OCR Text |
Show 1889.] THE BODY-CAVITY IN LIZARDS, E T C 455 referred to, between which lie the two pairs of air-sacs that reservoirs diaphragmatiques or moyens (intermediate air-sacs Huxley), as constituting a single diaphragm ; and Milne-Edwards says (loc. cit.), "l'appareil diaphragmatique des oiseaux se compose de deux portions ou diaphragmites." This unity of the whole apparatus, which is very apparent when the development is followed (cf § III.), cannot, I think, be too clearly kept in view, especially when homologies are being discussed. In this paper I refer to all from the anterior or dorsal face of the pulmonary aponeurosis to the peritoneum covering the posterior or ventral face of the oblique septum, as the Avian diaphragm, in contradistinction to the Mammalian diaphragm, with which, in agreement with Huxley's verdict, it is argued in the sequel it has little, if any, true homology. With regard to the term "oblique septum" of Huxley. Ashe speaks (p. 562) of the four post-bronchial air-sacs of either side as being shut off by the " oblique septum " in a similar kind of way, I presume that he includes under this term the septum (y in the Plates) dorsal to the liver, above referred to. At all events in using this term, I refer merely to that septum which in the Fowl is (cf. p. 458), so to speak, blown away from the other part of the Avian diaphragm by the growth of the intermediate, or diaphragmatic, air-sacs. I would call this septum (y), which on either side forms the dorsal wall of the pulmohepatic recess, and into which a large part of the abdominal air-sac projects1, the oblique abdominal septum. It is true that the more lateral parts of this septum, in that they take their final form in connexion with the abdominal air-sac, might so far be held to resemble the " oblique septum " proper, in its relation to the diaphragmatic air-sac ; but this would not apply to the more median parts of the septum (y), and the nature of the two things is really very different. That which I have termed the oblique abdominal septum does really separate one part of the body-cavity from another, as described bySappey(footnote, p. 454 above), and is covered on both sides by peritoneum ; but the " oblique septum " is only part of a ccelomic septum, the other part being the pulmonary aponeurosis. The oblique abdominal septum we might perhaps regard as a backward continuation of the whole avian diaphragm with the abdominal air-sacs between its dorsal and ventral laminae, just as the intermediate or diaphragmatic air-sacs lie between the two laminae of the latter. III. ON THE DEVELOPMENT AND HOMOLOGIES OF THE VARIOUS SEPTA IN THE BODY-CAVITY OF T H E CHICK. We may now turn to consider the development of the septa in the body-cavity of the Fowl, to which reference has been made in the preceding section. 1 Sappey (see footnote, p. 454) says that these air-sacs rest on (s'appuient sur) the septum in question; but it is more correct to say that they project into it, in the end, as it were, blowing away a dorsal lamina from the rest. |