1882.] ORGANS OF APTERYX. 567 



with the vertebral column above, and the oesophagus and bronchus 

 below. Laterally, it is bounded by so much of the anterior facet of 

 the lung as lies above the level of the bronchus. Mesially, it is 

 widely separated from its fellow by the fibrous tissue continued from 

 the anterior end of the median vertical septum onto the oesophagus, 

 bronchi, and aorta. 



The prsebronchial air-sac in the Duck is fully three times as long, 

 and sends off prolongations to the vertebral column, which have been 

 described by Sappey. 



8. The vestibule, mesobronchium, and entobronchia of Apteryx 

 present no important differences from those of the Duck, except 

 that the branches of the entobronchia are less numerous. 



The ectobronchia, on the other hand, are much smaller, and only 

 the most anterior reaches the lateral surface of the lung, and there 

 divides into an ascending and a descending superficial branch of 

 small size ; the others break up into parabronchia before reaching the 

 surface of the lung. 



9. The parabronchia are much wider in Apteryx, the diameters 

 of their cavities varying from I'o millim. to 0*8 millim., while they 

 range from 0'8 to 0'4 millim. in the Duck Moreover the inter- 

 vening vascular parenchyma is relatively much narrower in Apteryx 

 than in the Duck. Hence a section of the lung of the former 

 appears much more coarsely spongy than one of the latter. In the 

 Duck, as in many other Carinate birds, the parenchyma around each 

 parabronchial canal is defined by linear interspaces from that of ad- 

 jacent parabronchia ; and in transverse section these boundaries have 

 a polygonal, usually hexagonal form. In Apteryx, the parenchyma 

 between the parabronchial tubes is continuous, and the interca- 

 pillary air-passages show no interruption. 



In the Duck, as in most Carinatse, the fossulse lead into branching 

 passages (intercellular passages of Rainey), which radiate towards the 

 periphery of the area of parenchyma which belongs to each para- 

 brouchium, finally ending in the intercapillary passages. In Apteryx, 

 the fossulse are mere shallow pits which open at once into the inter- 

 capillary passages. 



Thus the respiratory organs of Apteryx are thoroughly ornithic, 

 differing from those of other birds chiefly in the greater width and 

 smaller aggregate surface of the respiratory passages, in the rudi- 

 mentary condition of the pneumatic sacs, and in the much greater 

 strength of the pulmonary and septal aponeurotic expansions. 



Neither in Apteryx, nor in any other bird, has either of these the 

 slightest real resemblance to a Mammalian diaphragm. For, as has 

 been seen, the heart lies altogether behind both, and the muscular 

 digitations of the pulmonary aponeurosis are supplied by the inter- 

 costal nerves, the phrenic being absent. The vertical and oblique 

 septa really answer to the fibrous tissue of the posterior and middle 

 mediastinum in Mammals. 



In this, as in all other cases, the meaning of ornithic peculiarities 

 of structure is to be sought, not in Mammals, but in Reptiles. It 

 is only among Reptiles that we meet with pneumatic bones similar 



Proc. Zool. Soc— 1882, No. XXXVIII. 38 



