PHYSIOLOGY OF MOVEMENT. 



715 



curve will be produced as is represented by Fig. 279. The two con- 

 tractions are thus added together and the total shortening may be nearly 

 double that produced by a single contraction (Fig. 280). 



If a third stimulus is then allowed to pass into the nerve before the 

 second contraction has passed off, a third contraction will be added to 

 the second, and so on in the case of the fourth, fifth, or more. It will 

 be, however, noticed that while the second contraction may be nearl} 7 or 

 quite as extensive as the first, the third and fourth progressively decrease 

 in extent, until finally simply a broken line without any extensive increase 

 in contraction will indicate the entrance of the separate stimuli, the 

 stimuli merely serving to keep up the contraction already produced. 



When the stimulation ceases the muscle then rapidly passes into a 

 condition of rest, relaxation occurring very rapidly. 



^ I 



FIG. 281. MUSCLE THROWN INTO TETANUS WHEN THE PRIMARY CURRENT OF 

 AN INDUCTION MACHINE is REPEATEDLY BROKEN AT INTERVALS OF SIX- 

 TEEN IN A SECOND. (Foster.) 



(To be read from left to right.) 



The upper line is that described by the muscle. The lower marks time, the intervals between the ele- 

 vation indicating seconds. The intermediate line shows when the shocks were sent in, each mark corre- 

 sponding to a shock. The lever, which describes a straight line before the shocks are allowed to fall into 

 the nerve, rises almost vertically (the recording surface moving slowly) as soon as the first shock enters the 

 nerve at a. Having risen to a certain height it begins to fall again, but in its fall is raised once more by 

 the second shock, and that to a greater height than before. The third and succeeding shocks have similar 

 effects, the muscle continuing to become shorter, though the shortening at each shock is less. After a 

 while the increase in the total shortening of the muscle, though the individual contractions are still visible, 

 almost ceases. At b the shocks cease to be sent into the muscle, the contractions almost immediately disap- 

 pear, and the lever forthwith commences to descend. The muscle being only slightly loaded, the relaxa- 

 tion is very slow. 



When the separate stimuli do not follow each other more rapidly 

 than sixteen in a second, the contraction produced by one stimulus has 

 had time to undergo partial relaxation before the following stimulus 

 enters ; as a consequence, the point of the lever traces a broken line on 

 the traveling surface (Fig. 281). 



If, however, the stimuli follow each other more rapidly than this, an 

 apparently constant shortening is produced, in which no variation what- 

 ever in length can be made out. The gradual production of this state 

 of affairs indicates that this apparent^ constant and uniform contraction 

 is, nevertheless, made up of a large number of individual contractions 

 added to each other. Such a condition is spoken of as tetanus, and the 



