THE MUSCULAR SYSTEM 65 



a similar tracing is taken from a muscle with its circulation in- 

 tact, a fatigue level of working power will be found. This proves 

 that the blood furnishes substances to maintain the Working power 

 of muscles and removes the wastes. 



A "green" horse — that is, one not accustomed to steady 

 work — fatigues much quicker $han a "hardened" horse, because 

 his muscles are softer and carry an excess of fat. By working 

 such a horse moderately, with properly spaced periods of rest, 

 it is possible to build up the muscle cells so that he can do as much 

 work as the "seasoned" animal. Rest gives an opportunity 

 for the muscles to repair themselves. During rest, as has been 

 shown, the blood furnishes new food to build up the worn and 

 broken-down cells and oxygen to furnish life. It should be 

 remembered that fatigue increases with the square of the amount 

 of work done and not in direct proportion. Furthermore, there 

 is a limit to continued muscular effort, and harmful fatigue can 

 be avoided only by working the horse at a moderate rate, so as to 

 keep a balance between the products of muscular activity and the 

 ability of the blood to remove this waste material and provide 

 fuel. An animal should under no circumstances be worked until 

 exhausted, for it is inhuman as well as uneconomical. 



Rigor mortis is the condition of stiffness which the muscles 

 usually assume soon after death. It is probably due to the coagu- 

 lation of myosin and other muscle proteins through the formation 

 of sarcolactic acid. The process changes both the physical prop- 

 erties and the chemical reaction. The muscles become firm and 

 unelastic to the touch, and give an acid reaction. During death- 

 stiffening carbon dioxide, sarcolactic acid, and heat are produced. 

 In some cases the after-death temperatures are very high. The 

 muscles of the head are first affected. From here the change 

 rapidly spreads backward to the body muscles. The time when 

 rigor mortis appears depends upon the activity of the muscles just 

 before death; the greater this has been, the sooner rigor sets in 

 and the longer it lasts. Animals which have died from exhaustion 

 or severe febrile disease show little rigor mortis; it appears very 

 soon and passes off quickly. In such cases decomposition changes 

 set in almost immediately as the bacteria present in the body find 

 conditions suitable for their rapid growth and multiplication. 

 When rigor mortis disappears the muscles become soft and the 

 body limp. The phenomenon of contraction is occasionally 



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