118 AVES. 



which traverse the internal and inferior side of the lungs ; behind 

 these openings are situated others for the superficial tubes, and lesser 

 ones for the deeper. The superficial tubes pass almost completely 

 round the lung upon every side, and their external walls are very 

 thin and transparent. The deeper tubes resemble cylindrical pipes, 

 traverse the lungs in many directions, and are nearly straight and 

 parallel to each other ; they are the most numerous, and open in 

 such a manner into the superficial tubes, that they pass from the 

 upper to the under side of the lungs, and communicate laterally with 

 each other. From their walls being thick and remaining constantly 

 patulous, and from those of the superficial tubes into which they open 

 being transparent, the lungs of Birds acquire the perforated tubular 

 appearance which they present. The parietes of the tubes and 

 canals are covered with a most beautiful and delicate network of 

 small cavities and cells, with intervening septa mostly of an hexago- 

 nal form, and within the meshes lie other still smaller open cells. 

 The cells of the lungs in Birds are therefore never terminal cells, as 

 in the Mammalia, but open parietal cells from -Jth to ^th of a line in 

 diameter, upon which the vessels expand and thus come into contact 

 with the air. All the cells and tubes of the lungs communicate 

 naturally with each other, so that the lungs can be perfectly inflated 

 from any one point. 



Upon the surface of the lungs near to their posterior margin and 

 upon the inner side, there is observed, upon stripping off the pleura 

 in this situation, openings from five to seven in number, by means of 

 which the bronchi are brought into communication with the peculiar 

 Air-cells of the Bird. These highly remarkable receptacles for the 

 atmospheric fluid are membranous, being formed by reflected prolon- 

 gations of the pleura and peritoneum, and surround all the viscera. 

 These cells may be distinguished in general into the following prin- 

 cipal divisions, which are separated by membranous partitions, and 

 for the major part transmit air. 1st, The two empty lateral cells 

 which descend beneath the sternum as far as the pelvis, divide 

 again into an anterior and posterior, or frequently even into three 

 cells, and enclose no viscera. In the Passeres the two anterior 

 lateral cells coalesce into one, and communicate with the bronchial 

 cells. 2d, the two cells which enclose the lobes of the liver, do 

 not communicate with any air-opening, and therefore receive no 

 air, and merely result from the subdivision of the remaining air- 

 cells. 3d, An intestinal cell, also conveying no air, which includes 

 the intestinal canal, and is divided by the mesentery into two 



