MECHANICAL PHENOMENA OF RESPIRATION. 237 



chambers are lined by an extremely moist mucous membrane, 

 containing a large quantity of blood, and consequently very 

 warm ; it covers a number of folds (turbinated or spongy 

 bones) in passages (meatus), through which the air, as it 

 passes, is filtered, simultaneously becoming charged with 

 moist vapor, and being brought to the temperature of the body. 

 These considerations alone prove that respiration is naturally 

 performed through the nose, and not through the mouth, and 

 show the danger of breathing through the latter when in a 

 cold dry atmosphere. 



II. MECHANICAL PHENOMENA OF RESPIRATION. 



THE best method of exhibiting the arrangement of the 

 circulating reservoir was presented by a diagram, and we 

 shall find this plan equally useful in regard to the respiratory 

 system. We see, in this way, that the air-bearing tubes, 

 being placed side by side and the partitions left out, repre- 

 sent a very wide cone, having for 

 its base the alveolar surface which 

 we have already studied, and for 

 its summit the opening of the 

 nasal chambers (Fig. 76). 



This arrangement shows us 

 that when the air, no matter by 

 whatever mechanism, enters or 

 leaves this reservoir, the velocity 

 of its current will be very differ- 



ent in the different zones of the ^ of tonary eonfc . 

 cone, being more rapid as the 



zone is narrower (higher), and slower as the zone is wider 

 (nearer the base) ; and that at the base of the cone, on the 

 surface of the alveoli, the air is comparatively stagnant. In 

 spite of the number of our respiratory movements, the air at 

 the level of the breathing surface (alveolar) is never found 

 pure, but contains as much as 8 per cent of carbonic acid, 

 produced by former gaseous exchanges. 1 The upper part of 



1 The figure 8 per cent may appear too high, and yet there is 

 no doubt that it is below the truth. Grehant made it 7.5 per cent 

 by direct experiment, but he did not analyze the gas which is in 

 immediate contact with the respiratory surface; because, as we 



* T, Trachea. P, Cavity of the lung. E, E, Respiratory surface (pavement 

 epithelium of the alveoli. 



