Ill 



AIR-SACS 



165 



cer 



/ 



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

 pulmonary cavities the cervical from the first entobronchus, the 

 interolavioular 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 FlQ 92 . Diagrammatic view of the right lung of 

 lateral edge of the ventral a Fowl embryo of the tenth day as seen from 

 Surface of the lung, about the the ventral side > illustrating the origin of the 



e> ,i / air-sacs. (After Juillet, 1912.) 



level Of the attachment Of e ntobronchi are shaded. 



The four 



ab, abdominal air -sac; at, anterior thoracic air-sac; 



the bronchus, and those of 



the posterior thoracic .and cer, cervical air-sac; Eland E4, first and fourth 

 abdominal SaCS lyino- near bronchi ; ic, interclavicular air-sac ; mts, mesobronchus ; 



the hind end of the lung, f*W^W^^J** 



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- 



