MECHANICAL PHENOMENA OF RESPIRATION. 307 



thoracic cavity, differ greatly in importance, and, in this 

 respect, the transverse diameter far surpasses the two others. 

 (Sappey.) 



The quantity c, or air of respiration (ordinary), may also 

 be easily estimated : this is done by collecting the gas ex- 

 pelled from the lungs by a certain number of expirations, 

 measuring it, and dividing the quantity thus obtained by 

 the number of expirations. It is, however, difficult, during 

 the experiment, not to change the number and extent of the 

 respiratory movements. Grehant, nevertheless, succeeded in 

 obtaining perfect precision, by special controlling methods, 

 founded on analysis of the air exhaled, at the beginning and 

 at the end of the experiment; 1 he has thus estimated the 

 quantity c at -^0^ f a litre, which is nearly the same as the 

 standard J litre (Dalton, Valentin, Berard). 



The other two quantities, air of reserve (>), and residual 

 air (a), are much more difficult to determine: they can be 

 measured only in a roundabout way. The sum of these two 

 quantities (a -f- b) is first measured, and then that of one of 

 them (a) ; the value of the third unknown (b) is obtained 

 by subtraction. 



Grehant has estimated the sum a -(- b with the greatest 

 accuracy : his method is based on the same principle which 

 we have already seen employed in estimating the quantity 

 of blood contained in the circulating reservoir (see p. 111). 

 In order to measure the blood contained in the vessels, we 

 ascertain the degree of dilution which it undergoes by means 

 of the injection of a certain quantity of water; in order to 

 men sure the volume of air remaining in the lungs after an 

 ordinary expiration (a -}- b), the gases which are then con- 

 tained in the respiratory tree or system are carefully mixed 

 with a known quantity of hydrogen, and the analysis of the 

 mixture is then made by means of the eudiometer. Thus, 

 Jil'ter an ordinary expiration in the air, the person making the 

 experiment begins to breathe into a receiver, containing 500 

 cubic centimetres of pure hydrogen; after the fifth respiratory 

 movement, the mixture is perfected, that is, it is exactly the 

 same in the receiver and in the lung (see p. 288). It is only 

 requisite then to analyze the gases in the receiver to obtain, 

 by a simple calculation, the volume of air contained in the 

 lung at the beginning of the experiment, that is, after ordi- 



1 See u Journal de I'Anutomie," etc., de Charles Robin, 1861, 

 p. 512. 



