OCR Text |
Show 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 praebronchial 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 1*5 millim. to 0*8 millim., while they range from 0*8 to 0*4 millim. in the Duck Moreover the intervening 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 adjacent 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 Carinatae, the fossulae lead into branching passages (intercellular passages of Rainey), which radiate towards the periphery of the area of parenchyma which belongs to each para-bronchium, finally ending in the intercapillary passages. In Apteryx, the fossulae are mere shallow pits which open at once into the intercapillary 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 rudimentary 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 intercostal 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 |