THE MECHANICAL CHANGES OF MUSCLE 203 



THE EXTENSIBILITY OF MUSCLE 



Living muscle in a perfectly normal condition is distinguished by its 



slightj)ut perfect elasticity ; that is 

 tosay, it is considerably stretched by a 

 slight force (in the longitudinal direc- 

 tion), but returns to its original length 

 when the. extending weight is re- 

 moved. The length to which muscle 

 Fia. 62. Extensibility of india-rubber (a) is stretched is not proportional to 



S^Tmu^M^ f a fr g ' S gaStr C " ^weight used, but any given in- 



crement of weignt gives rise to less 



elongation the more the muscle is already stretched. The accompanying 

 curves show diagrammatically the elongation of muscle as compared with a 

 piece of india-rubber when the weight on it is uniformly increased. 



Dead muscle is less extensible and its elasticity is less perfect. A given 

 weight applied to a dead muscle will not stretch it so much as when the 

 muscle was alive, but the dead muscle does not return to its original length 

 when the weight is removed. 



A contracted muscle, on the other hand, is more extensible than a muscle 

 at rest. A gramme applied to a contracted gastrocnemius will cause greater 

 lengthening than if it 

 were applied to the 

 same muscle at rest. 

 The relation between 

 the excitability of a 

 muscle under the two 

 conditions of contrac- 

 tion and rest are shown 

 in the diagram in Fig. 

 63. 



At the Point y the 

 muscle is unable to 

 shorten at all against 

 a weight. It is evi- 

 dent from this diagram 



that the height of contraction of a muscle diminishes as the load is increased, 

 very rapidly if the muscle is after-loaded, less rapidly if the weight applied 

 the muscle be allowed to extend it at rest. It is evident however that j 

 either case the diminution in height is not in proportion to the load t 

 that the work done by the muscle, w x h, as the weight is, increased, n 

 first quickly, then more slowly to a maximum to sink finally t 

 inspection of diagram (Fig. 63) it will be seen that 



O.h < lO-hj < 20.h 2 < 30.h 3 > 40.h 4 > 50.h 5 , 



so that in this case the maximal mechanical work is obtained when the muscle 

 is loaded with about 30 gms. 



showing the length of a muscle under various 

 loads in the contracted condition by, and uncontracted 

 condition cy. The double lines a b, &c., represent the con. 

 tracted muscle, while the long single lines a c, Ac., show the 

 length of the inactive muscle. 



