ORGANS OF RESPIRATION. 



4!) 



12. The larynx is prolonged inferiorly into a long tube, called 

 the irucfu-a or wind pipe, which descends through the neck, and 

 enters into the thorax. This tube is formed by a series of cartil- 

 aginous rings, and is lined internally by a thin membrane, whicn 

 also lines the larynx, and is continuous with that of the pharynx. 

 The cartilaginous rings of the trachea are very elastic, and prevent 

 this air canal from being effaced, that is, from having its sides 

 pressed together, and thus offer an obstacle to the passage of the air. 



13. At its lower extremity, the trachea is divided into two 

 branches, one going to each of the two lungs; they are called 

 bronchia. 



14. Soon after they enter the lungs, these bronchi are sub- 

 divided, and ramify in an almost infinity of branches, so as to 

 furnish every pulmonary cell with a little branch, which opens 

 into it, and conveys there, the air necessary to respiration. 



15. The instrument which 

 causes the air to pass through 

 these tubes, and to enter the 

 lungs, or lo go out from them is 

 the thorax, (Fig. 12.) 



16. The mechanism by which 

 this phenomenon is produced is 

 very simple, and in almost every 

 respect resembles the play of a 

 pair of bellows, except that the 

 air escapes by the same passage 

 that it entered the lungs, which 

 is not the case in the bellows. 



17. The lungs are lodged in a 

 great cavity called the chest, or 

 thorax, the sides of which are 

 moveable, and so arranged, as to 



Fig 



mi 



d 



Explanation of Fty. 12. The thorax. On the right side of the cut are seen 

 the muscles which fill the spaces between the ribs; on the left side, they have 

 been removed, e. the vertebral column, the greater part of which is con- 

 cealr-d by the sternum, (s), e.c,c.e, ribs of the right side, mi, intercostal 

 muscles, s, the sternum, d, the diaphragm ; a part of which is seen below 

 thj thorax, and between the ribs on the right side, but it is concealed on the 

 left by the ribs and intercostal muscles. 



12. Wh;it is the trachea ? What is its structure ? 



13. What are the brorichiae? 



14. How do the branchiae terminate? 



15. What causes the air to pass through the lungs? 



16. To what is this mechanism comparable ? 



17. Where are the lungs situated ? What happens when tne ihorax 

 di l ate ? How does the air escape from the lungs ? 



