SECTION II 

 EXCITATION OF MUSCLE 



A MUSCLE may be caused to contract in various ways. Normally it con- 

 tracts only in response to impulses starting in the central nervous system 

 and transmitted down the nerves. But contraction may be artificially 

 excited in various ways in a muscle removed from the body. If we make 

 a muscle-nerve preparation (i.e. a muscle with as long a piece of its nerve 

 as possible attached to it), such as the gastrocnemius of the frog with the 

 sciatic nerve, we find we can cause contraction by various forms of stimuli- 

 mechanical, thermal, or electrical applied to the muscle or the nerve 

 (direct and indirect stimulation). Thus the muscle responds with a twitch 

 if we pass an induction shock through it or its nerve, or pinch either with a 

 pair of forceps. Or we may use chemical stimuli, and cause contraction 

 by the application of strong glycerin or salt solution to the nerve. 



These experiments do not prove conclusively that muscle itself is irritable. 

 It might be urged that, when we pinched or burnt the muscle we stimu- 

 lated, not the muscle substance- itself, but the terminal ramifications of 

 the nerve in the muscle, and that these in their turn incited the muscle to 

 contract. But the independent excitability of muscle is shown clearly by 

 the following experiment by Claude Bernard. 



A frog, whose brain has been previously destroyed, is pinned on a board, 

 and the sciatic nerves on each side exposed. A ligature is then passed 

 round the right thigh underneath the nerve, and tied tightly so as effectually 

 to close all the blood-vessels supplying the limbs, without interfering 

 with the blood-supply to the nerve. Two drops of a 1 per. cent, solution 

 of curare are then injected into the dorsal lymph-sac. After the lapse of a 

 quarter of an hour it is found that the strongest stimuli may be applied 

 to the left sciatic nerve without causing any contraction of the muscles it 

 supplies. On the right side, stimulation of the nerve is as efficacious as 

 before. Both gastrocnemii respond readily to direct stimulation, show mi: 

 that the muscles are not affected by the drug. Since both sciatic nerves 

 have been exposed to the influence of the curare, it is evident that the 

 difference on the two sides cannot be due to any deleterious effect on them 

 by the curare. We have also excluded the muscles themselves ; so we must 

 conclude that the curare paralyzes the muscles by affecting the terminations 

 of the nerve within the muscle, and probably the end-plates themselves. 



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