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 margiu 

 of the lung is very thin ; its dorsal margin, on the other hand, is 

 thick and rounded off iato 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 septutn, 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 eleva- 

 tion 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 tlie 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 for- 

 wards to the dorsal and anterior face of the pericardium, with which its 

 fibres become firmly connected on their way to their attachment to 



' 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. 



