RESPIRATION 3 



the amount of heat formed by an animal is nearly equivalent to 

 that formed in combustion of carbon when an equal quantity of 

 oxygen is consumed in respiration and combustion. He thus made 

 it clear that in the living body, just as in combustion, oxygen 

 combines with carbon and other substances, producing carbon 

 dioxide and other oxidation products : also that this combination 

 is the source of animal heat. 



He found in the course of experiments on man that during 

 muscular work the consumption of oxygen and output of carbon 

 dioxide is increased. Curiously enough, he expresses regret that 

 this should be so, as the laboring classes, who have least money 

 for buying food, consume more food than those who are better 

 off. 6 The essential connection between physiological work and 

 consumption of oxygen was still hidden from him, although, as 

 already seen, Mayow had fairly correct ideas on this subject. 

 It was not until 1845 that Mayer, 7 a German country doctor, 

 pointed out in connection with the general formulation of the 

 doctrine of conservation of energy, that in living animals, as in 

 steam engines, ordinary kinetic energy as well as heat has its 

 source in the potential energy liberated in the process of oxida- 

 tion. Oxidation is thus the ultimate source of the energy of animal 

 movements. Every exact experiment made since then on this 

 subject has confirmed Mayer's conclusion, and the increased 

 consumption of oxygen during muscular work became as intelli- 

 gible as it was on Mayow's crude theory. 



The discoveries with regard to the chemistry of respiration 

 raised the further question as to what the exact nature of the 

 combustible material is, and where the combination of oxygen 

 with combustible matter occurs. As regards the first question it 

 was evident that since on an average the composition of the adult 

 living body remains constant, and the excreta, as compared with 

 the food taken, contain very little combustible material, the 

 material oxidized must correspond to the oxidizable matter of 

 the food. This material was classified by Prout as belonging almost 

 entirely to one or other of three groups of substances, known now 

 under the names of proteins, carbohydrates, and fats. Of these 

 the former alone contains nitrogen, which is excreted in the urine 

 in the form, mainly, of urea when the protein is oxidized. Only 

 water and carbon dioxide are formed in the oxidation of carbo- 



8 Lavoisier and Sequin, Mem. de I'Acaci., p. 185, 1789. 



7 Mayer, Die orgamsche Bewegung in threm Zusammenhange mit dem Stoft- 

 wechsel, Heilbronn, 1845. 



