CHAP. XXIX.] CHANGES IN RESPIRED AIR. 405 



and the movements which regulate the supply of air, we may proceed 

 to consider the changes which occur in the air during its sojourn 

 in contact with the respiratory surface, and also the corresponding 

 changes in the blood circulating there. 



Changes in the respired Air. The air consists of a mixture of 

 oxygen and nitrogen, in the respective proportions of about 2O81 

 and 79*19 in 100 parts by volume, with the addition of a very 

 minute portion of carbonic acid gas, not exceeding one part by 

 volume in 2000. It contains watery vapour in variable quantity, 

 according to temperature and other circumstances ; and also non- 

 estimable quantities of other gaseous and volatile substances, of no 

 account in relation to the respiratory function. In air that has 

 been breathed, the temperature is found to have assumed nearly or 

 quite that of the blood, the quantity of moisture has nearly reached 

 the point of saturation, the proportion of oxygen has diminished, 

 and that of carbonic acid has increased, while that of nitrogen has 

 slightly increased, or has undergone little change. A small quan- 

 tity of animal matter has also been received from the air-passages, 

 as is proved by the brown colour assumed by sulphuric acid, 

 through which the respired air is made to pass. 



Aqueous Vapour exhaled. It results from the most careful ex- 

 periments, amongst which may be mentioned those of Valentin 

 and Brunner, and of Moleschott, that the air expired is usually 

 nearly saturated with moisture, and that the quantity of water thus 

 escaping from the system in twenty-four hours, may be estimated in 

 temperate climates at from twelve to twenty ounces. It is, of 

 course, impossible to ascertain, nor is it important, how much of 

 this quantity is derived from the respiratory surface, strictly so 

 called, arid how much from the moist and vascular surfaces of the 

 nasal, pharyngeal, and bronchial passages. 



Exhalation of Carbonic Acid by the Lungs. It is easy to shew 

 that carbonic acid gas is exhaled from the lungs, by breathing into 

 lime-water, which thus becomes turbid by the formation of inso- 

 luble carbonate of lime ; or into a phial, where a taper is then at 

 once extinquished. 



In 100 parts by volume of expired air (as an average of many 

 experiments), 



Carbonic Acid. 



Coathupe . . found . . 4-02. 

 Valentin and 



. R 



Brunner 



Vierordt . . . 4-33. 



Thomson . 4-16. 



