292 COMPARATIVE ANATOMY 



the ribs, and are capable of very little distension. They are pene^ 

 trated by a system of bronchi which will be described presently. 

 The lower surface of each lung is closely invested by a thin fibrous 

 membrane, the pulmonary aponeurosis,'^ into which are inserted a 

 variable number of muscular bands {costo-pulmonary muscles) : 

 these arise from the vertebral ribs, and are supplied by the inter- 

 costal nerves (Fig. 238). 



When the ventral body-wall of a Bird is removed, the heart, 

 stomach, liver, and intestine are seen pressed towards the mid-line, 

 and on either side of them a tightly-stretched fascia, the oblique 

 septum, is observable, which shuts them off from a paired lateral 

 stht-pulmonary chamber (Fig. 237). Other chambers are situated 

 in the anterior thoracic region, ventral to the lungs. Others, again, 

 are seen in the region of the heart and in the posterior part of the 

 abdominal cavity. These chambers are occupied by the air-sacs 

 with which certain of the bronchi communicate. 



The most posterior chamber on either side encloses an abdominal 

 (posterior) air-sac (Fig. 237). In Apteiyx, this is completely closed in by 

 the oblique septum, but in other Birds it gives rise to a large, distensible 

 diverticulum which extends backwards ventrally to the kidney, amongst the 

 viscera. 



In front of this there are two air-sacs lying above and externally to the 

 oblique septum, and constituting the main part of the sub-pulmonary 

 chamber ; these may be caUed the anterior and posterior intermediate sacs. 

 A transverse dividing-wall separates these, at the level of the cosliac 

 artery, and a second septum shuts off the anterior intermediate sac from the 

 one lying in front of it, to be described presently. The posterior inter- 

 mediate air-sac presents the simplest and most constant relations, and never 

 communicates with any of the neighbouring chambers, as is often the case 

 with the anterior intermediate. 



A pair of prehronchial air-sacs lies on either side of the oesophagus above 

 each bronchus, anterior to the hilum of the lung, and below this a siib-bronchial 

 sac is situated, which is separated behind from the anterior intermediate sac 

 by a septum. This is usually unpaired, the sac of either side fusing with its 

 fellow to form an interelavicidar chamber, bounded by the furcula ^ ; it com- 

 municates with neighbouring air-cavities which lie between the pericardium 

 and sternum and in the axilla, outside the body-cavity (axillary sac). 



The main trronchus (mesobronchium) runs close to the ventral surface of 

 the lung surrounded by the lung-parenchyma, and extends to its posterior 

 end, where, as a rule, it opens directly into the abdominal air-sac (Fig. 238). 

 From it a large lateral hronchiis is given off, which opens into the posterior 

 intermediate sac by one or two (e.g., in Passeres) apertures. Besides this 

 there are from four to six other lateral bronchi, all of which become 

 broadened out in a fan-like manner on the ventral surface of the lung. 

 These may be called entobronchia (bronchi divergentes) : they aU arise from 

 the anterior portion of the mesobronchium. The first of these radiates out 



^ The pulmonary aponeurosis, as well as the oblique septum, is often spoken of 

 as a "diaphragm" (comp. p. 141). The chamber (pleural cavity) in which the 

 lungs are situated is shut off from the rest of the abdominal cavity in Chelonians 

 and Crocodiles also. 



- In some Birds (e. (/., Rhea, Vulture, Adjutant) a median septum is present 

 separating the two sub-bronchial sacs. 



