42 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



said that the power of a muscle to do work varies directly with the 

 cross area, since this depends upon the number of constituent 

 fibers whose combined contraction determines the power of the 

 nmscle, and also varies directly with the length of the muscle, 

 since the greater the length of the fibers, other things being equal, 

 the greater will be the lift or shortening. Moreover, for any given 

 muscle it has been shown that the work done is increased by in- 

 creasing the length of the fibers, that is, there is an actual increase 

 in the energy of contraction when the fibers are extended by a 

 greater tension or an increased load. 



COMPOUND OR TETANIC CONTRACTIONS. 



Definition of Tetanus — When a muscle receives a series of 

 rapidly repeated stimuli it remains in a condition of contraction as 

 long as the stimuU are sent in or until it loses its irritabihty from 

 the effect of fatigue. A contraction of this character is described 

 as a compound contraction or tetanus. If the stimuli follow each 

 other with sufficient rapidity the muscle shows no external sign of 

 relaxation in the intervals between stimuli, and if its contractions 

 are recorded upon a kymographion by means of an attached lever 

 a curve is obtained such as is shown at 5 in Fig. 19. A con- 

 traction of this character is described as a complete tetanus. If, 

 however, the rate of stimulation is not sufficiently rapid the mus- 

 cle will relax more or less after each stimulus and its recorded 

 curve, therefore, will present the appearance shown in 1,2 3, and 4 

 of Fig. 19. A tetanus of this character is described as an incom- 

 plete tetanus. It is obvious that according to the rate of stimu- 

 lation there may be numerous degrees of incomplete tetanus, ae 

 shown in Fig. 19, extending from a series of separate single con- 

 tractions, on the one hand, to a perfect fusion of the contractions, 

 a complete tetanus, on the other. Tetanic contractions present 

 two peculiarities in addition to the mere matter of duration, 

 which is governed, of course, by the duration of the stimu- 

 lation: First, the more or less complete fusion of the contrac- 

 tions due to the separate stimuli. This, as stated above, is the 

 distinctive sign of a tetanus. Second, the phenomenon of sum- 

 mation in consequence of which the total shortening of the muscle 

 in tetanus may be considerably greater than that caused by a 

 maximal simple contraction. 



Summation. — The facts of summation may be shown most read- 

 ily by employing a device to send into the muscle two successive 

 stimuh at varying intervals. If the second stimulus falls into the 

 muscle at the apex of the contraction caused by the first stimulus, 

 then, even if the first contraction is maximal, the muscle will shorten 

 still farther; the first and second contractions are summated, giv- 



