7 i2 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES 



In all artificial stimulation there is an element of sudden or abrupt 

 change, of shock, in other words; but we cannot tell in what the 

 ' natural ' or ' physiological ' stimulus to muscular contraction in 

 the intact body really consists, nor how it differs from artificial 

 stimuli. All we know is that there must be a wide difference, and 

 that our methods of excitation must be very crude and inexact 

 imitations of the natural process. 



Direct Excitability of Muscle. The famous controversy on the 

 existence of independent ' muscular irritability ' has long been 

 forgotten, and has no further interest except for the antiquaries of 

 science, if such exist. The direct excitability of muscle in the modern 

 sense is not quite the same as the ' muscular irritability/ the dis- 

 cussion of which occupied Haller and his contemporaries. What 

 the modern physiologists have been called upon to decide is whether 

 muscular fibres can be caused to contract except by an excitation 

 that reaches them through their nerves. In this sense there can 

 exist no doubt that muscle is directly excitable, and some of the 

 proofs are as follows: 



(i) The ends of the frog's sartorius contain no nerves, yet they 

 respond to direct stimulation. (2) Certain chemical stimuli- 

 ammonia, for instance excite muscle but not nerve. (3) When 

 the motor nerves of a limb are cut they degenerate, and after a 

 certain time stimulation of the nerve-trunk causes no muscular 

 contraction, while the muscles, although atrophied, can be made 

 to contract by direct stimulation. (4) Finally, there is the cele- 

 brated curara experiment of Claude Bernard, which is described in 

 a somewhat modified form in the Practical Exercises, p. 784. A 

 ligature is tied firmly round one thigh of a frog, omitting the sciatic 

 nerve; then curara is injected, and in a short time the skeletal 

 muscles are paralyzed. That the seat of the paralysis is not the 

 contractile substance of the muscles itself is shown by their vigorous 

 response to direct stimulation. The ' block ' is not in the nerve- 

 trunk, nor above it in the central nervous system, for the ligated 

 leg is often drawn up that is, its muscles are contracted although 

 the poison has circulated freely in the sacral plexus and the spinal 

 cord. Further, if the nerve of the ligated leg be prepared as high 

 up above the ligature as possible, where the curara must undoubtedly 

 have reached it (just above the ligature the nerve has been isolated 

 and the circulation in it more or less interrupted), stimulation 

 of it will cause contraction of the muscles of the limb ; while excita- 

 tion of the other sciatic is ineffective. 



It can be also shown, by means of the negative variation or 

 current of action (p. 797), that a nerve-trunk on which curara has 

 acted remains excitable, and capable of conducting the nerve- 

 impulse. The conclusion, therefore, is that the curara paralyzes 

 neither nerve-fibre nor the contractile substance of the muscular 



