4 RESPIRATION 



hydrates or fats, and by the ratios and amounts in which nitrogen 

 compounds and carbon dioxide are excreted and oxygen consumed 

 we can calculate how much protein, carbohydrate, and fat is 

 being consumed in the body. 



As regards the second question there was for long much doubt. 

 It was, however, definitely shown by Magnus 8 in 1845 tnat mucn 

 gas is liberated from blood on exposing it to a vacuum, and that 

 less oxygen and more carbon dioxide are given off from venous 

 than from arterial blood. The mercurial blood gas pump was 

 then gradually perfected, mainly by Lothar Meyer, Ludwig, and 

 Pfliiger ; and it was gradually established that the oxygen which 

 disappears in the lungs is taken up by the blood almost entirely 

 in the form of a loose chemical compound with haemoglobin, the 

 colored albuminous substance in the red corpuscles. This compound 

 yields up part of its oxygen as the blood passes round the systemic 

 circulation, and returns to the lungs for a fresh charge, the charg- 

 ing being due to the higher partial pressure of oxygen in the 

 lungs, while the partial discharging in the systemic circulation 

 is due to the lower partial pressure there in consequence of con- 

 sumption of oxygen. The discharging is accompanied by a change 

 of color from scarlet to dark purple. Similarly carbon dioxide is 

 taken up mainly in the form of a loose chemical combination with 

 alkali, and discharged in the lungs as a consequence of the lower 

 partial pressure of the gas in the lungs. For a considerable time 

 there was much doubt as to how far the actual oxidation occurs in 

 the blood or in the tissue elements; but the investigations of 

 Pfliiger 9 about 1872 showed clearly that practically all the oxida- 

 tion occurs in the tissues. 



So far I have discussed from an abstract physical and chemical 

 standpoint the main outlines of discovery relating to respiration. 

 It is now necessary to consider these discoveries more closely, and 

 from a physiological standpoint. For a long time the brilliance 

 of Lavoisier's discovery as to the relation between respiration and 

 animal heat carried physiologists to some extent off their balance, 

 as it came to be believed that heat production is a more or less 

 blind mechanical process under no direct organic control, and 

 presumably dependent simply upon the supply of oxygen and 

 oxidizable material. Thus Liebig, who was not only a great 

 chemist but also a great chemical physiologist, concluded that 

 every increase in the food consumed or the amount of oxygen 



8 Magnus, Annalen der Physik, XL, 1838, and LXVI, 1845. 



9 Pfliiger, Pjliiger's Archrv, VI, p. 43, 1872. 



