GASES IN VENOUS AND ARTERIAL BLOOD. 457 



necessary to life, which is conveyed into the Hood" l excepting 

 that the vivifying principle is not named nor its other prop- 

 erties described, expresses what we now consider as one of 

 the two great principles of respiration. This is even more 

 strictly in accordance with fact than the idea of Lavoisier, 

 who supposed that all the chemical processes of respiration 

 took place in the lungs. Mayow also described the evolution 

 of gas from blood placed in a vacuum. 3 Many observers 

 have since succeeded in extracting gases from the blood by 

 various processes. Sir Humphry Davy induced the evolu- 

 tion of carbonic acid by raising arterial blood to the temper- 

 ature of 200 Fahr., and venous blood to a temperature of 

 112 ; 3 Stevens, 4 and others, disengaged gas by displacement 

 with hydrogen, nitrogen, or the ordinary atmosphere ; but 

 in spite of this, before the experiments of Magnus in 183Y, 

 many denied the existence in the blood of any free gas what- 

 soever. 5 



Magnus made some experiments upon the human blood, 

 extracting the gases by displacement with hydrogen ; but the 

 observations which, are most generally referred to by phys- 

 iologists were made upon the blood of horses and calves, 

 extracting the gases by the air-pump, and giving the com- 

 parative quantities existing in the arterial and venous blood. 

 These experiments were of great value as settling the ques- 

 tion of the existence of gases in the blood, either in a free 

 state, or very loosely combined with some of its organic con- 

 stituents ; and until very recently they have been universally 



1 See page 411, note. 



" See quotation in MILNE-EDWARDS, Physiologic, tome i., p. 438, note. 



3 SIR HUMPHRY DAVY, Works, London, 1839, vol. i., pp. 77-79. An Essay 

 on Light, Heat, and the Combination of Light, with a new Theory of Respiration. 



4 Loc. cit. 



6 Gmelin, Mitscherlich, and Tiedemann denied the existence of any free gases 

 in the blood. At one time Dr. John Davy held the same opinion, though he 

 finally recognized his error, and succeeded in extracting gas from the blood by 

 means of the air-pump (Researches, Physiological and Anatomical, London, 

 1839, vol. ii., p. 154). 



