342 MUSCLE IN ACTION. [llOOK I. 



time in a regular alternation of simple dark and simple bright bands. 

 There is however this difference : those parts of the fibre which 

 before were dark are now bright, and those parts which formerly 

 were bright are now dark. The fibre has emerged from the homo- 

 geneous stage with its bands interchanged as far as regards their tint ; 

 and this, therefore, may be called the stage of transposed lands. Their 

 transposition only affects the shade or tint of the stripes, the isotropous 

 and double-refracting elements of the fibre maintaining their original 

 relationship. This is well shewn in the figure on p. 314, where the 

 same contracting muscle is exhibited by ordinary and by polarized 

 light 



It may be added that Eugelmann believes he has demonstrated that, 

 in contraction, the volume of the main double-refracting zone increases 

 at the expense of the isotropous layers. 



If contraction is started at one point with a given 

 intensity it does not instantly extend over the whole fibre; 

 but travels along it as a wave with a velocity of 3 metres a second in 

 frogs. In the case of excised muscles, the wave suffers in its course a 

 diminution 1 of intensity. 



Latent When muscle is directly stimulated the contractile 



period, force does not at once begin to develope. An interval 



elapses between the application of the stimulus and the beginning of 

 contraction ; this is known as the period of latent stimulation, or latent 

 period. Its value was determined by Helmholtz to be about ^ sec., 

 but it is found to vary in different circumstances, and under favour- 

 able conditions it is said to become as short as ^ or ^fa of a sec. 2 



Course, or When once started, the force does not spring 



Curve, of suddenly to perfection, but developes in course of time. 



Contraction. if a resistance be opposed to the shortening of the 

 muscle it is clear that no contraction can occur until the contractile 

 force has grown large enough to overbalance the resistance ; hence, 

 the greater the resistance, the longer the interval which must elapse 

 between the moment of stimulation and the beginning of actual 

 contraction. During contraction the contractile force does work and 

 becomes spent. Hence a smaller resistance serves to check contraction 

 near its end than near its beginning. The rapidity of contraction is 

 not equal throughout its course; it first increases and then diminishes 

 until the summit of contraction is reached, as it usually is within 

 ^ or jlfo- of a second after stimulation. Beyond the maximum 

 the contractile force dies gradually away 3 ; this is rendered probable 

 by the course of re-extension under the influence of a weight, which 



. J See Hermann's Handbuch der Physiologie, Bd. i. Abth. i. p. 55. 



3 See Hermann's Handbuch, Bd. i. Abth. i. p. 36. 



3 Heidenhain, "Ueber Ad. Pick's experimentellen Beweis fiir die Giiltigkeit des 

 Gesetzes von der Erhaltung der Kraft bei der Muskelzusammenziehtmg." Pfluger's 

 Arch., Vol. ii. p. 426. 



