MUSCULAR ELASTICITY. 13? 



ole can actively contract or change its shape when a stimulus 

 .acts upon it, and that without being previously stretched. 

 It does not merely tend to return to a natural shape from 

 which it has been removed, but it assumes a quite new 

 natural shape, so that physiological contractility is a differ- 

 ent thing from mere physical elasticity; the essential differ- 

 ence 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 independ- 

 ently of any previous mechanical stretching. In addition 

 to their contractility, 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 indian -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 01 

 .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 

 Testing 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 muscle so far apart as to be pulling on 

 it when it begins to shorten. 



Physiology of Plain Muscular Tissue. What has hith- 

 erto been said applies especially to the skeletal muscles ; but 

 an the main it is true of the unstriped muscles. These 

 also are irritable and contractile, but their changes of form 

 are much more slow than those of the striated fibres. 

 Upon stimulation, a longer period of latent excitement 

 elapses before the contraction commences, and when finally 

 this takes place it is extremely slow, very gradually attain- 

 ing a maximum and then gradually dying away again. 



