

RESPIRATION. 

 Figure I. 



115 



Represents the Trachea, or windpipe, its bifurcation or division into two great branches, called 

 bronchial tubes or pipes (the seat of BRONCHITIS). They again divide into numerous branches, 

 too small to be seen with the naked eye. and terminate in a multiplicity of cells, which at every 

 inspiration are filled with atmospheric air, which comes in contact with venous or impure blood, 

 and changes it from a dark state to a bright, healthy vermillion or red color, and then is transmitted 

 back to the heart. 



Figure 2. 



Represents the vesicles or air bladders, the first round and natural ; ihs second, dissected or open. 



If we tie up tightly in a bladder a quantity of venous or dark 

 blood, we shall find, in a short time, that exposure to the air has 

 changed the color of the portion near the surface. The air has 

 passed through the bladder, and has converted the venous into red 

 or arterial blood. Exactly the same thing takes place in the lungs ; 

 for %ie air, in the air-vesicles, is separated from the blood in its 

 vessels by a membrane not more than the thousandth part of an 

 inch in thickness. But we shall find, immediately, that it is not the 

 blood alone that is altered in its qualities. The air also undergoes 

 alterations. The blood in the lungs becomes fit for supporting life ; 

 the air becomes unfit for this purpose. We must, therefore, describe, 

 1st, the means by which the air is brought into, and then removed 

 from, the lungs ; and, 2dly, the changes of composition that thence 

 occur in the air and in the blood. 



