AIR SACS. 



445 



The mesobronchium * a short distance before its termination 

 in the abdominal air-sac gives off a branch which passes to the 

 lung surface and dilates into the posterior thoracic air-sac. 

 This lies on the outer side of the anterior end of the abdominal 

 sacs on the ventral side of the hind end of the lung and behind 

 the lung. Its outer side 

 is united with the side Lh 



wall of the posterior part 

 of the thorax and of the 

 anterior part of the abdo- 

 men, and the anterior 

 part of its dorsal wall 

 with the lung surface. 

 Mesially the anterior dor- 

 sal end of it is in contact 

 with its fellow (forming 

 part of the median sep- 

 tum, see below). The 

 tubes which pass to the 

 other air-sacs are second- 

 ary bronchi and are called 

 entohronchia ; they come 

 off from a dilatation on 

 the m e s o bronchium, 

 called the vestibule, soon 

 after it enters the lung. 

 There are four entohron- 

 chia. The fourth runs 

 to the hind end of the 

 lung and ends there cae- 

 cally. The third runs 

 parallel with this ; it 

 gives ofiP close to its point 

 of origin from the vesti- 

 bule a tube which immediately dilates into the anterior thoracic 

 sac. This lies on the ventral surface of the middle part of the 

 lung to which its dorsal wall is adherent and extends back so as 

 to overlap the outer side of the anterior part of the posterior 

 thoracic sac. Its outer wall is adherent to the side walls of the 

 ♦ The following description of the bronchi refers mainly to the pigeon. 



uy 



Fia. 245. — Lungs and air-sacs of the pigeon (dia- 

 grammatic, after C. Heider). C connection of 

 tlie interclavicular sac with the sternal air-spaces ; 

 La abdominal air-sacs ; Lp meaian part (peri- 

 tracheal) of interclavicular air-sac with its diver- 

 ticula {7M into the humerus (7/) and between the 

 pectoral muscles (im) ; Lth thoracic air-sacs ; 

 P lungs ; Tr trachea. 



