THE TOTAL EXCHANGES OF THE BODY 



617 



31 7i Air circuit in Benedict's respiration 

 apparatus. 



side of the vessel is a corresponding series of vessels for the absorption of water and of 

 carbon dioxide. On the further side of these vessels is a gas meter. During an ex- 

 periment air is sucked through the whole apparatus by means of an aspirator or a water 

 pump, the amount of air passing through the apparatus being measured by the meter. 

 The animal is thus supplied with pure air freed from water vapour and from carbon 

 dioxide. Any water or carbon dioxide produced by the animal is absorbed by the 

 vessels interposed in the course of the outgoing air. These vessels are weighed at the 

 beginning of the experiment and at the end, and the difference in weights will there- 

 fore give the amounts of carbon dioxide and water which have been discharged by the 

 animal. 



The intake of oxygen by the animal is determined indirectly. Since it gives off 

 only carbon dioxide and water, and absorbs only oxygen during its stay in the chamber, 

 the loss of weight of the animal during 

 its stay in the chamber, subtracted from 

 the total amount of carbon dioxide plus 

 water it gives off, will represent the 

 amount of oxygen absorbed. 



The advantage of this apparatus is 

 that it can be fitted up in any labora- 

 tory, and is accurate for the purposes 

 to which it is applied. It is not, how- 

 ever, appropriate for long-continued 

 experiments or for experiments on 

 larger animals or on man himself. 

 Most of the data with regard to the 

 respiratory exchange under various cir- 

 cumstances have therefore been obtained 

 by one of the three following methods : 



II. THE METHOD OF REGNAULT AND &EISET. The principle of this method 

 consists in placing the animal that is to be the subject of investigation in a closed 

 chamber containing a given volume of air. The carbon dioxide produced by the animal 

 is absorbed by means of caustic alkali, and the oxygen consumed by the animal is made 

 good by allowing oxygen to flow into the chamber from a gasometer. The inflow of 

 oxygen is regulated so as to keep the pressure of air in the chamber constant. At 

 the end of the experiment the alkali is titrated and the amount of carbon dioxide 

 absorbed thus determined. The air in the chamber is also analysed so as to be certain 

 that it contains an excess neither of carbon dioxide nor of oxygen. The amount of 

 oxygen absorbed by the animal is known already, the oxygen which has been allowed 

 to flow in having been measured. 



A modification of this method has been devised by Benedict and is especially 

 applicable to clinical purposes. In this method the individual who is the subject of 

 the experiment breathes through a nose-piece into a wide metal tube, the mouth being 

 kept closed. The metal tube forms part of a closed system through which a current 

 of air is maintained by means of a pump. In the course of the current of air are inter- 

 posed vessels for the absorption of carbon dioxide and of water, and the volume of 

 gas in the system is maintained constant by admitting oxygen to it in proportion as 

 the oxygen of the system is used up in respiration. In Fig. 317 is given a diagrammatic 

 scheme of the air circuit, and in Fig. 318 a diagram of the arrangement of the whole 

 respiration apparatus, showing the nose-piece for breathing, the tension equaliser, 

 the air-purifying apparatus, and the oxygen cylinder. The tension equaliser, A, is 

 attached to the ventilating pipe near the point of entrance of the air into the lungs. 

 It consists of a pan with a rubber diaphragm (which may be conveniently made from 

 a lady's bathing-cap). As the air is drawn into the lungs the rubber diaphragm sinks, 

 to rise again with expiration. The respiratory movements can thus proceed without 

 altering appreciably the pressure within the closed system of tubes. By the admission 

 of oxygen the supply of oxygen is adjusted so as to keep the bag from becoming either 

 too much distended or too much flattened. As the air leaves the lungs and passes 



2C* 



