented by curara. He therefore concludes 

 that, since nicotine produces its effects by a direct action on 

 muscle, and not by an action on nerve-endings or on any special 

 structure (such as the protoplasmic mass or ' sole ' at the nerve- 

 ending in many animals) interposed between the nerve and the 

 muscle, no such special structure existing in the frog (Fig. 221), 

 curara must also act directly on the muscle. But obviously 

 curara does not paralyze the general contractile substance of 

 the muscle, else the curarized muscle would not contract on direct 

 stimulation. Langley accordingly assumes that, in addition to 

 the contractile or ' general ' substance, ' receptive ' substances 

 exist in the fibre, through which the excitation is transferred to 

 the contractile substance when the motor nerve is stimulated. 

 He pictures these receptive substances as ' side-chains ' of the 

 contractile molecule, in accordance with Ehrlich's theory of 

 immunity (p. 29), and distinguishes those in the neighbourhood 

 of the nerve-ending from those present throughout the muscle 

 fibre. Both the slow local tonic contraction and 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 ascertained by observing 

 the points which respond most readily to nicotine. 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 

 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- 

 membranosus is generally used on account of their long parallel 

 fibres. For indirect excitation, a muscle-nerve preparation, com- 

 posed 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 continues 



