98 THE HUMAN BODY 



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 con- 

 traction, and this is always provided for from the first, by having 

 the origin and insertion of the muscles so far apart as to be pull- 

 ing on it when it begins to shorten. 



The Electrical Phenomena of Muscle. When a living muscle 

 is carefully exposed and suitable electrodes connected with a 

 sensitive galvanometer or electrometer are applied to its surface 

 the entire surface is found to be isoelectric, i. e., having a uniform 

 electric potential. If, however, an injury such as cutting or 

 burning is inflicted upon any part of the muscle the injured sur- 

 face is found to possess a different potential from the surround- 

 ing uninjured surfaces. This difference of potential is shown by 

 movements of the indicator of the galvanometer or electrometer. 

 These movements are usually in such a direction as to indicate 

 that the injured region has a lower potential than uninjured parts 

 of the same tissue. This difference of potential existing between 

 injured and uninjured living tissue is often referred to as the 

 current of injury, although no current actually flows unless the 

 two regions are connected by an electrical conductor. No cur- 

 rent of injury can be obtained by connecting living tissue with 

 dead tissue. Only while the injured tissue is in act of dying does 

 it exhibit the altered potential which may give rise to an injury 

 current. 



The explanation, of the change of electric potential accompany- 

 ing an injury to living tissue is found in the fact that the death 

 process which follows injury involves extensive chemical changes 



