816 OF THE STRUCTURE AND FUNCTIONS OF MUSCULAR TISSUE. 



traction at its middle portion represents a parabola modified by the elasticity 

 of the muscle. The period of time which elapses from the instant that the 

 stimulus is applied until the contraction is complete, increases generally with 

 the amount of shortening, with the heaviness of the weight, and with the 

 exhaustion of the muscle. In the Frog the entire time occupied in the con- 

 traction and subsequent elongation of a muscle is about one-third of a sec- 

 ond, of which 



Of a second. 

 . 0.02 

 . 180 

 elongation " .... 105 





The period of latent excitation amounted to 

 " " contraction " 



.305' 



From the results of various experiments, Helmholtz satisfied himself that to 

 develop their greatest force, muscles require a longer time than when slight 

 efforts only are made. 2 



667. Mechanism of Muscular Contraction. From the inquiries of Mr. Bow- 

 man, it appears that the act of contraction usually commences at the ex- 

 tremities of the fibre ; but it frequently occurs also at one or more intermediate 

 points. The first appearance is a spot more opaque than the rest, caused by 

 the approximation of a few of the dark points of some of the fibrill?e ; this 

 spot usually extends in a short time through the whole diameter of the fibre; 

 and the shading caused by the approximation of the transverse stride increases 



FIG. 292. 



Muscular Fibre of Dytiscus, showing the contracted state in the centre ; the strise approximated ; the 

 breadth of the fibre increased ; and the myoleinma raised in bullse on its surface. 



in intensity. The stride are found to be two, three, or four times as numerous in 

 the contracted as in the uncontracted part, and are also proportionally nar- 

 rower and more delicate. The line of demarcation between the contracted 

 and uucontracted portions is well defined ; but as the process goes on, fresh 

 strise are absorbed as it were from the latter into the former. The contracted 

 part augments in thickness, but not in a degree commensurate with its di- 

 minished length, so that its solid parts lie in smaller compass than before, 

 the fluid which previously intervened between them being pressed out in 

 bulhe under the sarcolemma. Marey and Aeby have shown that when a 

 muscle is excited by passing a current of electricity through it, every part 

 contracts simultaneously, as proved by the identity in point of time of trac- 



hus pointed out various differences between the white and the red muscles of the 

 rabbit, and amongst others, that tin 1 period of latent excitation of the white muscles 

 of the rabbit is ^ 5 d of a second, whilst for the red muscles it is as much as T \th of a 

 second. 



1 See Holmholtx in Muller's Archiv, 1850, 1852; and Volkmann, in Leipzig. Be- 

 richte Math. Phys. Class., 1851. Place has mon> recently (Nederland. Arehief, Bd. iii, 

 18<i7, Heft 2) estimated the period of latent excitation in the muscles of frogs at only 

 0.005 sec., and he finds the rapidity of propagation of the excitation through muscles 

 to be about one meter per second. 



2 For a very minute investigation on this point, see Kliinder, Ueber den zeitlichen 

 Verlauf der Muskelzuckung, in the Arbeiten aus dem Kieler physiologischen Insti- 

 tut., 1868, pp. 107-130. 



