THE RESPIRATORY APPARATUS I97 



thick. The sides of the lungs are covered with connective tissue 

 which attaches them to the costal walls. 



The concave, or inferior, face also called the diaphragmatic or 

 visceral face is directed downward. The diaphragm separates it 

 from the abdominal viscera. The surface is covered by connective 

 tissue which closely attaches it to the diaphragm. It is perforated 

 by the five tubules which bring the posterior air-sac into communi- 

 cation with the lungs. 



The borders of the lungs extend parallel to the long axis of the 

 body. The internal border is rectilinear, thick, and rounded. The 

 external border is convex, thin, and sharp. 



The anterior extremity terminates in a sharp point which occupies 

 a space formed by the ribs externally and the inferior spines inter- 

 nally. The posterior extremity is somewhat rounded and extends 

 as far back as the anterior border of the kidneys. 



As soon as the bronchi enter the lungs they become broadened, 

 the cartilaginous rings disappear, and they continue as membranous 

 channels whose diameters gradually decrease, as they extend back- 

 ward, to the point where they terminate in the ostium caudate, at 

 which point they are surrounded by a cartilaginous ring. The 

 ostium caudale brings the tubules into communication with the 

 ventral air-sacs. 



Twelve air tubules have their origin from each common bronchus, 

 or trunk. Four are given off from the internal wall of the main 

 bronchus by a series of openings arranged in a row. Seven are given 

 off from the external wall by a second series similar to that of the 

 first. The twelfth extends from the inferior wall, and immediately 

 takes a course downward and outward and communicates with the 

 posterior diaphragmatic air-sac. This may be considered as the 

 terminal branch of the trunk. 



AU of these secondary canals, except the last, pass toward the 

 periphery of the lung. They divide and subdivide at the periphery, 

 covering it \\ith their ramifications. The canals extending from the 

 inner wall are distributed to the inferior face of the lung. Those 

 extending from the outer wall are distributed to the outer face of the 

 lung. ■ The first constitute the diaphragmatic and the second the 

 costal bronchial tubes. 



The four diaphragmatic bronchial tubes are numbered in the order 

 in which they are given off. The first is carried forward horizontally, 

 the second transversely inward, the third obUquely inward and back- 



