138 ^ ^ THE HUMAN BODY. 



natural shape from which it has been removed, but it assumes 

 a quite new natural shape, so that physiological contractility 

 is a different thing from mere physical elasticity; the essen- 

 tial difference being that the coiled spring or a 'stretched band 

 only gives back mechanical work which has already been spent 

 on it, while the muscle originates work independently of any 

 previous mechanical stretching. In addition to their contrac- 

 tility, however, muscles are highly elastic. If a fresh muscle 

 be hung up and its length measured, and then a weight be 

 hung upon it, it will stretch just like a piece of india-rubber, 

 and when the weight is removed, provided it has not been so 

 great as to injure the muscle, the latter will return passively, 

 without any stimulus or active contraction, to its original 

 form. In the Body all the muscles are so attached that they 

 are usually a little stretched beyond their natural resting 

 length; so that when a limb is amputated the muscles divided 

 in the stump shrink away considerably. By this stretched 

 state of the resting elastic muscles two things are gained. In 

 the first place when the muscle contracts it is already taut, 

 there is no "slack" to be hauled in before it pulls on the 

 parts attached to its tendons: and, secondly, as we have 

 already seen the working power of a muscle is increased by 

 the presence of some resistance to its contraction, and this is 

 always provided for from the first, by having the origin and 

 insertion of the muscles so far apart as to be pulling on it 

 when it begins to shorten. 



The Electrical Currents of Muscle. When a muscle is 

 exposed in the body or carefully removed from it and suitable 

 electrodes connected with a sensitive galvanometer are applied 

 to different parts of its surface, there is nearly always to be 

 found evidence of a difference of electric potential between 

 different parts of the muscle. These differences give rise to 

 currents which are shown by the galvanometer to travel 

 through the wires of the circuit from any central portion of 

 the muscle to any part nearer one end, or from any part of the 

 belly to a tendon. The less injured the muscle the more 

 feeble are these currents, and in very fresh and very carefully 

 exposed muscles they may be absent altogether. They are 

 probably altogether absent from perfectly uninjured resting 

 muscles, and when present in a resting muscle are due to the 

 fact that any more living part of a muscle is electrically posi- 

 tive to a more injured or dead. When a muscle is exposed 



