58 THE HORSE IN HEALTH AND DISEASE 



slowly revolving cylinder. A fatigue tracing of this kind shows 

 that continuous work by a muscle removed from the body and, 

 therefore, not supplied with blood, finally results in the muscle's 

 refusal to respond to stimulation. On the other hand, if a similar 

 tracing is taken from a muscle with its circulation intact, a fatigue 

 level of working power will be found. This proves that the blood 

 furnishes substances to maintain the working power of muscles. 



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

 — fatigues much easier than a "hardened" horse, because his mus- 

 cles 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. It should be remembered that there is a 

 limit to continued muscular effort, and that harmful fatigue can be 

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

 keep a balance between the products of muscle activity and the 

 ability of the blood to remove this waste material. An animal 

 should under no circumstances be worked until exhausted, if for 

 no other reason than that it is not economical. 



Rigor mortis is the condition of stiffness which the muscles 

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

 lation of certain muscle proteins through the formation of sarco- 

 lactic acid and myosin. 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 becomes limp. 



INVOLUNTARY MUSCLES 

 Involuntary muscles are smooth or unstriped. They occur in 

 the walls of all the hollow organs of the body, such as the stomach, 

 intestines, bronchi, bladder, arteries, and veins. Because they are 

 found almost exclusively in soft organs they are known as visceral 

 muscles. In structure they differ from the voluntary muscles 



