THE PHENOMENON OF CONTRACTION. 39 



cross-area is estimated at from 0.7 kilogram to 3 kilograms, while 

 that of a human muscle of the same size is estimated by Hermann 

 at 6.24 kilograms. Taken as a whole, the human muscle is a better 

 machine for work, but it seems possible, although exact figures are 

 lacking, that the absolute power of the muscles of some insects 

 reckoned for the same unit of cross-area would be much greater 

 than in human muscle. 



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 stimuli are sent in or until it loses its irritability 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, as 

 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 maxi- 

 mal simple contraction. 



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

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

 stimuli 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- 

 ing a total shortening greater than can be obtained by a single stim- 

 ulus (see Fig. 20). The extent of the summation in such cases 

 varies with a number of conditions, such as the intervals between the 



