THE GCELOM 45 



distinct from, and present in addition to, the horizontal 

 septum. 



Homologies of Oblique and Horizontal Septa. Sir RICHAKD 

 OWEN in the year 1838 described the oblique septa of the 

 Apteryx 1 as 'a well-developed diaphragm.' His figure, 

 indeed, is highly suggestive of the mammalian diaphragm, 

 more so than are (in our opinion) the actual structures 

 observable in the dissection of the bird. OWEN anxinin'<l 

 that the structures corresponded, contenting himself with 

 indicating the principal differences between the avian dia- 

 phragm and the mammalian, and pointing out how Apteryx 

 was nearer to mammals than any other bird. This assumption 

 was undoubtedly based upon current opinions of the day, for 

 in the ' Le9ons d'Anatomie Comparee ' of CUVIER (ed. '2, 

 vol. vii. p. 21) such a comparison appears to be drawn. As 

 OWEN justly observed, the imperforateness of the oblique 

 septum of the Apteryx is more mammalian than in other 

 birds, even struthious, where the abdominal air sacs project 

 beyond it ; the obliquity of its direction, too, is paralleled in 

 the dugong and manatee, and it is furthermore less oblique 

 than it is in other birds. 



SAPPEY and MILNE-EDWARDS also use variants of the 

 word ' diaphragm ' to describe what we term in the present 

 work, following HUXLEY, the oblique septa. But for the two 

 first-named observers the costo-pulmonary (see under descrip- 

 tion of lungs, below) muscles also form part of the diaphragm. 

 The older observers, impressed with certain resemblances 

 (such as warm-bloodedness) between birds and mammals, 

 regarded them as more nearly akin than is at present the 

 belief. HUXLEY devoted much work to the demonstration 

 of the nearer relationship between birds and reptiles a 

 relationship which is now generally held. In redescribing 

 the respiratory organs of Apteryx HUXLEY pointed out 

 their thoroughly ornithic character, and remarked that ' in 

 this, as in all other cases, the meaning of ornithic peculiari- 

 ties of structure is to be sought not in mammals, but in 

 reptiles.' It is on a priori grounds likely enough that there 



1 ' On the Anatomy of the Southern Apteryx,' Trans. Zool. Soc. ii. 276. 



