MECHANICAL PHENOMENA OF RESPIRATION. 309 



being equal to a half-litre, about one-third of the air inhaled 

 is given back to the atmosphere, Avhile two-thirds of pure air 

 enter the lung, and renew its contents by mixing with 

 them). The coefficient of pulmonary ventilation is, there- 

 fore, 5 3 3 Yj = 0.145 ; or a little more than -fa. It varies, 

 however, with the volume of the lungs, and with the volume 

 of inspiration. Grehant has obtained extremely interesting 

 results from this point of view. Thus he found that an in- 

 spiration of litre renews the air in the lungs better than two 

 inspirations of 300 cubic centimetres, forming, together, 600 

 cubic centimetres. " This is the reason that in certain affec- 

 tions of the chest, in which patients inhale frequently, but 

 not deeply, the air is not renewed so perfectly as in the 

 normal state ; 40 inspirations, of 300 cubic centimetres each, 

 not producing such entire renovation of the air as 20 inspira- 

 tions of 500 cubic centimetres." 



Such is the estimate of the quantities of air introduced 

 into the lung: the frequency with which the movements 

 producing this renovation of the air are performed is easily 

 ascertained ; we breathe 13 or 14 times in a minute, thus 

 making the number of inspirations 20,000 in 24 hours ; as 

 each inspiration introduces ^ litre of air into the lungs, we 

 breathe altogether 10,000 litres of air in :\ day. The quantity 

 of blood brought into contact with this air has a very simple 

 numerical relation to it, being 20,000 litres, or rather 10,000 

 litres of globules (1 litre of blood = J litre of globules, or 

 cruor -\- litre of liquor). 



The differences of pressure^ caused by the mechanical 

 working of the thorax, and which are intended to produce 

 the movements of the air, are also very slight in the normal 

 condition : if, for instance, we represent the exterior pressure 

 (atmospheric pressure), in the state of repose, by 100, the 

 Ultra-pulmonary pressure will be ICO also. The dilatation 

 produced by inspiration, however, causes the interior pres- 

 sure to descend to 99 5, and thus the interior air penetrates 

 the lung (\ a litre, as we have said). When the normal 

 expiration occurs, the intra -pulmonary pressure rises to 

 100.5, and a quantity of gas equal to that which has entered 

 the lungs, is thrown off. 



In forcible respiratory movements, however, these figures 

 are much larger: thus inspiration may reduce the interior 

 pressure to 75, while expiration may increase it to 130 or 

 135 ; in other words, in a very forcible inspiration, the interior 

 pressure differs from the exterior by , and in forcible expira- 



