662 



PHYSIOLOGY 



nitrogen, after a few days a condition known as nitrogenous equilibrium is set 

 up, i. e. the total nitrogen of faeces and urine is exactly equal to the total 

 nitrogen of the food. The same thing applies to the sulphur, as is shown 

 in the following Table (quoted by Tigerstedt) : 



In order to express the nitrogenous metabolism in terms of protein, 

 we use the factor employed in estimating the amount of protein in the 

 food, i. e. we multiply the total nitrogen of the excreta by 6-25. This will 

 give the total protein which has been broken down during the period of the 

 experiment. Much more important from the energy standpoint is the deter- 

 mination of the total processes of oxidation of the body, information on 

 which is given by a comparison of the oxygen intake with the output of 



Soda Lime H,S0 4 



FIG. 327. Haldane-Pembrey respiration apparatus, 

 c, chamber for animal ; M, gas meter. 



carbon dioxide and water. The estimation of these substances presents 

 much greater difficulties than the investigation of the nitrogenous exchange 

 and involves the use of some form of respiration apparatus. 



The followjng are the chief methods which have been employed for this purpose : 

 I. THE METHOD OF HALDANE. This method is extremely convenient when 

 dealing with the gaseous exchanges of small animals, such as mice, rats, guinea-pigs 

 or rabbits. The animal is placed in the chamber c, which may be simply a wide- 

 mouthed bottle (Fig. 327). This dhamber is supplied with a thermometer, and can 

 be kept at any desired temperature by immersion either in warm or cold water. On 

 the inlet side of the bottle is a series of tubes or bottles, some of which contain 

 sulphuric acid and pumice-stone, while the others contain soda lime. On the outlet 

 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 pun- air tVeed 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 wal-r which have been discharged by 

 the animal. 



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



