6 



STRIATED MUSCLE. 83 



under the name of cadaveric rigidity, which rigidity is 

 owing to the coagulation of the albuminous substance of the 

 muscle (myosin) by the acids which it has formed. The 

 muscle may also pass into the state of spontaneous rigidity, 

 after a continued activity which produces great excess of 

 acid : mineral acids, heat 50 c., any thing, in short, which 

 coagulates the myosin, either produces or hastens this rigid- 

 ity. We have already remarked that an injection of serum 

 or of alkaline liquid entirely prevents or delaya it (p. 71). 

 The sort of retraction which the muscles undergo during 

 this rigidity is owing to the contraction or solidification of 

 the coagulated myosin ; the muscle is then extremely fragile, 

 and only ceases to be so when the coagulum becomes lique- 

 fied by putrefaction. Of course, the muscle then becomes 

 alkaline again by means of the ammonia resulting from its 

 decomposition. 



Irritants. The agents which excite the irritability of 

 the muscle are very numerous. As their mode of action is 

 not exactly known, they have been divided and classed 

 simply as chemical, physical, and physiological. 



The chemical excitants are very numerous; almost any 

 chemical agent can cause a muscle to pass from the first into 

 the second form. We will only observe that these agents 

 must, in general, be very much diluted, and that some among 

 them, ammonia, for instance, have, when thus diluted, no 

 influence upon the motor nerves, which is a fresh proof that 

 muscular irritability belongs really, not to the nerves, but to 

 the muscles. 



Among physical excitants we must rank first, electricity, 

 and especially currents, whatever be their source (see p. 30) ; 

 other physical excitants, often employed in experiments, are 

 pinching, a blow (Heidenhain), or pricking. Most people 

 have seen how fresh meat, in a butcher's stall, will palpitate 

 under the influence of a current of air, a breath of wind. 

 Changes of temperature, especially cold, must also be reck- 

 oned among these excitants : cold is often employed in sur- 

 gery to produce contraction of the smooth muscular elements 

 of the arteries (see circulation ; physiology of the arterial 

 coats). Indeed, light itself is an excitant of muscle, as has 

 been shown by Brown-Sequard, in his experiments on the 

 pupil of the eye. 



Finally, the physiological excitant, whose object in the 

 organism is to make the muscle pass from the first into the 

 second form, is represented by the action of the motor 

 erves. 



