196 HAND-BOOK OF PHYSIOLOGY. 



. t The quantity of water exhaled from the lungs in twenty-four hours 

 ranges (according to the various modifying circumstances already men- 

 tioned) from about 6 to 27 ounces, the ordinary quantity being about 9 

 or 10 ounces. Some of this is probably formed by the chemical combina- 

 tion of oxygen with hydrogen in the system; but the far larger propor- 

 tion of it is water which has been absorbed, as such, into the blood from 

 the alimentary canal, and which is exhaled from the surface of the air- 

 passages and cells, as it is from the free surfaces of all moist animal mem- 

 branes, particularly at the high temperature of warm-blooded animals. 



6. A small quantity of ammonia is added to the ordinary constituents 

 of expired air. It seems probable, however, both from the fact that this 

 substance cannot be always detected, and from its minute amount when 

 present, that the whole of it may be derived from decomposing particles 

 of food left in the mouth, or from carious teeth or the like; and that it 

 is, therefore, only an accidental constituent of expired air. 



7. The quantity of organic matter in the breath is about 3 grains in 

 twenty-four hours. (Ransome.) 



The following represents the kind of experiment by which the fore- 

 going facts regarding the excretion of carbonic acid, water, and organic 

 matter, have been established. 



A bird or mouse is placed in a large bottle, through the stopper of 

 which two tubes pass, one to supply fresh air, and the other to carry off 

 that which has been expired. Before entering the bottle, the air is 

 made to bubble through a strong solution of caustic potash, which absorbs 

 the carbonic acid, and then through lime-water, which by remaining 

 limpid, proves the absence of carbonic acid. The air which has been 

 breathed by the animal is made to bubble through lime water, which at 

 once becomes turbid and soon quite milky from the precipitation of cal- 

 cium carbonate; and it finally passes through strong sulphuric acid, 

 which, by turning brown, indicates the presence of organic matter. The 

 watery vapor in the expired air will condense inside the bottle if the sur- 

 face be kept cool. 



By means of an apparatus sufficiently large and well constructed, 

 exp Briments of the kind have been made extensively on man. 



METHODS BY WHICH THE KESPIRATORY CHANGES IN THE AIR ARE 



EFFECTED. 



The method by which fresh air is inhaled and expelled from the lungs 

 has been considered. It remains to consider how it is that the blood 

 absorbs oxygen from, and gives up carbonic acid to, the air of the alveoli. 

 In the first place, it must be remembered that the tidal air only amounts 

 to about 25. 30 cubic inches at each inspiration, and that this is of course 

 insufficient to fill the lungs, but it mixes with the stationary air by diffu- 

 sion, and so supplies to it new oxygen. The amount of oxygen in expired 

 air, which may be taken as the average composition of the mixed air in 



