64 PRINCIPLES OF VETERINARY SCIENCE 



power and no differentiation is made between fuel. In the or- 

 ganism the food is not only heat-producer, but tissue-builder, 

 repairer, and generator of new organisms of a similar type. 

 From these statements it is clear that muscle is a chemical-engine 

 and not a heat-engine. 



A muscle supplied with fuel has been found by various methods 

 of study to produce about 66% per cent, of the heat which the 

 same material would produce when burned. The balance, or 

 33^3 per cent., is convertible into work. Most steam engines 

 can use only about 6 per cent, of the energy in fuel for mechanical 

 work, the rest is lost as heat. Accordingly, muscles are more 

 economical workers than steam engines and in. addition they have 

 the further advantage of being able to utilize part of the liberated 

 heat to maintain the temperature of the body, which is necessary 

 for life-processes. 



Muscular fatigue follows continued work. This is either due 

 to the accumulation of products of activity, or to the using up 

 of substances from which energy is derived. The latter results 

 in muscular exhaustion. If time be allowed for rest, the 

 accumulated waste products are removed by the blood and 

 lymph, a fresh supply of nutrients is brought to the organ, 

 there is a rapid renewal of irritability and contractility, and a 

 feeling of "fitness" prevails. The experiment of tying off one 

 leg of a frog and injecting extract of a fatigued muscle shows 

 clearly the effect that these products have on muscular activity. 



Hand-rubbing the legs of horses is beneficial because the blood- 

 and lymph-vessels are stimulated to increased activity in the 

 removal of waste products, and in causing the blood to circulate 

 more freely. Fatigue may be in part overcome by providing a 

 feed of a readily available carbohydrate, such as molasses, which 

 is quickly converted into glucose, in which form, it may be at 

 once utilized by the cells of the body for nutritive purposes. 



Experimentally, the course of fatigue in a muscle may be 

 studied by causing an isolated muscle to contract to its limit, at 

 regular intervals, against the resistance of a spring. The length 

 of the successive contractions is recorded on the smoked surface 

 of a 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 mus- 

 cle's refusal to respond to stimulation. On the other hand, if 



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