56 MUSCULAR CONTRACTION. 



gradual in the frog, rapid in man and mammalia. It is 

 associated with chemical changes which resemble those 

 which take place in contraction. 



Partial rigor can be induced in living muscle by arrest, and removed by 

 restitution, of the circulation. Muscle becomes rigid at about 45 C. in 

 'frogs, 48 to 50 in mammals. Rigor occurs sooner after death in exhausted 

 muscles than in others : all rigid muscles are acid. 



In doing work muscle shortens and thickens. Its vol- 

 ume diminishes very slightly and it becomes more exten- 

 sible. Every muscular contraction results from excitation 

 either extrinsic or intrinsic. An instantaneous extrinsic 

 excitation of a muscle by its nerve produces a single 

 contraction, called a twitch. The contraction begins a 

 certain time after the excitation (period of latent excita- 

 tion of du Bois-Reymond) ; it rapidly increases to a maxi- 

 mum and then gradually subsides. After contraction 

 has ceased the muscle is nearly as long as before, and 

 soon quite as long. 



By the myographic method (see Practical Exercises) 

 a single contraction may be investigated with reference to 

 the time after excitation at which it begins, to its duration 

 and character, and to the modifications produced by 

 changes in its physiological condition, or in its temperature. 

 If two or more instantaneous excitations of a muscle 

 through its nerve follow each other, the effect is augmented 

 by each successive excitation; but the increment produced 

 by any single excitation is always less than that produced 

 by its predecessor. The effect of a series of equal exci- 

 tations following each other at very short intervals of 

 time, although apparently continuous, consists in reality 

 of a succession of instantaneous contractions, of which 

 the frequency is the same as that of the excitations. This 

 condition is called, in physiological language, Tetanus. 

 The number of single contractions per second of which a 

 tetanic or voluntary contraction is constituted may be 

 judged of by the " tone " heard in the contracting muscle. 



