STIMULATION OF MUSCLE 739 



they are anatomically similar to those in the nerve-trunk till near 

 their terminations, are similarly affected by curara and it is a 

 justifiable assumption the seat of the curara paralysis must either 

 be the nerve-ending or some mechanism, physiological if not 

 anatomical, interposed between the nerve-ending and the con- 

 tractile substance. Now, Langley has shown that the contractions 

 caused by the local application of dilute nicotine solution to points 

 of the skeletal muscles of the frog, both in normal muscles and in 

 muscles whose motor nerves and nerve-endings have degenerated 

 after section of the nerves, are prevented by curara. He there- 



Fig- 245. -Frog's Motor Nerve-Ending (Wilson). A, B, C, three muscle-fibres. The 

 medullated nerve a loses its medullary sheath and breaks up on B at i. It gives 

 off at 2 a large non-medullated branch, which also breaks up on B. The nerve- 

 endings send ultraterminal fibrillae to A, B, and C, some of which were seen to 

 end in small knobs. A separate non-medullated nerve, n, is shown, which forms 

 a small plexus on B, one fibre of which penetrates to a lower plane than the other, 

 and ends by forming a knob under the sarcolemma. 



fore 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. 245), 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 tx^ansferred to the con- 

 tractile substance when the motor nerve is stimulated. He pictures 

 these receptive substances as ' side-chains ' of the contractile mole- 

 cule, in accordance with Ehrlich's theory of immunity (p. 31), 

 and distinguishes those in the neighbourhood of the nerve-ending 



