68 TEXT-BOOK OF PHYSIOLOGY. 



Weight. Height. Work Done. 



o grams 14 mm. o gram-millimeters. 



50 grams 9 mm. 450 gram-millimeters. 



100 grams 7 mm. 700 gram-millimeters. 



150 grams 5 mm. 750 gram-millimeters. 



200 grams 2 mm. 400 gram-millimeters. 



250 grams o mm. o gram- millimeters. 



From the preceding figures it is evident that the mechanical work of a 

 muscle increases with increasing weights up to a certain maximum, and 

 then declines to zero. Equally when the muscle contracts to its maxi- 

 mum without being weighted, and when it does not contract at all from 

 being overweighted, no work is done. Between these two extremes the 

 muscle performs varying amounts of work. 



Absolute Muscle Force. The maximum amount of force which a 

 muscle puts forth during a contraction is naturally measured by the amount of 

 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 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 144 sq. centi- 

 meters 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 muscle 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 grad- 



