OCR Text |
Show 1882.] ORGANS OF APTERYX. 563 mesenteric arteries, which thus appear in the middle line of the oblique septum when this is viewed from below and behind. The vena cava inferior enters the pericardium just behind the attachment of the oblique septum to it. Immediately after the bronchus has entered the lung, it enlarges somewhat to form a dilatation, which has been termed the vestibule. A trunk which continues the direction of the bronchus through the centre of the parenchyma of the lung backwards, leaves the posterior end of the vestibule, and ends at the superior end of the posterior ventral margin in the posterior ostium, by which it opens into the posterior air-sac. This trunk may be termed the mesobronchium, by way of distinction from certain others which have a superficial position on either the lateral or the mesial face of the lung; and which may be distinguished, respectively, as ecto- and ento-bronchia. Toward the middle of its course the mesobronchium gives off a wide branch, which passes backwards and downwards to the posterior ventral edge of the lung, and opens there by a very wide posterior intermediate ostium into the corresponding air-sac. The entobronchia (" bronches diaphragmatiques," Sappey) are four in number. They take their origin in wide openings of the dorsal wall of the vestibule. The fourth is hindermost, runs almost directly backwards to the posterior end of the lung, and ends there caecally. Branches are given off only from its ventral wall. The third entobronchium runs parallel with this, giving off a number of close-set branches from its dorsal side, which are distributed over the superior facet. Close to the entrance of the bronchus, this presents the large anterior intermediate ostium for the corresponding air-sac. The second entobronchium passes directly dorsad, and ramifies over the middle of the superior facet. A wide branch descends to the subbronchial ostium. The first entobronchium curves sharply round the entrance of the bronchus, and from its anterior or convex wall gives off a number of branches to the anterior part of the superior facet and to the anterior inferior facet. One of these branches passes directly forwards, and opens by the prabronchial ostium into the praebronchial air-sac; while the inferior end of the trunk opens below the bronchus, into the subbronchial ostium, and, in that way, communicates with the subbronchial air-sac. Thus the mesobronchium and the first entobronchium are each connected with two air-sacs-the former with the posterior two, and the latter with the anterior two ; while the middle air-sac (anterior intermediate) communicates with the third entobronchium so close to the vestibule that it might almost be said to open into the latter. Behind the vestibule, the mesobronchium gives off successively several (usually six or seven) branches, which are directed laterally and dorsally towards the lateral or costal face of the lung. These are the ectobronchia (" bronches costales," Sappey). Those parts of the walls of the various bronchia and their ramifica- |