38 STRUCTURE AND CLASSIFICATION OF BIRDS 



the thick septum, in a cavity formed by the splitting of its 

 layers. Anteriorly the horizontal septum, passing forwards, 

 lies beneath the liver, coming into relations on each side 

 with a cavity which will be referred to later as the ' pulmo- 

 hepatic recess.' 



If, therefore, the abdominal walls of the bird have been 

 cut through anteriorly to the attachment of the horizontal 

 septum to the abdominal walls, the only abdominal viscera 

 exposed will be the gizzard and the liver lobes. These 

 latter are separated from each other by the median vertical 

 'falciform ligament,'' which is continued backwards to divide 

 the cavity into right and left halves. 



If, on the other hand, the abdominal walls of the bird 

 have been cut through posteriorly to the attachment of the 

 horizontal septum to the abdominal walls, the viscera 

 exposed will be the intestines and kidneys and not the 

 liver. 



In some birds for instance, in the duck, and in many 

 charadriiform birds the horizontal septum is so short 

 behind the gizzard that the latter is closely attached to the 

 abdominal parietes by what looks at first sight almost like a 

 pathological adhesion, due to peritonitis. On the other 

 hand in many storks, in Chauna, Cariama, struthious and 

 other birds, the horizontal septum is very extensive, reaching 

 back to the immediate neighbourhood of the cloaca. Various 

 intermediate stages are offered by other birds. 



The Oblique Septa. Eeference has been already made to 

 these structures, which are present in all birds, and concerning 

 whose homologies there is some divergence of opinion. Their 

 structure and relations are as follows : On either side of the 

 body is a tough fibrous sheet of membrane, which runs an 

 oblique course (hence HUXLEY'S name 1 of oblique septum'), 

 entirely enclosing and shutting off from the abdominal cavities 

 (dorsal and ventral) the lungs and air sacs, with an excep- 

 tion to be noted immediately. These oblique septa have, 

 as HUXLEY pointed out, a tent-like arrangement, coming 

 into contact with the median septum in front of the heart, 



1 On the Respiratory Organs of Apteryx,' P. Z. S. 1882, p. 560. 



