THE MECHANICAL RESPONSE. 



355 



lengthen in exact accordance 

 with Hooke's law, i.e. the suc- 

 cessive increments of length 

 which are produced by equal 

 increments of load are not per- 

 fectly equal. In other words, 

 it is not perfectly elastic. It 

 is, however, so nearly elastic 

 that the statement above given 

 is sufficiently correct. 



In all experiments the ex- 

 ternal work actually done is 

 represented by the lifting of 



a weight, and estimated in 

 gramme-millimetres by multi- 

 plying the weight by the height 

 to which it is lifted; but the 

 amount so measured varies 

 according to the mechanical 

 conditions under which the 

 effort is made. In order to 

 obtain the most favourable 

 result, these must be such as 

 to enable the muscle, through- 

 out the period during which it 

 is being excited, to act to the 

 best advantage. Now, in a 

 muscle excited under isotonic 

 conditions, the stress rapidly 

 diminishes as the muscle 

 shortens. If, for example, it 

 shortens from its natural length 

 when unexcited to its natural 

 length in tetanus, the tension 

 diminishes pari passu with the 

 length, from a considerable 

 amount to zero. If the load 

 is nearly as much as it can lift 

 at the beginning of the effort, 

 there will be a slight initial 

 shortening followed by a long 

 period during which at first no 

 work is being done by it, but 

 afterwards " negative " work is 

 being done on it. If the load 

 is very small, the muscle will 

 at once nearly complete its 

 contraction, but will thereby 

 do little work ; for, though the 

 lift is great, the load is too 

 small to yield a considerable 

 product. If the original load 

 is reduced to half, the result of 



Fig. 190. — Cosine lever, combined with writing 

 lever and tension recorder. H H ', steel 

 writing lever, 32 cms. long, rotating on the 

 axle A, which bears two pulleys, of 4 and 

 8 mm. diameter respectively, from one of 

 which a scale-pan L is suspended. M', the 

 muscle preparation, fixed at z ; its free end, k, 

 being attached by a fine wire to one arm of 

 the writing lever at a distance half-way be- 

 tween the axis and the writing point s', i. c. 

 8 cms. from A. DD', wires from the second- 

 ary coil of an inductorium. To the extremity 

 of the other arm of the writing lever, at H ', i.e. 

 16 cms. from A, one bar of the cosine lever is 

 connected by a long thread, the whole length 

 of which (about 60 cms.) is not shown in the 

 figure. M, the movable block on this bar to 

 which the thread is attached. C, axle of the 

 cosine lever, fixed to a stand, so that the two 

 brass cross-bars shall be in the same vertical 

 plane as the lever, HH. L, sliding weight 

 of 200 grms. on the second cross-bar. S, 

 scale - pan for additional weights. °A A', 

 stops on the stand for preventing the weight 

 L from acting on the muscle before and after 

 the experiment respectively. hh', bamboo 

 tension-recording lever, with its fixed axle a 

 bearing a small pulley, only 1 mm. in dia- 

 meter, round which is strung a pliable but 

 inextensible thread, ending in a brass hook c, 

 by means of which it can be attached to a 

 pin d on the length-recording lever HH, at 

 the same point at which the free end of the 

 muscle is attached, and readily detached. 

 //', steel spring fixed at /, its other end, 

 /', being connected with the lever h h' by a 

 stiff wire. s, writing point of lever hh'. 

 na, position that hh' would assume on ex- 

 citation of the muscle. — After Fick. 



the first part of the effort will be much 



