The airway (bronchial) system of the avian lung comprises of a three-tiered system of air conduits. The IPPB gives rise to four sets of SB as it transits the lung proximal-distally (Figs. 40 and 43). The architecture of the bronchial sys-
Fig.45A,B. Different views of the epithelial lining of the primary bronchus of the domestic fowl, Gallus gallus variant domesti-cus, showing conspicuous folding (arrows) and presence of ciliated epithelial cells. (A from Maina and Africa 2000)
tem in the avian lung fundamentally differs from that in the mammalian lung. Unlike in the BRLu where the airway system forms by invariant dichotomous bifurcation, in the avian lung, a continuous bronchial system forms by sprouting of the SB from the IPPB (Figs. 7 and 8). In a hoop-like arrangement, the PR interconnect the SB, establishing continuity of the bronchial system.While the so-called respiratory tree or bronchioalveolar tree of the mammalian lung is blind-ended (Fig. 21), with alveoli forming terminal 'grapefruit-like' arrangements (Fig. 44), in the avian lung the ACs are essentially continuous anastomosing air conduits (Sect. 3.4).
After piercing the horizontal septum, the EPPB enters the lung at the hilus, a site near the junction of the cranial and middle thirds of the lung (Fig. 39). There, it relates to the pulmonary artery (PA) and vein (PV; Fig. 43A). The IPPB passes through the lung in a rather curved manner (Figs. 40B). In transit, it changes in cross-sectional area (e.g. King 1966). The airway terminates on the caudal margin of the lung by entering the abdominal air sac (AAS).
Supported by cartilaginous plates, the IPPB is lined by a pseudostratified simple columnar epithelium with characteristic longitudinal folds that support ciliated cells and mucous-secreting goblet cells (Figs. 45 and 46).
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