46 



PHYSIOLOGY 



CHAP. 



to a minimum if the muscle is laid horizontally on mercury, after 

 first dipping it into oil to diminish the surface friction. 



Again, the muscle does 110 work when it is loaded with such 

 a heavy weight that it is unable to raise it. In the first case the 

 energy developed by the excitation is exhausted in the contraction, 

 in the second in the tension of the muscle ; but in both cases 

 no external mechanical work, but only internal mechanical work 

 is done. 



On calculating the external work done by a muscle in raising 

 regularly increasing weights, it is found that it increases quickly 

 at first, and then more slowly, until it reaches a certain maximum, 

 after which it diminishes again and finally becomes nil on reaching 

 the weight which the muscle is unable to lift. 



Fig. 26 illustrates 



grms. 



i 50 



.. 100 



.. <50 



.. 200 



.' 250 



" 300 



.. 350 



i 400 



.. 450 



>, 500 



min. 5 



6 

 5 



4 

 3 



2 5 

 2 



grin. mm. 



.. 550 



.. 700 



.. 900 



,.000 



N<000 



> i. 900 



" 675 



ii ., 800 



,,675 



,, BOO 



Fio. 26. Diagram showing work done by muscle frog's gastrocnemius. (A. D. Waller.) 



these experimental results, which can be verified on every muscle 

 that is loaded and stretched before contraction, or merely while it 

 contracts. 



It may thus be stated that there is a given weight for every 

 muscle, at which it reaches its maximal yield of work, and that 

 with diminution or increase of the load the work becomes 

 gradually less till it finally reaches zero. This law of course 

 applies also to all groups of muscles which co-operate in the work 

 performed. 



The resistances encountered by the different muscles concerned 

 in complicated action vary ; the degree of shortening which they 

 undergo varies also. Generally speaking, the strength of a muscle, 

 i.e. the weight it is able to lift, increases in proportion to its 

 diameter, that is, the number of fibres it contains. Since work is 

 the product of the weight and the height to which it is raised, it 

 follows that, other things being equal, the work of a muscle is in 

 proportion with the product of its length and cross-section, viz. 



