616 



PHYSIOLOGY 



rise to as much as 4 per cent, of the total nitrogenous output of the body. 

 Although a trace of ammonia has been described as occurring in the expired 

 air, the amount is so minute that any loss of nitrogen by the lungs can be 

 neglected. That the loss both by lungs and skin under ordinary circum- 

 stances can be disregarded is shown by the fact that it is possible to account 

 directly for the whole nitrogen of the body by a comparison of the compo- 

 sition of food with that in the urine and faeces. If, for instance, an animal 

 is kept on a sufficient diet which contains a perfectly regular amount of 

 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 nitro- 

 gen 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 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 following 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 



Soda Lime H 2 SO 4 



SwdaLimo. H 2 S0 4 



FIG. 316. Haldane-Pembrey respiration apparatus, 

 c, chamber for animal ; M, gas meter. 



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

 mouthed bottle (Fig. 316). This chamber 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 



