138 



ELEMENTARY PHYSIOLOGY 



LESS. 



the chest, it cannot produce such a separation without 

 developing a vacuum between these two layers. To effect 

 this, the elastic tissilfe must pull with a force greater than 

 that of the external air (or fifteen pounds to the square 

 inch), an effort far beyond its powers, which do not equal 



Fio. 43.— Sternum viewed from the Front. 



1-T, points of attachment of first seven ribs ; cl. points of attachments of 

 clavicles (collar-bones) ; x, lower projecting end of sternum. 



one-fourth of a pound on the square inch. But the 

 moment a hole is made in the pleura, the air enters into 

 its cavity, the atmosj)heric pressure inside the lung is 

 equalised by that outside it, and the elastic tissue, freed 



