MOVEMENTS OF KESPIRATION. 



161 



Fig. SO. 



tion. The second stage is the translation of the fresh air from the larger 

 bronchial tubes to the ultimate air-cells : this is accomplished on the 

 principle of gaseous diffusion. The third stage is the passage from the 

 air-cells into the blood : this is through the wall of the cell, the wall of 

 the blood-vessel, and the sac of the blood disc ; it involves passage 

 through membranes, and implies their condensing action. Each of these 

 three stages we have now to consider. 



1st. The introduction of fresh air into the trachea and larger ramifi- 

 cations of the bronchial tubes is accomplished by muscular The effect of the 

 contraction, which calls into operation atmospheric pressure, pressure of the 

 In tranquil respiration the diaphragm is nearly sufficient for 

 this purpose. This muscle, forming the convex floor of the chest, as soon 

 as it contracts, assumes more nearly a plane figure, thereby increasing 

 the content of that cavity ; and, just as in a common bellows, when the 

 lower board is depressed, the air flows in through the pipe, so^on the de- 

 scent of the diaphragm, the air flows in through the trachea, forced by 

 the external pressure. 



An experimental illustration of the manner in 

 which the air is introduced into the cavity of the 

 lungs by the descent of the floor of the chest, 

 and then expelled by its elevation, is represented 

 in Fig. 80, in which a a is a tube of glass half 

 an inch or more in diameter, and six or eight 

 inches long, to the lower end of which a blad- 

 der, b, is tightly attached. The tube is passed 

 through the neck of a bell-jar, c <?, air tight. A 

 large glass reservoir of water, filled to the height 

 d d, receives the bell-jar, as shown in the figure. 

 When the jar is depressed in the water the air 

 is expelled from the bladder, and when the jar 

 is raised the air flows in. By alternately ele- 

 vating and depressing the bell, the bladder exe- 

 cutes movements like those of the lungs, of which, 

 indeed, it is a representation; the glass tube be- 

 ing the trachea, the bell-jar the walls of the chest, 

 and the rising and falling water-level the rising 

 In this illustration the bladder is, of course, per- 



Mechanism of respiration. 



and falling diaphragm. 



fectly passive, as was at one time supposed to be the case with the lungs : 

 an erroneous opinion, which will presently be corrected. 



In the mature period of life, and especially in deep respiration, the ac- 

 tion of the diaphragm is insufficient for the introduction of air, Manner of in _ 

 and a still farther volume is obtained by raising the ribs, which troducing the 

 increases the dimensions of the chest from right to left, and air> 



