550 A MANUAL OF PHYSIOLOGY 



used, and will be discussed in detail. A prick, a cut, or a 

 blow are examples of mechanical stimuli. A fairly strong 

 solution of common salt or a dilute solution of a mineral 

 acid will act as a chemical stimulus, which always tends to 

 cause, not a single contraction, but a tetanus. Sudden 

 cooling or heating acts as a stimulus for muscle, but thermal 

 stimulation is somewhat uncertain. In all artificial stimula- 

 tion 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 discussion of which occupied 

 Haller and his contemporaries. What the modern physio- 

 logists 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 the 

 proofs are as follows : 



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

 apex of the frog's heart contains neither nerves nor nerve- 

 cells, yet both respond to direct stimulation. (2) Certain 

 chemical stimuli ammonia, for instance do not act on 

 nerve, but excite muscle. (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 contrac- 

 tion, 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. 617. A ligature is tied firmly round one thigh 

 of a frog, omitting the sciatic nerve; then curara is injected, 



