MUXLER] THE AIR-SACS OF THE PIGEON 371 



In the cavum subpulmonale, loculus quartus, lies the saccus abclo- 

 minalis. 



Before entering on the description of these sacs we will consider 

 more closely the organ? from which the air-sacs phylogenetically and 

 ontogenetically arise, the lungs. 



The Lungs (Figs. 9, 10; Figs, i, 2, 11, 12, P) 



Each of the two symmetrical lungs has the shape of a three-sided 

 pyramid with posterior base, and a dorsal, a medial, and a ventral 

 face. 



The dorsal face of the lung pyramid is convex and of greater 

 extent than the others. It extrudes in the intercostal spaces, the 

 ribs lying in deep, transverse, furrow-like depressions of the lung 

 (fig. 10, C). One of the two edges of the lung-pyramid bounding 

 this face extends along the vertebral column ; the other, which is 

 convex downward, extends along the ribs in a line ventral from their 

 uncinate processes. At its lowest point it usually reaches the line 

 of junction between the vertebral and sternal portions of the ribs. 



The medial face of the lung pyramid is the smallest in extent. It 

 lies in the median plane of the animal, and is in contact with the 

 mediastinum that separates the two lungs. 



The ventral face of the lung* pyramid (fig. 9) is more irregular 

 than the two others. Its anterior portion (fig. 9, SICL) is convex, 

 its posterior (fig. 9, SI) concave. Both portions, but especially the 

 anterior one, have a rough appearance, and are in places markedly 

 furrowed. These irregularities are caused by the bronchial branches, 

 many of which lie very near this ventral surface and are covered, on 

 their ventral side, with only a thin membrane. As long as the pleura 

 is intact the ventral surface of the lung appears as if it were really 

 as smooth (fig. 9, PL,) as it is generally figured and described in the 

 text-books. If, however, the pleura is removed, the true character 

 of this surface is at once revealed. 



Near the center of the ventral face of the lung pyramid the 

 bronchus enters. Within the lung it retains for a considerable dis- 

 tance its cartilaginous character, although rings are not present in 

 its wall. From the place of entrance it extends in an arch, con- 

 vex towards the ribs, to the ostium posterius (figs. 8, 9, 10, 12, OP), 

 situated near the middle (somewhat nearer the ribs than the median 

 plane) of the posterior border of the lung. The part of the bronchus 

 lying within the lung has been named by Huxley the mesobronchinni. 

 The anterior portion of this mesobronchium is distended to the 

 vestibulum (fig. 10, V), which in many birds has the shape of a 



