106 ANIMAL CHEMISTRY LECTURE V. 



strong, indeed, not more than one-tenth of the force liberated by 

 the combustion of the coal used for raising steam is realised as 

 available motion. Be this as it may, it appears that by burning, 

 or consuming, or oxidating a given weight of muscle in our 

 bodies, we obtain a quantity of available motive force which 

 could only be produced by the combustion of at least five or 

 six times its weight of coal in the most perfect steam-engine 

 that ever was constructed. 



(113.) The superior economy of muscle over any artificial con- 

 trivance, as a motive machine, seems to depend in great measure 

 upon the circumstance of the force liberated by its oxidation 

 being expressed directly in motion, instead of first appearing in 

 some intermediate form of energy. Thus, in a steam-engine, the 

 immediate effect of the oxidation is not motion, but heat, some of 

 which eventually or intermediately appears as external motion. 

 In this case, the combination first produces heat, and the heat is 

 afterwards transformed into motion ; whereas, in muscular tissue, 

 the combination first produces motion, which is afterwards, in 

 many cases, transformed into heat. The force liberated by the 

 combustion of the muscular fibre of the heart, for instance, is 

 expressed directly in the contraction of its ventricles, and the 

 consequent propulsion of the blood through the greater and lesser 

 circulations. But by the time the blood gets back to the heart, 

 it has given up all its motion, and requires to be again propelled 

 by another contraction of the ventricles, and so on. Now, what 

 becomes of the motion received by the blood at each contraction ? 

 It appears in the form of heat. The blood circulating through 

 the vessels and capillaries undergoes a certain amount of friction. 

 It is brought to a state of rest gradually by the hindrance to its 

 motion, just as a bullet is brought to rest suddenly by the hin- 

 drance to its motion. In both cases, that which was motion 

 becomes heat, and, in the latter case, the quantity of heat finally 

 produced by the friction of the blood is generated as truly by the 

 combustion of the heart-fibre, as if it had been burnt directly 

 in a furnace. Under certain circumstances, indeed, as in the 

 attempt to lift a heavy weight, the oxidation of muscle within 



