1909. | SUBDIVISIONS OF THE BODY-CAVITY IN BIRDS. 225 
closely applied to one another, and so have obliterated the con- 
nective tissue of the post-pulmonary septum which at first separated 
them. The pulmonary aponeurosis and the oblique septum, as 
already shown, together constitute a single partition across the 
celomic cavity, into the thickness of which the intermediate and 
posterior air-sacs push their way as they develop; the latter are 
never at any stage surrounded by a part of the body-cavity. 
Roché [15] and, in the last year, Miiller [14] describe the sub- 
divisions of the ccelom in the same way. 
Beddard [5] describes two interesting variations ip the adult 
arrangement of the oblique septa. In the Emu, for example, he 
says, ‘‘ the posterior part of the oblique septum is free from the 
abdominal walls, ending, in fact, in a free edge within the 
abdominal cavity, this edge being really continuous with the 
horizontal septum” (post-hepatic septum). This condition I 
believe to be due to the fact that in the Emu the posterior parts 
of the post-pulmonary septa fail to unite with the lateral body- 
walls, as they do in most forms, but retain throughout life the 
embryonic relations shown in text-fig. 29,p.217. Beddard proceeds 
to say, “the oblique septum is thus merely a fold of the horizontal 
septum; they form one continuous structure.” In the adult the 
two septa are really continuous, but it must be borne in mind 
that from its development only the anterior dorsal moiety of the 
post-hepatic septum is of the same nature as the post-pulmonary 
septum, the posterior ventral portion having a quite separate 
origin. 
The other modification of the relations of the oblique septa 
occurs in many—and possibly all—Passerines. Here the septa 
of each side, ‘instead of being attached independently to the 
sternum, become fused with the falciform ligament in the middle 
line, and form a horizontal sheet of membrane covering over the 
two lobes of the liver. The original (?) attachments of the oblique 
septa are not, however, in these birds entirely lost; a much 
fenestrated membrane—sometimes, indeed, reduced to a thread 
or two—remains to remind the anatomist of the more prevalent 
conditions. In the Rook, however, they are completely preserved. 
But the attachment of the falciform ligament to the sternum in 
the middle line is lost.” This condition of the oblique septa 
appears to be due to the backward growth, on the ventral side 
between the sternum and the pericardium, of diverticula of the 
median interclavicular air-sac. These in the adult seem to form 
a single sac, but are presumably of paired origin, since they open 
into the interclavicular air-sac by distinctly paired orifices; and 
Bertelli [7] has shown that this air-sac itself arises in the embryo 
as a double structure. These diverticula would seem to push their 
way behind the peritoneum lining the ventral body-wall, and so 
carry it inwards until it comes to invest closely the pericardium 
and liver-lobes, as Beddard shows to be the case in the adult bird. 
The persistence or not of the sternal attachments of the oblique 
septa would in this case be of no especial significance, since it 
