THE PHENOMENON OF CONTRACTION. 41 



curve has the general form indicated in Fig. 18. Three facts are 

 expressed by this curve: First, that if the muscle lifts no weight 

 no work will be done; this follows theoretically from the formula 

 W = L H, in which W represents the work done, L the load, and 

 H the lift. If either L or H is equal to zero the product, of course, 

 is zero; that is, no external work is done; the chemical energy 

 liberated in the contraction takes the form of heat. Second. 

 There is an optimum load for each muscle with which the greatest 

 proportion of work can be obtained. Third. When the load is just 

 sufficient to counteract the contraction of the muscle no work is 

 done, H in the above formula being zero. This amount of load 

 measures what Weber called the absolute power of the muscle. 

 As will be seen from the above curve, it is measured by the 

 weight which the muscle cannot lift and which, on the other 

 hand, cannot cause any extension of the muscle while contracting. 

 Or, in more general terms (Hermann), the absolute power of a 

 muscle is the maximum of tension which it can reach without 

 alteration of its natural length. This absolute power can be 

 measured for the muscles of different animals and for convenience 

 of comparison can then be expressed in terms of the cross-area 

 of the muscle given in square centimeters. Weber has shown 

 that the absolute power of a muscle varies with the cross-area, since 

 this depends upon the number of constituent fibers whose united 

 contraction makes the contraction of the muscle. Expressed in 

 this way, it is found that the absolute power of human muscle is, 

 size for size, much greater than that of frog's muscle. For in- 

 stance, the absolute power of a frog's muscle of 1 square centimeter 

 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 senes 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- 



