114 EXTENSIBILITY, ELASTICITY, AND WORK OF MUSCLE [CH. X. 



gramme-millimetres, the next 20x6 = 120 gramme-millimetres, and 

 so on, while the last on the right, 100 x 3 = 300 gramme-millimetres. 

 It is thus seen that the height of a muscle-curve is no measure of the 

 work done by the muscle unless the weight lifted is taken into 

 account as well. 



The following figures are taken from an actual experiment done 

 with the frog's gastrocnemius (Weber) : 



FIG. 132. Dynamometer. 



The work increases with the weight up to a certain maximum, 

 after which a diminution occurs, more or less rapidly, according as 

 the muscle is fatigued. 



Similar experiments have been made in human beings, weights 

 being lifted by the calf muscles, or elbow muscles, leverage being 



allowed for. In the higher 

 animals the energy so ob- 

 tained compared with the frog 

 is about twice as great for 

 the same volume of muscular 

 tissue. 



Fig. 132 represents a com- 

 mon form of dynamometer for 

 clinical use, employed in test- 

 ing the muscles of the arms 

 and hands. It is squeezed by the hand, and an index represents 

 kilogrammes of pressure. 



The muscle, regarded as a machine, is sometimes compared to 

 artificial machines like a steam-engine. A steam-engine is supplied 

 with fuel, the latent energy of which is transformed into work and 

 heat. The carbon of the coal unites with oxygen to form carbonic 

 acid, and it is in this process of combustion or oxidation that heat 

 and work are liberated. Although the analogy between muscle and 

 a steam-engine is by no means an exact one (see more fully p. 397), 

 nevertheless it may stand for our present purpose. In a steam- 

 engine a good deal of fuel is consumed, but there is great economy 

 in the consumption of the living muscular material. Take the 

 work done by a gramme (about 15 grains) of muscle in raising a 

 weight of 4 grammes to the height of 4 metres (about 13 feet) ; in 



