SYSTEMATIC ANALYSIS OF APPARATUS. 107 



386. THE NUMBER OF CELLS and the size of them 

 will vary according to the size of the Lungs and the 

 individual, and the amount of air that will act in a 

 given time upon the blood will vary accordingly. 



387. THE CELLS ARE COLLECTED, like the leaves on 

 a tree, into clusters called lobules, and the lobules into 

 clusters called lobes, and these again into the right and 

 left (halves of) lung ; the tubes from the cells, lobules, 

 lobes, and lungs unite to form the trachea, as the trunk 

 of a tree is formed by the uniting of the branches. 



388. THE MODE OF FILLING AND EMPTYING THE 

 CELLS is as simple as their structure. 



389. IF AN ELASTIC BAG WITH AN OPEN MOUTH HAVE 

 THE PRESSURE OF THE AIR REMOVED FROM ITS EXTERNAL 



SURFACE, the pressure of the air through the mouth will 

 distend the bag ; if then the external pressure is allowed 

 to act again, it will balance the internal pressure, and 

 the elasticity of the bag will throw out the air. 



390. SUPPOSITION : Let 15 = External pressure. 



" 15 =r Internal pressure. 

 " 10 = Elasticity. 



391. WHEN EXTERNAL PRESSURE, 15, AND ELASTIC- 

 ITY, 10, ACT TOGETHER, 25, they overcome Internal pres- 

 sure, 15, and expel the air; take away External pressure, 

 15, and Elasticity, 10, is overcome by Internal pressure, 

 15, and the lung is distended. 



392. To CAUSE BREATHING, viz., the passing of air 

 into the cells and out again, all that is necessary is to 

 contrive some means to remove the pressure of the air 

 from and return it against the external surface of the 

 lungs, which is the easiest thing in the world to do. 



393. PLACE THE LUNG in an air-tight box, with the 

 pipe that connects with the air-cells extending through 

 and closely fitting an opening in the side of the box ; this 

 must also be so constructed as to be readily enlarged in 

 all directions and again returned to its previous size. 



886. What said of ? 38T. How ? 388. What ? 889. What ? 890. What 

 Is ? 891. What is said ? 892. What necessary ? 393. How ? 



