224 A MANUAL OF PHYSIOLOGY 



air, or, in general, the investigation of the gaseous inter- 

 change between the blood and the air in the lungs. (2) The 

 analysis and comparison of the gases of arterial and venous 

 blood, of the other liquids, and of the solid tissues of the 

 body, with a view to the determination of the gaseous inter- 

 change 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 

 composition 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). 



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

 rabbits bv 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 caustic potash 

 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) Larger and more elaborate arrangements, such as Pettenkofer's 

 great respiration apparatus, in which a man can remain for an in- 

 definite period, working, resting, or sleeping. Smaller chambers of 

 the same kind have also been used for animals. In Pettenkofer's 

 apparatus air is drawn through by an engine, its volume being 

 measured by a gasometer. But as it would be far too troublesome 

 to analyze the whole of the air coming from the chamber, a sample 

 stream of it is constantly drawn off, which also passes through a 

 gasometer, 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. But the oxygen has to be calculated by difference, 

 and all the errors fall upon it. 



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

 which is the most suitable of any at least, for small animals. It 

 depends upon the absorption of carbon dioxide by soda lime. See 

 Practical Exercises, p. 276. 



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

 saturated with watery vapour, and contains about 4 per cent, 

 more carbon dioxide and 4 to 5 per cent less oxygen than 

 the inspired. There may be In addition in expired air small 

 quantities of hydrogen or ammonia, but these are probably 



