MUSCLE 633 



muscle, but thermal stimulation is somewhat uncertain. It is 

 not quite settled whether the contraction which can be obtained 

 from a muscle when it is subjected to brief local heating to a 

 ' thermic shock/ as some writers prefer to say (e.g., by the mo- 

 mentary glow of a platinum wire below but not touching it) is an 

 ordinary muscular contraction, or a physical, although transient, 

 contracture analogous to that caused by ;ertain drugs (Waller). 

 Smooth, like striped, muscle is susceptible to electrical, mechani- 

 cal, thermal, and chemical stimulation. In addition, in certain 

 situations it can be excited by light (phouc stimulation), as in the 

 case of the excised iris of fish and amphibia. 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 in- 

 exact 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 anti- 

 quaries of science, if such exist. The direct excitability of 

 muscle in the modern sense is not quite the same as the ' muscular 

 irritability,' the discussion 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 celebrated curara experiment of Claude Bernard, 

 which is described in a somewhat modified form in the Practical 

 Exercises, p. 706. 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 



