40 



STRUCTUUE AND CLASSIFICATION OF BIRDS 



attachment of the falciform ligament. Dorsally they are 

 attached to the parietes. As a general rule the abdominal 

 air sac is the only one of the posterior air sacs (see fig. 23) 

 which is not enclosed within the oblique septa ; in all birds, 

 so far as is known, except the Apteryx, the wall of this air 

 sac ' has been apparently driven out, like a hernial sac, 

 between the peritoneum and the parietes, and projects into 

 the abdominal cavity.' In Apteryx the air sac in question 

 is completely enclosed by the oblique septum. Another 

 exception to the statement made above as to the completely 



I 



FIG. 25. DIAGRAMMATIC TBANSVEKSE SECTION OF EMU, TO SHOW THE PRO- 

 JECTION OF OBLIQUE SEPTUM. 



rt, as a free fold ; 6, falciform ligament. 



dissepirnental nature of the oblique septa occurs in a few 

 birds (e.g. emu and Cariama), in which the posterior end 

 of the oblique septum, though firmly attached to the dorso- 

 lateral parietes, is not so attached ventrally, but projects 

 into the abdominal cavity as a free fold. In these cases the 

 free fold is double (see fig. 25), the inner half being conti- 

 nuous with the horizontal septum. To the possible signi- 

 ficance of this fact we propose to return later. 



The oblique septa are, as has already been stated, mem- 

 branous, but they are occasionally and partially invaded or 

 covered by muscular tissue. HUXLEY speaks of ' unstriped 



