GASES OF THE BLOOD 517 



composition of the expired air, for the latter contains much air 

 from the " dead space " of the respiratory tract together with 

 alveolar air. It is necessary to know the composition of this 

 alveolar air, and thus ascertain the pressures of oxygen, carbon 

 dioxide, and nitrogen, to which the blood has been exposed. Like- 

 wise in the case of the blood, both arterial and venous, it is not 

 only necessary to determine the quality but also the pressure, 

 or, as it is often called, the tension, of its several gases. Is the 

 pressure of oxgyen in the venous blood less than that of the oxygen 

 in the alveolar air ? Is the pressure of carbon dioxide in the venous 

 blood greater than that of the carbon dioxide in the alveolar air ? 

 These are important questions, which at present cannot be definitely 

 answered. Some observers maintain that the respective pressures 

 are such, that according to physical and chemical laws oxygen 

 must pass into, and carbon dioxide out of, the venous blood in 

 the lungs ; others as strongly insist that the pressure of oxygen 

 in the blood is above, and that of carbon dioxide below, the 

 pressures of the corresponding gases in the air of the pulmonary 

 alveoli. The former find a sufficient explanation of the gaseous 

 exchange in ordinary physical and chemical processes ; the latter 

 regard the exchange as a special physiological process, a secretion 

 of gas, which cannot be explained by our present knowledge of 

 physics and chemistry. Around this subject has been and still is 

 waged the contest between the Mechanical and Vitalistic Schools 

 of Physiology. Before an attempt is made to discuss this difficult 

 question, it will be necessary to consider the simple qualitative 

 results of the analyses of the gases of venous and arterial blood. 



Gases of the Blood. The presence of gases in the blood was 

 first demonstrated in 1636 by Boyle, who found that fresh defibrin- 

 ated blood gave off bubbles of gas when it was exposed to the 

 vacuum of an air-pump. The importance of this experiment was 

 grasped by Mayow, who considered the gas to be nitro-aerial gas, 

 that is, the gas now known as oxygen. The next advance was made 

 by Priestley about the year 1776 ; he noticed that blood placed in 

 an atmosphere of hydrogen or nitrogen gave off oxygen. A few 

 years later the question, whether the blood did or did not contain 

 gas, became the subject of many experiments and much debate ; 

 some observers, among whom may be mentioned Humphry Davy 

 and his brother, John Davy, obtained carbon dioxide and oxygen 

 or carbon dioxide alone ; others could find no gas. The contra- 



