1882.] ORGANS OF APTERYX. 563 



mesenteric arteries, which thus appear in the middle Hne of the 

 obhque septum when this is viewed from below and behind. The 

 vena cava inferior enters the pericardium just behind the attach- 

 ment 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 entobvonchium runs parallel with this, giving off a 

 number of close-set branches from its dorsal side, which are distri- 

 buted over the superior facet. Close to the entrance of the bronchus, 

 this presents the large anterior intermediate ostium for the corre- 

 sponding 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 Jirst 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 ivcebronchial ostium into the prsebronchial 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- 



