140 EESPIRATION. 



of the blood and our knowledge of the composition of the air and the properties of 

 oxygen. 



The discovery of the properties of oxygen and carbonic acid, although bearing upon the 

 great question under consideration, were simply isolated facts and failed to develop any 

 definite idea of the changes of the air and blood in respiration. The application of these 

 facts was made by the great chemist, Lavoisier, who was the first to employ the delicate 

 balance in chemical investigation, and whose observations mark the beginning of an 

 accurate knowledge of the function of respiration. With the balance, Lavoisier showed 

 the nature of the oxides of the metals ; he discovered that carbonic acid is formed by a 

 union of carbon and oxygen ; and, noting the consumption of oxygen and the produc- 

 tion of carbonic acid in respiration, advanced, for the first time, the view that the one 

 was employed in the production of the other. Although, as would naturally be expected, 

 the doctrines of this great observer have been modified with the advances in science, he 

 developed facts which will stand forever, and which have served as the starting-point of 

 all our knowledge on this subject. From that time, physiologists began to regard respi- 

 ration as consisting in the appropriation of oxygen and the exhalation of carbonic acid ; 

 and now the seat of this process is simply changed from the lungs to the tissues. From the 

 limited knowledge of the intimate phenomena of nutrition which obtained in his day, 

 Lavoisier could not be expected to entertain any other view than that the carbonic acid 

 produced was the result of a direct union of oxygen w r ith carbon in the blood. It is 

 only since investigations have made manifest the great complexity of the processes of 

 nutrition, that some are unwilling to believe that carbonic acid is produced in so simple 

 a way as it appeared to Lavoisier. 



Composition of the Air. Pure atmospheric air is a mechanical mixture of 79*19 parts 

 of nitrogen with 20'81 parts of oxygen (Dumas and Boussingault). It contains, in addi- 

 tion, a very small quantity of carbonic acid, about one part in 2,000 by volume. The air 

 is never free from moisture, which is very variable in quantity, being generally more 

 abundant at a high than at a low temperature. In 1840, Schonbein discovered in the 

 air a peculiarly odorous principle called ozone, which he conceived to be a compound of 

 oxygen and hydrogen, but which is now pretty well shown to be an allotropic form of 

 oxygen. Oxygen obtained by decomposing water by the Voltaic pile is in this condition. 

 It exists in very small quantity in the air, and, as far as we know, plays no important 

 part in the function of respiration. Its chief interest has been in its theoretical rela- 

 tions to epidemic diseases. Floating in the atmosphere, are a number of excessively- 

 minute organic bodies. Various odorous and other gaseous matters may be present as 

 accidental constituents of the atmosphere. 



In considering the function of respiration, it is not necessary to take account of any 

 of the constituents of the atmosphere except oxygen and nitrogen, the others being 

 either inconstant or existing in excessively minute quantity. It is necessary to the regu- 

 lar performance of the function, that the air should contain about four parts of nitrogen 

 to one of oxygen and have about the density which exists on the general surface of the 

 globe. When the density is very much increased, as in mines, respiration is usually 

 more or less disturbed. By exposure to a rarefied atmosphere, as in the ascent of high 

 mountains or in aerial voyages, respiration may be very seriously interfered with, from the 

 fact that less oxygen than usual is presented to the respiratory surface and the reduced 

 atmospheric pressure diminishes the capacity of the blood for holding gases in solution. 



Magendie and Bernard, in experimenting on the minimum proportion of oxygen in 

 the air which is capable of sustaining life, found that a rabbit, confined under a bell- 

 glass, with an arrangement for removing the carbonic acid and water exhaled as fast as 

 they were produced, died of asphyxia when the quantity of oxygen became reduced to 

 from three to five per cent. 



A few experiments are on record in which the human subject and animals have been 



