RESPIRATION. 



can pass through it from the air-cells to (he blood-vessels, 

 and from the blood-vessels to the air-cells; but it is suffi- 

 ciently thick to prevent the passage of fluids. 



FIG. XX. Interweaving of the Air-tubes and Blood-vessels in 

 the Lungs. 



a. Windpipe. 



6, c. Right and left lung. 



d. Heart. 



, . Divisions of the great air- 

 tubes going to the right and left 

 lungs. 



/, /. Pulmonary arteries carry- 



ing the blood from the heart to tho 

 lungs. 



g, g. Pulmonary veins, carrying 

 the blood from the lungs to the 

 heart. 



h. h, h, h. Air-cells at the termi- 

 nations of the air-tubes. ^ 



286. When the ribs are lifted and the chest expanded at 

 the sides, and the diameter thereby increased, and when the 

 diaphragm is drawn down, and the chest enlarged below, 

 there must be a vacuum within this cavity to be supplied by 

 air. The only passage into the chest is through the mouth 

 and windpipe, and into the lungs; consequently, when the 

 cavity of the chest is enlarged, the air rushes into the air- 



