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Show 566 PROF. HUXLEY ON THE RKSPIRATORY [June 20, each side, a layer of unstriped muscular fibres. I have not been able to make out any such fibres in Apteryx. 6. The most remarkable difference, however, lies in the development of the air-sacs in the two birds. In Apteryx, as in the Duck, the attachment of the dorsal end of the dissepiment between the intermediate loculi corresponds with the exit of the cceliac artery from the dorsal median septum; and the relation of the air-sacs in front of and behind this dissepiment to the bronchia which open into them is such, that there can be no question of their homology with the anterior and the posterior intermediate air-sacs in the Duck, notwithstanding the vastly greater size of the latter. Hence the air-sac in front of the anterior intermediate in Apteryx must be the homologue of the subbronchial in the Duck ; and the position of this sac and its relation to the subbronchial ostium leave no doubt that such is the case. But while, in Apteryx, the subbronchial air-sac does not extend beyond the front margin of the sternum, and is floored by that part of the oblique septum which lies at the sides of the fore part of the pericardium and is attached to the front edges of the sternum, in the Duck only a small part of the sac is thus related to the oblique septum, and even this extends much further backwards and more towards the median line than in Apteryx. In front, each of these sacs has enlarged forwards to the space between the furcula and the sternum, and there has opened into its fellow in the middle line. Thus the two subbronchial air-sacs are fused into one great air-chamber, and their mesial walls are so closely applied to the trachea and great vessels as to invest them like a serous coat. Moreover the common sac sends prolongations into the axillae and elsewhere, and communicates with the pneumatic cavities of the adjacent bones. A similar modification has taken place in the posterior air-sacs of the Duck, but has been carried to a still greater extent. In Apteryx the whole of this sac is enclosed between the oblique septum and the pulmonary aponeurosis, the dissepiment between its loculus and that of the posterior intermediate sac being situated almost midwav between the second dissepiment and the posterior extremity of the pneumatic chamber. In the Duck, on the contrary, the dorsal end of this dissepiment is attached close to the posterior extremity of the lung, and thence slopes very obliquely backwards. The capacity of the posterior intermediate air-sac thus becomes greatly increased. But, as the capacity of the posterior air-sac is also vastly greater than in Apteryx, its posterior wall has been, apparently, driven out, like a hernial sac, between the peritoneum and the parietes, and projects into the abdominal cavity. It would be incorrect, therefore, to say that the abdominal air-sac is absent in Apteryx : it is just as much present as in any other bird ; but its small size and the small relative development of the posterior intermediate sac permit it to occupy a different position. 7. The first or praebronchial air-sac has hitherto been overlooked in Apteryx. It is of a long ovate or spindle-shape, 21 millim. Ion"- by 8 millim. wide in the middle, and lies between the longus colli |
