GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 71 



work done; but as this varies with the degree to which the muscle is weighted, 

 another measure has been adopted, to which the term absolute muscle 

 force or static force has been given. The absolute force is measured by the 

 weight which a muscle can raise and hold at its natural length after it has 

 been extended by this weight. In Fig. 22 this weight would be on the ab- 

 scissa B b' where it is cut by the extension curve of the active contracted 

 muscle. The absolute force is also measured by the weight which is just 

 sufficient to prevent the muscle from shortening when stimulated. 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 dependent 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 its volume (obtained by dividing its actual weight by 

 the specific weight of muscle-tissue, 1.058) by the average length of the 

 fibers. Assuming that the muscle weighs 609 grams, its volume would be 

 576 c.c.; and if it be further assumed that the fibers have an average length 

 of 4 centimeters the transverse section would contain 114 sq. centimeters 

 each of which would have a length of 4 centimeters. 



For purposes of comparison it is customary to refer the absolute 

 force to the units of area viz., one square centimeter. Rosenthal esti- 

 mates 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 10 kilograms for the muscles of the leg and 7 to 8 kilograms for the 

 muscles of the arm. 



Summation Effects. 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 interval preceding each 

 stimulus be sufficiently long to enable the musclq to recover from the effects 

 of the previous contraction, there will be no change in the form or the char- 

 acter of the contraction 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 contraction immediately 

 follows which is added to or superposed on the first; the effect produced will 

 be greater than that produced by either stimulus separately. (See Fig. 32.) 



A third stimulus applied during the relaxation of the second contraction 

 produces a third contraction which adds itself to the second, and so on. 

 The increment of increase in the extent of the successive contractions gradu- 

 ally diminishes, however, until the muscle reaches a maximum of contrac- 

 tion. The superposition of the second contraction on the first, the third on 

 the second, and so on, is termed summation of contractions or effects. Experi- 

 ment has shown that the greatest effect of a second stimulus that is, the 

 highest contraction is produced when the stimulus is applied during the 

 last third of the period of rising energy, when the sum of the two contractions 

 is almost twice as great as the first contraction would have been. (Fig. 31.) 

 The effects following both maximal and submaximal stimuli indicate that 

 the muscle cannot attain its maximum of shortening except through a 

 summation of several stimuli. If a second maximal stimulus enters a muscle 

 during the latent period following the first, the effect produced will be no 



