MUSCLES AND PHENOMENA OF CONTRACTION. 125 



obtained only in the digestion of the muscle substance 

 itself. Special preparations of broth in which the muscle 

 substance has been at first digested artificially, and thus 

 made an ingredient of the broth have, of course, their full 

 nutritive value. 



Elasticity. 



The ordinary muscles possess to some extent an elas- 

 ticity which enables them when they are stretched by any 

 weight to return to their former position as soon as the 

 stress is removed. Naturally in the body all muscles are 

 slightly stretched, even when entirely relaxed, for a muscle 

 cut from one of its connections at once draws up and 

 becomes materially shorter. This is one of the dangers 

 accompanying the fracture of a bone, for as soon as the 

 controlling action of the bone is destroyed the muscles pull 

 themselves together, and so the ends of the bone may be 

 materially displaced. The advantage of having the muscles 

 on a stretch all the time is, that as soon as they begin to 

 contract they begin to pull upon the bones at once and no 

 time or energy is lost in first pulling up any slack of the 

 muscle itself. 



Physiology of Muscular Contraction. 



Thus far the muscle has been considered as an inert 

 structure, and all that has been said would apply to a dead 

 muscle as well as one still living. But living muscles 

 possess the property to contract when they are properly 

 stimulated. 



MUSCLE STIMULI. 



The usual stimulus to usher in a muscular contraction 

 is a nerve stimulus, but it may be stimulated in other ways. 

 First, by mechanical stimuli: Sudden pressure, tearing, 

 pricking, etc., produce each time a contraction of the 

 affected muscle. Second, thermal stimuli: A sudden change 

 of temperature causes the muscles to contract. The limits 

 of such changes are from almost freezing up to about one 

 hundred and ten degrees. Above and below these tern- 



