138 EXTENSIBILITY AND WORK OF MUSCLE. [CH. xi. 



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

 grammes of pressure. 



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

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

 plied 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. Similar, though 

 more complicated, combustions occur in muscle. 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 1 3 feet) ; 

 in doing this work probably less than a thousandth part of the 

 muscle has been consumed. 



Next let us consider the relationship between the work and the 

 heat produced. An ordinary locomotive wastes about 96 per 

 cent, of its available energy as heat, only 4 per cent, being 

 represented as work. In the best triple-expansion steam-engine 

 the work done rises to 12-5 per cent, of the total energy. 



In muscle, various experimenters give different numbers. Thus, 

 Fick calculated that 33 per cent, of the mechanical energy is 

 available as work ; later he found this estimate too high, and 

 stated the number as 25 ; Chauveau gives 12 to 15 ; M'Kendrick 

 17. Thus muscle is a little more economical than the best 

 steam-engines ; but the muscle has this great advantage over any 

 engine, for the heat it produces is not wasted, but is used for 

 keeping up the body temperature, the fall of which below a certain 

 point would lead to death not only of the muscles but of the 

 body generally. 



So far we have been speaking as though the only active phase of muscular 

 contraction is the period of shortening. It is. however, extremely probable, 

 though not yet proved that lengthening is also an active process. This was 

 originally mooted by Fick. who pointed out that the fall of a muscle lever 

 during the relaxation period is of variable speed, and is obviously not due to 

 the passive elongation of the muscle by gravity ; the way in which this part 

 of the curve is varied by such agencies as temperature, and drugs like vera- 

 trine. also indicates that relaxation is an independent process. 



Isotonic and Isometric Curves. If in recording the contraction of a 

 muscle, the load is applied vertically under the muscle, its pull upon the 

 muscle varies during the successive stages of a single contraction, owing to 

 the inertia of the load. In order to avoid this variation in tension, it is 

 usual to apply the weight at a point close to the fulcrum of the recording 

 lever, so that when the lever is raised, the weight remains practically 

 stationary, and thus the error due to its inertia is avoided. In order to 

 apply the necessary tension to the muscle, the weight hanging 011 the lever 

 must be increased in the ratio of the distances of the muscle and weight 

 from the fulcrum. A twitch recorded under such circumstances is called 



