GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 73 



which constitutes the contraction. With the disappearance of the 

 former, the latter also disappears and the muscle resumes its pre- 

 vious passive condition. There is no evidence, however, that the 

 excitation process travels traj^ersely, that is, into adjoining fibers, 

 being rjreyented from doing so by the presence of the limiting 

 membranes, the sarcolemmata. The fact that each muscle-fiber 

 receives its own, or at least a branch of a nerve-fiber, and hence its 

 own nerve impulse or stimulus, would also indicate that the excitation 

 process can not be conducted longitudinally into adjoining fibers, 

 or at least with sufficient rapidity for the purposes of ordinary muscle 

 actions. Nevertheless if a long muscle, such as the sartorius, from a 

 curarized frog be stimulated at one end with an induced electric cur- 

 rent, the excitation and the contraction processes will be conducted 

 with extreme rapidity to the opposite end of the muscle. The rapidity 

 of conduction in human muscles has been estimated at from 10 to 13 

 meters per second, and in frog's muscle at from 3 to 4.5 meterTper 

 second. The contraction process, the thickening of the muscle, is 

 termed the contraction wave. As it is the result of the excitation 

 process and immediately succeeds it, its rate of conduction must be 

 the same as that given above. With appropriate apparatus the 

 duration of the wave at any given point has been shown to be, in the 

 frog's muscle, one-tenth of a second and its length three-tenths of a 

 meter. 



PHENOMENA ATTENDING A MUSCLE CONTRACTION. 

 PHYSICAL PHENOMENA. 



The most obvious change in a muscle during the contraction is 

 that relating to its form. The muscle not only becomes shorter, but 

 at the same time thicker. The extent to which it may shorten when 

 unopposed may amount to 3O~pef cent, or more of its original length. 

 Trie increase in thickness practically compensates for the diminution 

 in length, for there is no observable diminution in volume. The 

 change in form of the entire muscle results from a corresponding 

 change of form of its individual fibers as determined by microscopic 

 examination, each of which becomes shorter and thicker. The 

 successive changes in both the muscle and the individual fibers are 

 represented in Fig. 2 1 . 



When the contraction begins, the dim band increases and the 

 bright band diminishes in. width. This Engelmann attributes to 

 the passage of fluid material from the bright into the dim band. At 

 the time of relaxation there is a return of this material and the bands 

 assume their original shape and volume. As the contraction wave 

 reaches its maximum the optic properties of the bright and dim bands 

 change. The former now becomes darker and less transparent 



