Syrliur ot Fold. 291 



fowl tliey i-eai-h oiilv as far as tlio (>leventh or twelfth last tracheal 

 ring, while the Mvrinx does not begin until the sixth-last ring. Thus 

 these muscles of the coninion fowl have no very close relation to the 

 juenibranes of the syrinx. 



4. Respiratory Air Sacs. — The air sacs completely surround the 

 .syrinx. Tliis envelope is not a sini})le one, but a composite one, in 

 wliic'h tlie several air sacs air in intimate contact with each other, 

 and with the syringeal membranes. In development, they are seen 

 to l)e derived fi-om the third entobronchus of the lung. The third 

 ontobrouchus teiiiiinates in a stem, which subdivides into two main 

 .stems, of which one, opening into the interclavicular sac, goes to 

 form the syringeal air sacs, while the other opens out into the 

 anterior thoracic air sac. As the stem of the interclavii'idar or 

 .syringeal air sacs ascends from the third entobronchus of the lung 

 to the interbronchial region, it expands into several air sacs, which 

 take uj) different positions in relation to the parts of tlie syrinx. 



A large ventral sac passes over the ventro-cranial region of the 

 bronchus md triangular plate, and comes to occupy the ventro- 

 lateral rc'giou of the svi'inx. 



A large sac from the right side extends up so far as to lie close 

 beneath the pessulus, the sub-pessnlar air sac. On each side it is 

 in relation to the mendjranac internae. 



Another large sac passes dorsally from between the bronchi, and 

 then opens out into dorso-lateral sacs. The most cranial one is 

 large, and comes to occupy the dorso-lateral region of the syrinx. 



Other lu-anches pass off, and become related to structures apart 

 from the syrinx, one main branch and several smaller ones retur]i- 

 ing to the lung tissue, and so constituting the recuri'ent branches 

 of the interclavicular sac. The recent research of Juillet, 1012 

 (Chap. IV.), showing that the direct stem of the interclavicular sac 

 arises in conmion with the stem of the anterior thoracic sac, is liete 

 confirmed. What was formerly thought to be the direct bronchial 

 Ktem. he has shown to be the indirect recurrent interclavicular 

 branch. Thus the stem of the interclavicular sac, besides giving off 

 its recurrent branches, expands into the interclavicular region, 

 •where it gives rise to the syringeal air sacs. 



Embryonic Development. 



Passing now to the embryonic condition of the organ, a general 

 survey of its development will first be given, followed by a detailed 

 description of the gradual appearance, in time, of the several yiarts 

 <if the syrinx. 



