GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 83 



the weight which is sufficient to prevent the muscle from shortening. 

 This is best determined by the method of after-loading in which the 

 muscle is not extended by the weight previous to the contraction. 

 It has been found that the absolute force of a muscle is directly de- 

 pendent on the number and not the length of the fibers it contains 

 and proportional to the physiologic transverse section of the muscle. 

 The transverse section of a muscle is obtained by dividing the volume 

 1058 (obtained by dividing its weight by the specific weight of mus- 

 cle-tissue) by the average length of the fibers. 



For purposes of comparison it is customary to refer the absolute 

 force to units of diameter viz., one square centimeter. Rosenthal 

 estimates the force for the square centimeter of the muscle of the frog 

 at from 2 to 8 kilograms ; for the muscles of man at 6 to 8 kilograms ; 

 Koster at about ten kilograms for the muscles of the leg and 7 or 8 

 kilograms for the muscles of the arm. 



Action of Successive Stimuli. If a series of successive stimuli 

 be applied to a muscle, the effect will vary according to the rapidity 

 with which they follow one another. As previously stated, if the 



FIG. 31. SUMMATION CURVE. FIG. 32. SUMMATION CURVE. 



interval preceding each stimulus be sufficiently long to enable the 

 muscle to recover from the effects of the previous contraction, there 

 will be no effect for a long time except a slight increase, in the early 

 period, of the irritability as shown by the increased height of the 

 curve or shortening of the muscle. If, however, a second stimulus 

 be applied to a muscle during the period of relaxation, a second con- 

 traction immediately follows which is added to or superposed on the 

 first; the effect produced will be greater than that produced by either 

 stimulus separately. 



A third stimulus applied during the relaxation of the second con- 

 traction produces a third contraction which adds itself to the second, 

 and so on (Fig. 31). The increment of increase in the extent of the 

 successive contractions gradually diminishes, however, until the muscle 

 reaches a maximum of contraction. The superposition of the second 

 contraction on the first, the third on the second, and so on, is termed 

 summation of e fleets. Experiment has shown that the greatest effect 

 of a second stimulus that is, the highest contraction is produced 



