Ill 



AIE-SACS 



165 



,EI 



E4 



The air-sac rudiments sprout out (Fig. 92) from the main 

 pulmonary cavities— the cervical from the first entobronchus, the 

 interclavicular and anterior thoracic jointly from the third ento- 

 bronchus, the posterior thoracic and the abdominal from the meso- 

 bronchus. Later on additional secondary communications between 

 the air-sac cavity and the pulmonary cavities are established (except 

 in the case of the cervical air -sac) by means of the recurrent 

 bronchi of Juillet. These arise in the ordinary fowl about the 

 tenth day of incubation in 

 the form of "outgrowths of 

 the wall of the air-sac either 

 near its tip (interclavicular 

 and anterior thoracic) or just 

 before it emerges through the 

 general surface of the lung 

 (posterior thoracic and abdo- 

 minal) as shown in Fig. 92. 



These outgrowths burrow 

 into the superficial layer of 

 the lung, branch and become 

 joined up, in a manner the 

 details of which have not yet 

 been worked out, with the 

 system of parabronchi. The 

 communications are visible 

 in suitable preparations of 

 the adult lung as groups of 

 openings, each group leading 

 into the lung from the appro- 

 priate air -sac — those of the 

 interclavicular and anterior 

 thoracic lying towards the 

 lateral edge of the ventral 

 surface of the lung, about the 

 level of the attachment of 

 the bronchus, and those of 

 the posterior thoracic and 

 abdominal sacs lying near 

 the hind end of the lung, 

 close to the direct opening between it and the corresponding air-sac. 

 It would appear that the function of these recurrent channels 

 is to conduct the air forced out of the air-sacs in the expiratory 

 effort through the system of air-capillaries, the muscular coat of the 

 parabronchi doubtless playing an important part in directing the 

 passage of the air through the system of air-capillaries rather than 

 through the parabronchi themselves. 



The formation of the air-sacs does not exhaust the remarkable 

 proliferative powers of the wall of the lung in Birds. Further out- 



at 



Fig. 92. — Diagrammatic view of the right lung of 

 a Fowl embryo of the tenth day as seen from 

 the ventral side, illustrating the origin of the 

 air-saes. (After Juillet, 1912.) The four 

 entobronchi are shaded. 



a&, abdominal air -sac; at, anterior thoracic air-sac; 

 eer, cervical air -sac; El and E4, lirst and fourth ento- 

 bronchi ; ic, interclavicular air-sac ; mes, mesobronchus ; 

 p.t, posterior thoracic air-sac ; r, recurrent bronchus. 



