THE MUSCULAR SYSTEM 57 



the activity of the muscle. It is measured in calories by a specially 

 constructed apparatus, termed the "calorimeter." Work is a result 

 of muscular activity. It is also accurately estimated by dehcately 

 controlled instruments. Locomotion and the pulling of a load are 

 examples of direct work. Indirect work is seen in the modifica- 

 tions of the visceral organs. About two-thirds of the energy liber- 

 ated when a muscle contracts is in the form of heat, the other one- 

 third may be applied as external work. 



The living animal requires fuel material to generate heat and 

 perform work. This is furnished by the combustion of the nutri- 

 ents digested from the hay and grain which it consumes. The 

 transformation of stored energy into heat and work in the animal 

 body takes place according to the same general laws of chemistry 

 which govern the generation of these powers in artificial motors 

 like steam and gasoline engines. A muscle supplied with fuel 

 produces about 66f per cent, of the heat which the same material 

 would produce when burned. The balance, or 33f per cent., is 

 converted into work. Many factors influence the proportion of 

 energy produced, but it follows the great law of conservation of 

 energy in nature. The body neither manufactures nor destroys 

 energy. All that it produces is derived from its food. 



Fatigue of the muscle follows continued work. This is either 

 due to the fact that the products of activity, principally sarcolactic 

 acid, accumulate in the muscle, or to the using up of substances 

 from which energy is derived. The latter results in the condition 

 termed "exhaustion." As soon as the accumulated waste products 

 are removed by the blood and lymph, and a fresh supply of nutri- 

 ents is brought to the organ, there is a rapid renewal of irritability 

 and contractility and a feeling of "fitness" prevails. 



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

 and Ijonph-vessels are stimulated to increased activity in the re- 

 moval of waste products, and in causing the blood to circulate more 

 freely. Fatigue may be in part overcome by providing a feed of 

 readily available carbohydrates which are convertible into glu- 

 cose, in which form they may be at once utiUzed 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 



