756 CHEMISTR Y OF RESPIRATION. 



This conclusion is supported by the work of Pfliiger's pupils, Finkler and 

 Oertmann, 1 who found that artificial respiration and apnoea 2 produced no 

 alteration in the absorption of oxygen by rabbits. The respiratory exchange 

 is determined by the activity of the tissues, and not by the frequency of 

 respiration, or the amount of oxygen contained in the blood. 



Other changes in the respired air. It has been shown that in 

 a man at rest the air respired undergoes a reduction in oxygen to about 

 16 per cent., and an increase in carbon dioxide to about 4 per cent. ; in 

 addition, the temperature of the inspired air is raised to that of the 

 body, and this generally occurs before the air reaches the smaller 

 bronchi. At this temperature the air is saturated with moisture, and 

 shows when dried a slight reduction, al^out -^ in volume, when it is com- 

 pared with the inspired air, and both are measured at and 760 mm. 

 This decrease in volume is due to the combination and retention of some 

 of the oxygen in the tissues, to the oxidation of some substances which 

 leave the body otherwise than by the lungs, and to the combination of 

 oxygen with hydrogen to form water. The oxygen does not reappear 

 entirely as oxygen in combination with carbon to form carbon dioxide ; 



CO 



this is shown by the respiratory quotient, -. , which in omnivorous and 



U 2 



carnivorous animals is about O'S. The effect of diet and other con- 

 ditions upon the respiratory quotient is considered elsewhere in this 

 work, and it has been shown 3 that, under certain conditions, marsh- 

 gas, hydrogen, and nitrogen may be discharged by the lungs. 



THE EFFECT OF KESPIRATION UPON THE BLOOD. 



Historical. The discovery of Harvey that every portion of blood passes 

 through the lungs during each complete circulation, confirmed the idea of the 

 early physiologists, that respiration produced important changes in that fluid ; 

 Harvey 4 himself thought that the blood discharged some noxious substances 

 as well as aqueous vapour into the air of the lungs. 5 



In 1669, Lower 6 observed, on opening the thorax of a living animal, and 

 keeping up artificial respiration, that the change of colour from venous to 

 arterial took place in the capillaries of the lungs ; the blood in the right 

 ventricle was dark, and if the artificial respiration ceased it passed through the 

 lungs to the left ventricle without attaining an arterial hue ; venous blood, 

 when exposed to air outside the body, acquired an arterial colour. Mayow, 7 

 even earlier than 1674, maintained that this change from venous to arterial 

 colour was due to the absorption by the blood of the nitro-aerial gas (oxygen) 

 from the air in the lungs, but his work was neglected and forgotten. 



About the year 1776, Priestley 8 made a series of experiments, in which he 

 showed that dark blood clot became red more rapidly in oxygen than in air, 

 but the red colour was reduced to purple when the clot was placed in nitrogen, 

 hydrogen, or carbon dioxide ; these alterations in colour also took place when 

 the blood clot was separated from the air by a piece of moistened bladder, or 

 by a thin film of milk. These changes were supposed by Priestley to be 



1 Arch.f. d. ges. Physiol., Bonn, 1877, Bd. xiv. S. 38. See also Pfluger, ibid., S. 9. 



2 See also Hanriot and Richet, Compt. rend. Acad. d. sc., Paris, 1887, tome civ. p. 1327. 



3 This article, pp. 700, 729. 



4 "DeMotu Cordis." 



5 For older theories see p. 692, and the references there given. 



6 "Tractatus rle Corde," Londini, 1669, pp. 175, 181. 



7 "Tractatus Primus," Oxon., 1674, p. 148. 



8 Phil. Trans., London, 1776, pt. 1, p. 226, 



