SECTION II 

 EXCITATION OF MUSCLE 



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

 it contracts 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 stimulated, 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 to effectually 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, showing 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 ; 



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