368 



OF RESPIRATION. 



This distension is entirely accomplished by the action of the muscles exter- 

 nal to the thorax, or partly forming its parietes. 



287. The mechanism of the respiratory acts may be rendered clearly in- 

 telligible by the accompanying diagram, in which the trachea and lungs (5, 

 6, Fig. lu'l) are inclosed in a glass vessel with three openings. The lower 



FIG. 161. 



Apparatus constructed by Douders to show the Mechanism of Respiration. 



opening (2) is closed by a bladder or piece of caoutchouc, representing the 

 diaphragm, and capable of being either pushed into the cavity of the vessel 

 or of being drawn down by a weight. The upper opening is closed by a cork, 

 through which a glass tube (4), firmly attached to the trachea, passes. The 

 lateral opening (7) is connected with a manometer (c, d) by a tube (b), having 

 a small lateral branch (a) opening to the air. If the openings (a- and 4) be 

 now stopped whilst the bladder is extended by the weight (3), the rarefaction 

 of the air in the glass vessel will be made apparent by the rise of the mer^ 

 cury in the arm (c) of the manometer. It is obvious that the pressure of 

 the'air within and without the lungs will be different, and consequently, on 

 opening the orifice of the tube (4) air will rush through it and the trachea 

 to equalize the pressure as far as the elasticity of the lung will permit. Such, 

 with the exception that in the living body no air intervenes between the sur- 

 face of the lung and the walls of the thorax, and therefore no rarefaction, 

 but only a tendency to a vacuum occurs, are the conditions present in the 

 ordinal 1 )' act of inspiration. On the contrary, with the same exception, the 

 act of expiration may be imitated by pushing the bladder into the vessel ; 



