| OCR Text |
Show 470 MR. G. W. BUTLER ON THE SUBDIVISION OF [Nov. 19, general intestinal (or post-hepatic) cavity. In fig. 43 (which represents the dorsal half of the trunk-region of one of these small animals that had been cut horizontally), (z) indicates a blunt wire or " seeker " passed through this channel of communication from the post-hepatic cavity (3) to the pulmohepatic recess (2). The posterior aperture (fTV) may be sometimes easily detected. It is the position of this channel that is specially noteworthy. The opening into the post-hepatic cavity is not a proper foramen of Winslow, as is the case in the bird up to the ninth day. It lies dorso-externally to the dorsal attachment of the fatty so-called " spleen," and has the same relation to the vena cava inferior, while in the case of the foramen of Winslow the relations to the latter are the reverse of this. On the other hand, the aperture here resembles, in its relation to the vena cava inferior, the dextral foramen in the post-hepatic septum of Tupinambis (cf. p. 467, & figs. 31, 32, & 34, o), though in the case of this Lizard, owing to the absence of pulmohepatic ligaments, there is no corresponding recess for it to lead into (cf. woodcut B, p. 465). The omental sac proper (if, as is probable, one is originally formed) would appear to have become obliterated in connexion with the tight matting together of the coils of the alimentary canal in the region of the stomach and duodenum. I regret that I have failed to make out exactly what it is that Huxley (4, p. 568) suggests may, in the Crocodile, represent the oblique septum of birds. But if the homologies advocated in this paper be correct, the oblique septum, together with the pulmonary aponeurosis, should be represented by the tissue covering the postero-mesial face of the lungs ventrally, and on the right side forming the antero-dorsal wall of the pulmohepatic recess. VI. CONCLUSIONS. 1. The avian diaphragm is a single structure completed as such (in the chick) about the tenth day of incubation and only subsequently divided into two parts by the growth of the " diaphragmatic " or " intermediate" air-sacs. 2. The facts described would appear to give support and definition to the view that the main part of the avian diaphragm is not homologous with the diaphragm of mammals, but with tissue, which in the latter group is called mediastinal. 3. The post-hepatic septum of the bird is composed of two parts. One, developing backwards and ventralwards from the posterior border of the lung in connexion with the growth of the abdominal air-sacs, seems to be quite rudimentary in other types which have not a like disposition of air-sacs. The other, spreading on either side dorsally and forwardly to meet the former, appears to be homologous with the greater part or the whole of the post-hepatic system of the Crocodilia, and of the Teiidee among Lizards. 4. So far as the subdivision of the body-cavity is concerned, the Crocodiles seem comparable to the Lizards rather than to the Birds. |
