RESPIRATION. 



129 



282. At the junction of the neck with the chest, and just 

 behind the top of the breast-bone, the great air-tube, the 

 windpipe, is divided into two tubes, or bronchi. One of 

 these goes to the right, the other to the left lung. After 

 entering their respective lungs, they divide into smaller 

 branches or tubes, as represented in Fig. XIX. These 



FIG. XIX. Windpipe and Air- Vessels of the Lungs. 



a. Windpipe. 



b. Left lung. 



c. Great brancn of the air-tube 

 going to the right lung. 



rf, d, d. Minute branches of the 

 air-tubes. 



e, e. Air-cells at the minute 

 termination of the air-tubes, mag- 

 nified. 



tubes do not divide so minutely as the blood-vessels, but ter- 

 minate somewhat abruptly in a great number of minute cells. 

 (Fig. XIX. 6, e.) These cells are estimated to be one hun- 

 dredth of an inch in diameter. They are so numerous as to 

 be distributed to every part of the lungs ; and the extent ot 

 the inner surface of the whole, collectively, is estimated vari- 

 ously by physiologists. Some suppose it to be 20,000 square 

 inches ; others thirty times the whole surface of the body. 

 But all agree that there is a very extensive surface presented 

 to the action of the air. 



283. These air-tubes and air-cells are lined with a mucous 

 membrane of exceeding delicacy, which, during the whole of 



