308 PULMONARY MUCOUS TISSUE. 



nary respiration, or, in other words, calculate the volume a -f 

 b. For persons whose ages are between 17 and 35 years, 

 Grdhant obtained, in this manner, quantities varying between 

 2^^ litres, and 3^^ litres. (Grehant calls this quantity 

 the pulmonary capacity: this is not the meaning usually 

 attached to this expression : if we refer to what was said 

 above, we shall find that the pulmonary or vital capacity 

 represents the sum b -|- c -f- d ; while that settled by Gre- 

 hant represents the sum a -j- b. 



The quantity a remains to be determined, and this, too, 

 Grehant enables us to do. " In order to decide this, I intro- 

 duce a half-litre of air into a receiver (with a stop-cock) ; 

 after an expiration made in the air, I inhale this gas, and 

 then make as long an expiration as possible into the receiver : 

 I then measure the volume of the exhaled gases ; I find that 

 is 1.8 litre. The pulmonary capacity (a -f- b, say 2.34 litres) 

 is increased by the inspiration of J litre, and diminished by 

 1.8 litre: what remains in the lungs is, therefore, 2.34 litres 

 -f- 0.5 1.8 litres = 1.04 litres." Thus the quantity a (re- 

 sidual air), which includes, it must be remembered, the 

 volume of the buccal cavity, is about equal to one litre. 1 



The same experiment gives us the value of #, or the air in 

 reserve. We have thus all the data necessary to solve the 

 physiological problems relating to the quantities a, &, c, d. 



One of the most important of these problems is that of the 

 ventilation of the lung, which Grehant was the first to solve. 

 The quantity of fresh air, which, after each movement of ven- 

 tilation, remains in the unit of volume in the ventilated space, 

 is called the coefficient of ventilation : the lung is a space of 

 this kind, and the respiratory movement really forms a ven- 

 tilating movement. The coefficient of ventilation is, there- 

 fore, the quotient obtained by dividing the quantity (x) of 

 pure air remaining in the lung after a normal expiration and 

 inspiration, by the known volume of the lung after such ex- 

 piration (a -|- b = 2.365 1., for instance). Grehant discovered, 

 by means of the inspiration of hydrogen, already mentioned, 

 that the quantity x =. on an average 0.3*28 1. (that is to say 

 that, when an ordinary inspiration or expiration is made, each 



1 We follow the example of most physiologists in calling this 

 quantity " residual air," but we must forewarn the reader that 

 Grehant gives it the name of ** air in reserve," a name which more 

 naturally applies to the quantity b. (See " llevue des Cours Scien- 

 tifiques." Aout, 1871.) 



