48 



THE AXIMAL AS A MACHIXE. 



ment of the periods of employment to best effect are 

 as important as the supply of the best coal and the 

 periodical stoppage and repair of the machinery in a 

 mill or factory. 



The elimination of nitrogen and carbon dioxide is, in 

 some as yet uncertain way, a gauge of the quantity of 

 useful and lost Avork of the animal machine. Liebig 

 states in his Animal Chemistry (1843) that the excre- 

 tion of nitrogen is proportional to the destruction of 

 tissue, and Lehman states in his turn that he finds this 

 excretion increased by exercise. Fick and AVislicenus, 

 on the other hand, assert that the animal is a heat- 

 engine, and that the performance of work affects the 

 elimination of nitrogen slightly, but increases that of 

 carbon dioxide enormously, and this view is confirmed 

 by Frankland and by Houghton : Avhile Dr. Parkes in- 

 directly gives similar testim.ony in the statement that 

 work is done by the consumption of other than nitroge- 

 nous foods, the elimination of nitrogen being due to 

 waste of tissue ; that of carbonic acid to combustion 

 resulting in thermodynamic action. Incidentally, 

 Liebig also confirms this idea by his statement that 

 the exhalation of nitrogen goes on long after work has 

 ceased."^ Dr. Pavy, on the other hand, found by ex- 

 periment on two well-knoAvn pedestrians that their 

 excessive exertion produced greatly increased elimi- 

 nation of nitrogen — apparently a consequence of the 

 breaking down of tissue. He concludes that the body 

 is a true heat-engine, but he finds it capable apparently 



*See Frankland's Origin of Muscular Power; Houghton in the 

 Lancet, iS63 ; Parkes in the Medical Times, 1871 ; Flint on Muscular 

 Power, 1872 ; Pavy on Muscular Power, 1878. 



