7 5 6 CHEMISTR Y OF RESPIRA TION. 
This conclusion is supported by the work of Pfliiger's pupils. Finkler and 
Oertmann, 1 who found that artificial respiration and apncea - 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, about ^ in volume, when it is com- 
pared with the inspired air, and both are measured at 0° 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, -t=\ — > which in omnivorous and 
carnivorous animals is about 08. 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 Respiration 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. 
In 1669, Lower 6 observed, on opening the thorax of a living animal, ami 
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, acquire 1 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./, d. cjes. Physiol., Bonn, 1S77, Bd. xiv. S. 3S. See also Pfluger, ibid., S. 9. 
2 See also Hauriot and Richet, Gompt. read. Acad. d. se., Paris, 1SS7, tome civ. p. 1327. 
3 This article, pp. 700, 729. 
4 "De Motu Cordis." 
5 For older theories see p. 692, and the references there given. 
6 "Tractatus de Corde," Londini, 1669, pp. 175, 181. 
7 "Tractatus Primus," Oxon., 1674, p. 148. 
8 Phil. Trans., London, 1776, pt. 1, p. 226. 
