6 PHYSIOLOGY CHAP. 



estimation of the total consumption of protein, we confine 

 ourselves to determining the total amount of nitrogen in the 

 excreta by Kjeldahl's method, and pay no attention to the 

 sulphur and phosphorus. If we multiply the nitrogen by 6'25, 

 we can calculate, as we have already seen, the sum total of 

 protein consumed. 



In order to estimate the total consumption of non-nitrogenous 

 substances, fats and carbohydrates, we may confine ourselves to 

 determining the total amount of carbon in the excreta. Since 

 the proportion of nitrogen to carbon in protein is as 16 to 5 2 -8, 

 we can easily calculate the carbon in the protein consumed, and 

 obtain by subtraction the carbon resulting from the decomposition 

 of the non-nitrogenous organic substances. 



In order to complete the list of materials taken in and 

 discharged, and obtain a balance of account, we must not only 

 analyse the food, urine, and faeces, but also determine, at least 

 approximately, the total quantity of oxygen absorbed, and carbon 

 dioxide and water discharged, by the lungs and skin. 



For the estimation of the quantity of oxygen, carbon dioxide, and water- 

 vapour absorbed or eliminated by the lungs and skin, the animal or man is 

 shut up in an airtight chamber, and the changes which the breathing of the 

 subject produces in the air during its passage through the enclosed space are 

 determined. 



Various kinds of apparatus have been suggested by physiologists for the 

 study of gaseous exchange in animals and man. We will describe the most 

 important of these, referring more particularly to those which can be used 

 for man. They may be arranged in three groups, according to the different 

 principles on which they are based. 



(a) The quantities of oxygen absorbed and carbon dioxide discharged by 

 the lungs and the skin are determined at the same time. 



(6) The carbon dioxide only is determined directly. 



(c) The respiratory gases from the lungs only are determined. 



Each of these methods possesses, as we shall see, its own advantages, and 

 may be used in preference to the others in certain cases. 



(a) The classical apparatus of the first group is that used originally for 

 small mammals by Regnault and Reiset. As will be seen from the diagram, 

 Fig. 1, it consists of a glass bell-jar A, in which the animal to be experimented 

 upon is placed, surrounded by a larger cylinder, also of glass, kept full of 

 water in order to limit the variations of temperature. Through the upper 

 aperture of the bell-jar, which is hermetically closed with a stopper, pass 

 four glass tubes, two of which are connected with the apparatus for the 

 absorption of the carbon dioxide produced by the animal. One of these 

 tubes ends above, the other is continued almost to the bottom of the bell- 

 jar ; they communicate with two receptacles C and (7 1 , which are half-filled 

 with a titrated solution of caustic soda. By means of the contrivance K 

 these two receptacles are alternately lowered and raised, so that, while the 

 one which is lowered withdraws air from the bell -jar, the other, which is 

 raised at the same time, restores exactly the same amount. The air, when 

 it comes into contact with the soda, loses its carbon dioxide, the quantity of 

 which is determined at the end of the experiment by another titration of the 

 solution of soda. 



The third tube, which passes through the stopper of the bell-jar, conveys 

 pure oxygen, which is driven from the receptacle N (or N f or N" which take 



