470 MUSCULAR EXCITABILITY. 



able amount of acid formed in the muscle, which dissolves the coagulated myosin. 

 After a time putrefaction sets in, accompanied by the presence of micro-organisms 

 and the evolution of ammonia and putrefactive gases (H 2 S, N, C0 2 184). 

 [Hermann and Birerfreund attach much importance to the resolution of rigor 

 mortis independently of putrefaction.] 



According to Onimus, the loss of excitability which precedes the onset of rigor mortis occurs 

 in the following order in man: left ventricle, stomach, intestine (55 minutes); urinary bladder, 

 right ventricle (60 min.); iris (105 min.); muscles of face and tongue (180 min.); the extensors 

 of the extremities (about one hour before the flexors); the muscles of the trunk (five to six 

 hours). The oesophagus remains excitable for a long time ( 325). 



296. MUSCULAR EXCITABILITY. By the term excitability or irrita- 

 bility of a muscle, is meant that property of a muscle in virtue of which it responds 

 to stimuli, at the same time becoming shorter and correspondingly thicker. The 

 condition of excitement is the active condition of a muscle produced by the applica- 

 tion of stimuli, and is usually indicated by the act of contraction. Stimuli are 

 simply various forms of energy, and they throw the muscle into a state of excite- 

 ment, while at the moment of activity the chemical energy of the muscle is trans- 

 formed into work and heat, so that stimuli act as " liberating " or "discharging 

 forces." [These " discharging forces " may themselves be very feeble, but they 

 are capable of causing the manifestation of the transformation of a large amount 

 of energy.] The normal temperature of the body is most favourable for main- 

 taining the normal muscular excitability ; the excitability varies as the tempera- 

 ture rises or falls. 



As long as the blood-stream within a muscle is uninterrupted, the first effect 

 of stimulation of a muscle is to increase its energising power, partly because the 

 circulation is more lively and the blood-vessels are dilated, but after a time, the 

 energising power is diminished. Even in excised muscles, especially when the 

 large nerve-trunks have already lost their excitability, the excitability is increased 

 after a stimulus, so that the application of a series of stimuli of the same strength 

 causes a series of contractions which are greater than at first {Wundt). Hence, 

 we account for the fact that, although the first feeble stimulus may be unable to 

 discharge a contraction, the second may, because the first one has increased the 

 muscular excitability (Fick). 



Effects of Cold. If the muscles of a frog or tortoise be kept in a cool place, they may remain 

 excitable for ten days, while the muscles of warm-blooded animals cease to be excitable after 

 one and a half to two and a half hours. (For the heart see 55.) A muscle, when stimulated 

 directly, always remains excitable for a longer time when its motor nerve is already dead. 



[Independent Muscular Excitability. Since the time of Albrecht v. Haller, and 

 R Whytt, physiologists have ascribed to muscle a condition of excitability which 

 is entirely independent of the existence of motor nerves, but is dependent on certain 

 constituents of the sarcous substance. Excitability, or the property of responding 

 to a stimulus, is a widely distributed function of protoplasm or its modifications. 

 A colourless blood-corpuscle or an amoeba is excitable, and so are secretory and 

 nerve-cells. In the first case, the application of a stimulus results in motion in 

 an indefinite direction, in the second in the formation of a secretion, and in the 

 third in the discharge of nerve-energy. In the case of muscle, a stimulus causes 

 movement in a definite direction, called a contraction, and depending on the con- 

 tractility of the sarcous substance. There are many considerations which show, that 

 excitability is independent of the nervous system, although in the higher animals, 

 nerves are the usual medium through which the excitability is brought into action. 

 Plants however are excitable, and they contain no nerves.] 



Numerous experiments attest the " independent excitability " of muscle: 1. 

 There are chemical stimuli, which do not cause movement when applied to motor 

 nerves, but do so when they are applied directly to muscle ; ammonia, lime water, 



