450 MANUAL OF PHYSIOLOGY. 



1. Mechanical Stimulation. Any sudden blow, pinch, etc., of 

 a living muscle causes a momentary contraction, which rapidly 

 passes off when the irritation is removed. 



2. Thermic Stimulation. If a frog's muscle be warmed to over 

 30 C. it will begin to contract, and before it reaches 40 C. the 



'muscle will pass into a condition known as heat rigor, which will 

 be mentioned presently. If the temperature of a muscle be re- 

 duced by C., it shortens before it becomes frozen. 



3. Chemical Stimulation. A number of chemical compounds 

 also act as stimuli when they are applied to the transverse section 

 of a divided muscle. Among these may be named (1) the min- 

 eral acids (HC1, .1 per cent.) and many organic acids; (2) salts 

 of iron, zinc, silver, copper, and lead ; (3) the neutral salts of the 

 alkalies of a certain strength ; (4) weak glycerin and weak lactic 

 acid, which only excite nerves when concentrated ; (5) bile also 

 is said to stimulate muscle in much weaker solutions than it will 

 nsrve fibres. 



4. Electric Stimulation. Electricity is the most convenient form 

 of stimulation, because we can accurately regulate the force of 

 the stimulus. The occurrence of any variation in the intensity 

 of an electric current passing through a muscle causes it to con- 

 tract. The sudden increase or decrease in the strength of a cur- 

 rent acts as a stimulus, but a current of exactly even intensity 

 may be made to pass through a muscle without exciting any con- 

 traction. The common method employed is that of opening or 

 closing a circuit of which the muscle forms a part, so as to make 

 or break the current; and thus a variation of intensity, equal to 

 the entire strength of the current used, takes place in the muscle, 

 and acts as a stimulus. 



The irritability of muscle substance is not so great as that of 

 the motor nerves, that is to say, a slight stimulus will make the 

 muscle contract when applied to its nerve, while the same stimulus 

 will have no effect if applied to the muscle directly. In experi- 

 menting on the contraction of muscle, as already stated, the in- 

 tervention of the nerve is commonly used, the stimulus, by means 

 of an electric current applied to the nerve, being more conve- 

 niently and completely distributed to the muscle than when ap- 

 plied directly. 



