BREATHING 277 



monary arteries conduct the blood from the right side 

 of the heart into capillaries which are wrapped about 

 minute air-sacs. Two cell-layers are here interposed 

 between the blood and the air. The gases have to pass 

 through the capillary wall and also the wall of the air- 

 sac. But the wall of the capillary is of the utmost 

 delicacy and the epithelium of the air-sac is of the same 

 nature. If the blood actually fell in drops through an 

 air-chamber it would probably be no more thoroughly 

 aerated than it is in fact. 



Each lung is to be regarded as consisting of a system 

 of blood-vessels intertwined with vessels containing air. 

 The resulting tissue is peculiarly light and yielding. 

 It is for this reason that the lungs of the lower animals 

 are colloquially called "the lights." If they are thrown 

 upon water they float with a great part of their mass 

 above the surface. The two lungs fill the greater 

 portion of the chest cavity. The left lung has a some- 

 what smaller volume than the right, chiefly because the 

 heart subtracts space from that half of the thorax. 

 On the other hand, the left lung is rather longer from top 

 to bottom than its fellow. Roughly speaking, the two 

 taken together, with the heart and the connecting vessels, 

 form a cone with its base upon the diaphragm. 



The air which we breathe reaches and leaves the lungs 

 by way of the trachea. This is a tube extending down- 

 ward from the larynx which, it will be recalled, is at 

 the root of the tongue and ventral to the upper part 

 of the esophagus. The trachea continues in front of 

 the esophagus to a point within the chest where it 

 forks into branches, the bronchi, leading into the two 

 lungs. The trachea and the bronchi have rather rigid 

 walls and are kept open at all times. This firmness is 

 owing to bows of cartilage imbedded in their tissues. 

 The contrast between these carriers of air and the 

 esophagus is marked, for the latter is nearly or quite 

 collapsed unless something is passing through it. 



Air may reach the larynx through the mouth or the 



