OCR Text |
Show 1882.] ORGANS OF APTERYX. 561 ends of the vertebral ribs. The lowest point of the ventral margin lies on one of these ribs, not far from its articulation with the sternal rib; and the hinder part of this margin, or the posterior ventral margin, slopes upwards and backwards, while the anterior ventral margin is inclined upwards and forwards from this point, which may be called the ventral angle of the lung. The whole ventral margin of the lung is very thin; its dorsal margin, on the other hand, is thick and rounded off into the outer or lateral face of the lung, which is convex from above downwards. The dorsal margin and the lateral face are closely applied to the parietes ; and the former presents deep notches, into which the necks and dorsal portions of the bodies of the ribs are received. The mesial face of the lung is divisible into three facets :-a superior, which is closely applied to the lateral faces of the vertebrae and to a fibrous lamella, the median vertical septum, which proceeds from the ventral faces of the vertebrae in the middle line ; and an anterior inferior and a posterior inferior facet, which occupy that slightly concave face of the lung which is turned towards the thoracic cavity. These last facets are divided by a ridge or elevation of the surface of the lung, which ascends from the ventral angle to the insertion of the bronchus. This is always situated at some considerable distance from the anterior end of the lung. The anterior inferior and posterior inferior facets of the lung are closely invested by a thin fibrous membrane, which may be termed the pulmonary aponeurosis (" diaphragme pulmonaire," Sappey; " diaphragmite anterieur," Milne-Edwards). The mesial edge of this is continuous with the median vertical septum; the lateral edge is inserted into the parietes of the thorax immediately beyond the ventral edge of the lung. Around the pneumatic apertures, or ostia, the pulmonary aponeurosis is closely adherent to their margins; it is, in fact, perforated by them. Broad flat bands of striated muscle (the costo-pulmonary muscles) take their origin from the vertebral ribs *, at some distance below the attachment of the pulmonary aponeurosis, and, proceeding obliquely dorsad and forwards, spread out and are inserted into that part of the aponeurosis which covers the posterior inferior facet. They are supplied with branches of the intercostal nerves. This pulmonary aponeurosis with its muscles is one of the two structures which have been compared to the mammalian diaphragm. The second so-called " diaphragm " (" diaphragme thoraco-ab-dominal," Sappey; " diaphragmite thoraco-abdominal," Milne- Edwards) is a more or less aponeurotic fibrous membrane, continuous with the ventral edge of the median dorsal septum and suspended by it, like the roof of a tent, across the thoraco-abdominal cavity. In the middle line, this oblique septum slopes downwards and forwards to the dorsal and anterior face of the pericardium, with which its fibres become firmly connected on their way to their attachment to 1 In many birds there are additional " sterno-pulmonary" muscles, which proceed from the antero-lateral part of the sternum to the aponeurosis covering the anterior inferior facet. |