384 COMPARATIVE ANATOMY 



which divide into finer and finer branches (" air-capillaries ") 

 with which the blood-capillaries are interwoven. The delicate air- 

 passages communicate with one another, so that none of the 

 alveoli apparently end blindly (Fig. 294, A). The relative delicacy 

 of the bronchial network is proportionate to the power of flight. 



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

 stomach, liver, and intestine are seen in the middle, and on either 

 side of them is a tightly- stretched fascia, the oblique septum, which 

 shuts them off from a paired lateral sub-pulmonary chamber 

 (Fig. 290). 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 cavit}^. 

 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. 290). In Apteryx, 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 called the anterior and posterior thoracic sacs. A transverse 

 dividing-wall separates these, at the level of the cceliac artery, 

 and a second septum shuts off the anterior thoracic sac from 

 another one lying in front of it. The posterior thoracic air- sac 

 presents the simplest and most constant relations, and never com- 

 municates with any of the neighbouring chambers, as is often the 

 case with the anterior thoracic sac. A pair of cervical air-sacs lies 

 on either side of the oesophagus above each bronchus, anterior 

 to the hilum of the lung, and below this a sub-bronchial sac is 

 situated, which is separated behind from the anterior thoracic sac 

 by the septum already referred to. This sac is usually unpaired, 

 that of either side fusing with its fellow to form an inte7 clavicular 

 chamber, bounded by the furcula * ; it communicates with neigh- 

 bouring air-cavities which lie between the pericardium and ster- 

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



The main bronchus (mesobronchium) as a rule opens directly 

 into the abdominal air-sac (Fig. 291). From it a large lateral 

 bronchus t is given off, which opens into the posterior thoracic sac 

 by one or two apertures. The first entobronchium radiates out 

 anteriorly to the hilum of the lung, and one of its branches 

 opens into the cervical sac. Almost without exception, a large 

 aperture or ostium is present on the wall of the third entobron- 

 chium, communicating with the anterior thoracic air-sac. A 

 branch of the second entobronchium opens externally to the 

 hilum of the lung into the sub-bronchial sac. 



1 In some Birds (e.g. Hhea, Vulture, Adjutant) a median septum is present 

 separating the two sub-bronchial sacs. 



