

MECHANICAL PHENOMENA OF RESPIRATION. 303 



sion becomes voluntary, for it is only in the larynx that the 

 foreign bodies or mucosities are perceived ; lower down, the 

 sensations produced by their presence are very slight, and 

 give rise to no reflex actions. The larynx is the starting- 

 point of the reflex or voluntary phenomena which produce 

 expulsion by means of this same mechanism of varying cur- 

 rents of air, but only with much greater energy ; it is here 

 that coughing is produced, and, higher up (in the pharynx and 

 the nasal chambers), sneezing; and higher still (in the nos- 

 trils), the action of blowing the nose: all these consisting in 

 a slow inspiration through a dilated orifice, and a sudden 

 expiration through an orifice, narrowed, either by the con- 

 traction of its own muscles, or by a more or less distant 

 mechanism. 



III. PHYSICAL AND MECHANICAL CONSEQUENCES OF RESPIRA- 

 TION. 



A. Mechanical effects produced upon, the lung. 



We have already studied the numbers representing the 

 varying conditions of the blood in regard to the intra-pulmo- 

 nary air; we must remember that the respiratory surface, 

 whose area is equal to 200 square metres, is essentially repre- 

 sented by a blood network of 150 square metres; that this 

 network represents a mass of 2 litres of blood ; that this 

 blood is so constantly renewed that 20,000 litres of blood 

 pass through the lung in 24 hours (Fig. 79). We have now 

 to specify the results of respiration in regard to the quantity 

 of air brought in contact with the blood, and the numerical 

 statistics of the agencies by which the air is renewed. 



The pulmonary cone represents a reservoir, the total 

 average capacity of which amounts to 4 or '5 litres, when 

 filled as full as possible; that is, when the deepest inspiration 

 is made; on the other hand, when the strongest possible 

 expiration is made, there still remains in the lungs from 1 to 

 1 \ litres of residual air, which cannot be exhaled in any way, 

 because the lung, as we have seen, can never quite attain its 

 natural form. The difference between this second figure and 

 the first represents the quantity of air which may be intro- 

 duced into the lung and then driven out, by means of the 

 most energetic respiratory movements: this is what is called 

 the vital capacity (or pulmonary capacity, or, better still, 

 respiratory capacity) ; it is equal to 3^ litres. This figure is 

 of some importance, for it indicates the magnitude of the 



