818 OF THE STRUCTURE AND FUNCTIONS OF MUSCULAR TISSUE. 



enables the duratiou of each of the impulses to be estimated. The arrange- 

 ment of the apparatus is such that the lower lines represent the tracing ob- 

 tained when the muscle is in the perfectly recent state, whilst the upper ones 

 represent it when more or less exhausted. It will be noticed that for each 

 galvanic shock there is a single instantaneous contraction represented by an 

 elevation with rounded summit, and that the period of ascent corresponding 

 to the period of shortening of the muscle (on the left) is more rapid than 

 the period of descent of the line, the latter corresponding to the period of 

 relaxation. In proportion as the muscle becomes fatigued three peculiari- 

 ties may be noticed : the duration of the contraction or shock augments ; the 

 period of ascent becomes prolonged, and the height of the wave or the 

 amount of shortening of the muscle increases. This last, however, is only 

 transient, since, if the tracings be continued, the wave height diminishes 

 though its width continues to increase. The tracings present the same fea- 

 tures whether the excitation be applied to the nerve supplying the muscle, 

 or to the muscle itself. It is very interesting to remark that the phenomena 

 of exhaustion are not produced in the living body within moderate limits, 

 indicating that reparation is constantly taking place. Of all the muscles of 

 the body, the heart alone in its ordinary contraction gives a tracing corre- 

 sponding to that obtained from a voluntary muscle on the application of an 

 electrical shock that is to say, the heart constitutes the only instance where 

 each contraction is definite and single, and it presents the same characters 

 in all animals. In all other instances, what is termed a muscular contrac- 

 tion consists of a series of shocks or impulses, as shown in Fig. 294, in which 



FIG. 294. 



a tracing is shown such as may be obtained from a muscle caused to con- 

 tract by a voluntary effort, or to which excitations at regular intervals are 

 applied. Here the effects of the successive shocks are in the first instance 

 superadded to one another till a certain degree of contraction is obtained, 

 which remains permanent with that amount of stimulation. If the electrical 

 or mechanical 1 excitations are made to succeed each other more rapidly 

 (Fig. 295), the successive shocks unite more quickly and completely, and 

 the total contraction is greater in degree. Finally, if the excitations are 

 repeated more than a certain number of times per second, varying with the 

 animal and the state of the muscle, the several shocks fuse completely into 

 one another, and tetanus is produced in which no vibration is perceptible 

 ( Fig. 2!((l }. The muscles of different animals respond differently to electrical 

 excitation. In the case of the bird, 2 permanent contraction or tetanus is not 



1 As in Heidenhain's mechanical Tetano-motor, in which a little hummer is set in 

 motion by an electro magnetic current, and a tetanic condition can be established in 

 the muscles which will last for two minutes or more, till the irritability of the nerve 

 is destroyed. 



2 Marey, Rev. des Cours. Scicnt, vol. iv, 1867, p. 215. 



