THE MECHANICAL RESPONSE OF MUSCLE 231 



muscle of the frog or tortoise were treated in the same way, a stimulus 

 applied at A would cause a contraction which would travel across the 

 bridge at the upper end and extend to B. 



It was shown by Gotch that, if each of the three roots which make up 

 the sciatic nerve and send fibres to the gastrocnemius be stimulated in turn, 

 it is often impossible to evoke a maximal contraction of the gastrocnemius, 

 however strongly each root be stimulated. Keith Lucas has shown that if 

 stimuli in gradually increasing strength be applied to the motor nerve (containing 

 only seven to nine fibres,) which supplies the dorso-cutaneus muscle of the frog, 

 the contraction of the muscle increases, not gradually, but by a series of steps. 

 This can only be explained by assuming that the smallest effective stimulus 



100 



150 



200 



FIG. 66. Curve showing relation of height of contraction of dorso-cutaneus 

 muscle to strength of stimulus. Ordinates = height of contraction ; 

 abscissa = strength of stimulus. (K. LUCAS.) 



excites perhaps four out of the seven nerve fibres, those immediately in contact 

 with the electrodes. With increasing strength of current the stimulus becomes 

 effective for the three fibres lying next to these, and finally still further increase 



of current may excite all the fibres making up the nerve (Fig. 66). 







LOAD. The height of contraction of a muscle diminishes as the 

 load is increased. This diminution in height is at first very slight 

 and is not proportional to the load, so that the work done by the 

 muscle, which is measured by the product of the weight lifted and the 

 height to which it is raised, w X h, with increase of weight rises at 

 first quickly, then more slowly to a maximum, and then, on further 

 increasing the load, sinks. 



This will be rendered clearer by reference to the diagram (Fig. 67) 

 representing the lengths of the resting and contracted muscle with 

 various loads. The lines h , h l5 &c., are the actual height of contrac- 

 tion of the muscle when loaded with weights of 0, 10, 20 grm., &c. 

 The work in each case is given by h X 0, h t X 10, h 2 X 20, h 3 X 30, 

 &c. By inspection it will be seen that 



O.h <10.h 1 20.h 2 30.h 3 > 40.h 4 > 50.h 5 . 

 In this case therefore the maximum of mechanical work is obtained 



