68 



PHYSIOLOGY OF MUSCLE AND NERVE 



until the venous blood has acquired considerable anounts of carbon 

 dioxid. 



The Wave of Contraction. A long muscle generally receives its 

 nerve fibers from a place about midway between its two ends, while 

 a short and compact muscle usually receives them at its upper pole. 

 It is of course essential that its constituent fibers contract at about the 

 same time, otherwise the best mechanical results cannot be obtained. 

 For this reason, the nerve terminals are commonly distributed in such 

 a way that the impulses reach the individual fibers at about the same 

 time and produce, therefore, a contraction which, to all appearances, 

 occurs practically simultaneously throughout the muscle. 



It can easily be shown, however, that the contraction of striated 

 muscle starts at the point stimulated and progresses from here to its 



FIG. 36. THE WAVE OF CONTRACTION. 



M, sartorius muscle of frog, A and B, two levers placed horizontally upon muscle; 

 S, stimulating electrodes; T, time; K, kymograph. When stimulated at S, lever A is 

 raised first. 



more distant segments. Thus, if the sartorius muscle of a frog, or one 

 of the long muscles of the neck of a turtle, is placed flat upon a board 

 with two writing levers resting horizontally upon its two ends, a stimu- 

 lus applied to one of its ends first of all produces a rise of that lever 

 which lies nearest the seat of the stimulation (Fig. 36). No special 

 record of the time need be taken, because the interval between the 

 contractions of the two poles of the muscle is quite apparent even 

 without this. It is advisable, however, to curarize the muscle before- 

 hand so that the wave of excitation cannot be spread by means of the 

 intra-muscular nerve fibers. From this fact it may be deduced that 

 the contraction travels over muscle in the form of a wave possessing a 

 definite velocity. If the distance between the two levers is compared 

 with the difference in the time between the two contractions, the speed 

 with which this wave is propagated, can easily be determined. Ac- 



