156 



ELEMENTS OF PHYSIOLOGY 



lungs to expand, and the air to rush in ? The air cannot 

 be pulled in, for it has no cohesion, its parts do not stick 

 together. It is found that the air has considerable weight, 

 for the height of the atmosphere is at least forty miles, and 

 the air above is pressing down on that below. When the 



chest walls are moved outward 

 against the weight of the outer 

 air, the space in the chest is in- 

 creased, and the air already in the 

 chest expands to fill the greater 

 space. The air, when expanded, 

 is lighter, and exerts less pressure 

 than before, and the denser air 

 outside, having greater pressure, 

 presses inward until the air in the 

 lungs is as dense as it was before 

 the lungs were enlarged. Thus 

 do we allow the air to come into 

 our lungs ; we do not draw it in, 

 but make space for it, and the at- 

 F,G. ,39. -Diagram of riand "osphere outside presses it in. 



260. Expiration. This is the 



reverse of inspiration. The space 



within the chest is diminished, and 



^ ^ j n ^ } {& CQm ed 



and becomes denser than the air 

 outside. This denser air has 

 greater pressure than the outside air, and presses out 

 through the air passages until enough has passed out to 

 restore the equilibrium, making the pressure equal without 

 and within (Figs. 137, 139). 



261. But how are the Lungs made Smaller ? In ordinary 

 quiet breathing, the lungs become smaller, owing chiefly to 

 the elasticity of the parts involved. When the air rushes 

 in during inspiration, it enlarges the air cells by stretching 



Seventh Ribs, in Connection 



tion the Latter is carried 



upwards and forwards. 

 The expiratory position is indicated 



by continuous lines, the inspiratory 



