124 THE WORK DONE. [BOOK i. 



In a muscle viewed as a machine we have to deal not merely 

 with the height of the contraction, that is with the amount of 

 shortening, but with the work done. And this is measured by 

 multiplying the number of units of height to which the load is 

 raised into the number of units of weight of the load. Hence it 

 is obvious from the foregoing observations that the work done 

 must be largely dependent on the weight itself. Thus there is a 

 certain weight of load with which in any given muscle, stimu- 

 lated by a given stimulus, the most work will be done ; as may 

 be seen from the following example : 



Load, in grammes 50 100 150 200 250 



Height of contractions in millimeters 149 7 5 2 

 Work done, in gram-millimeters ... 450 700 750 400 



77. The Influence of the Size and Form of the Muscle. Since 

 all known muscular fibres are much shorter than the wave-length 

 of a contraction, it is obvious that the longer the fibre, the greater 

 will be the shortening caused by the same contraction wave, the 

 greater will be the height of the contraction with the same 

 stimulus. Hence in a muscle of parallel fibres, the height to 

 which the load is raised as the result of a given stimulus applied 

 to its nerve, will depend on the length of the fibres, while the 

 maximum weight of load capable of being lifted will depend on 

 the number of the fibres, since the load is distributed among 

 them. Of two muscles therefore of equal length (and of the 

 same quality) the most work will be done by that which has the 

 larger number of fibres, that is to say, the fibres being of equal 

 width, which has the greater sectional area ; and of two muscles 

 with equal sectional areas, the most work will be done by that 

 which is the longer. If the two muscles are unequal both in 

 bngth and sectional area, the work done will be the greater in the 

 one which has the larger bulk, which contains the greater number 

 of cubic units. In speaking therefore of the work which can be 

 done by a muscle, we may use as a standard a cubic unit of bulk, 

 or, the specific gravity of the muscle being the same, a unit of 

 weight. 



We learn then from the foregoing paragraphs that the work 

 done, by a muscle-nerve preparation, will depend, not only on the 

 activity of the nerve and muscle as determined by their own 

 irritability, but also on the character and mode of application of 

 the stimulus, on the kind of contraction (whether a single spasm, 

 or a slowly repeated tetanus or a rapidly repeated tetanus) on the 

 load itself, and on the size and form of the muscle. Taking 

 the most favourable circumstances, viz. a well-nourished, lively 

 preparation, a maximum stimulus causing a rapid tetanus and an 

 appropriate load, we may determine the maximum work done by 

 a given weight of muscle, say one gramme. This in the case of 

 the muscles of the frog has been estimated at about four gram- 

 meters for one gramme of muscle. 



