RESPIRATION 241 



of the gaseous exchange between the tissues and the blood. We 

 shall take these up as far as possible in their order. 



The methods which have been used for comparing the com- 

 position of inspired and expired air are very various. 



(1) Breathing into one spirometer and out of another, the inspired 

 and expired air being directed by valves. The contents of the spiro- 

 meters are analyzed at the end of the experiment (Speck). In the 

 arrangement of Zuntz and Geppert, instead of the whole of the 

 expired air, a sample is collected for analysis during the entire dura- 

 tion of the experiment, while the total volume expired is measured by 

 a gas-meter. This is a very convenient method for observations on 

 man, especially in disease, but each experiment can only b3 carried 

 on at most for fifteen to twenty minutes. 



(2) A small apparatus, much on the same principle, was used for 

 rabbits by Pfliiger and his pupils. A cannula in the trachea was 

 connected with a balanced and self-adjusting spirometer containing 

 oxygen, and the inspired and expired air separated by potassium 

 hydroxide valves, which absorbed the carbon dioxide. The amount 

 of oxygen used could be read off on the spirometer, and the amount 

 of carbon dioxide produced estimated in the liquid of the valves. 



(3) Elaborate arrangements, such as Pettenkofer's great respira- 

 tion apparatus, and the still larger and more efficient modifica- 

 tions of it constructed since his time, in which a man, or even several 

 men, can remain for an indefinite period, working, eating, and 

 sleeping. Air is drawn out of the chamber by an engine, its volume 

 being measured by a gas-meter. But as it would be far too trouble- 

 some to analyze the whole of the air, a sample stream of it is con- 

 stantly drawn off, which also passes through a gas-meter, through 

 drying-tubes containing sulphuric acid, and through tubes filled with 

 baryta water. The baryta solution is titrated to determine the 

 quantity of carbon dioxide ; the increase in weight of the drying 

 tubes gives the quantity of aqueous vapour. A similar sample 

 stream of the air before it passes into the chamber is treated exactly 

 in the same way, and from the data thus got the quantity of carbon 

 dioxide and aqueous vapour given off can readily be ascertained. 

 The oxygen can be calculated, as the difference between the final 

 body-weight and the original body-weight plus the weight of the 

 carbon dioxide and water eliminated, but may also be directly 

 estimated by special methods. 



(4) Haldane and Pembrey have elaborated a gravimetric method, 

 which is very suitable for small animals. It depends upon the ab- 

 sorption of carbon dioxide by soda lime. (See Practical Exercises, 

 p. 293). In Atwater's so-called respiration calorimeter, which will 

 be referred to again under ' Animal Heat,' and by which, not only the 

 gaseous metabolism, but the heat production can be measured in 

 man, the carbon dioxide is estimated in the same way. 



The expired air is at or near the body temperature, and is 

 saturated with watery vapour. In ordinary breathing it contains 

 about 4 per cent, of carbon dioxide, while the inspired air only 

 contains a trace. The expired air contains 16 or 17 per cent, of 

 oxygen, the inspired air about 21 per cent. The percentage of 

 carbon dioxide in the alveolar air is, of course, greater than in the 

 ordinary expired air, since the relatively pure air of the dead space 



16 \: 



