THE SYRINX OF GALLUS DOMESTICUS 173 



It should be recalled that the first half-ring of each primary 

 bronchus and the last intermediate syringeal cartilage arch 

 caudad and cephalad respectively, thus forming two (one on 

 each side) oval spaces, occupied by the external tympanic mem- 

 branes (fig. 2). Caudally these membranes are attached along 

 the entire length of the cephalic borders of the first half-rings; 

 cephalically they are attached along the entire length of the 

 caudal borders of the last intermediate cartilages. Dorsally 

 and ventrally the membranes narrow and are attached to the 

 lateral borders of the corresponding pyramids of the pessulus. 

 The internal tympanic membranes are situated more caudad 

 (fig. 2), in the medial walls of the bronchi just below the tracheal 

 bifurcation and stretch between the dorsal and ventral free 

 ends of the half-rings. Cephalically they are attached to the 

 lateral borders of the shaft of the pessulus. Caudally they 

 extend to the level of the third half-rings where they are re- 

 placed by the heavy fibrous tissue of the medial bronchial 

 walls. 



Stretching across the interbronchial interval, just caudad to 

 the internal tympanic membranes, is another membranous struc- 

 ture, the bronchidesmus (figs. 1 and 2) of Garrod. The middle 

 portion of this is somewhat narrower than its ends, and may 

 be said to he between the level of the fifth and the level of the 

 eighth bronchial half-rings. Its ends, exceeding these levels and 

 extending for a variable distance above and below them, are 

 attached correspondingly to the dorsal third of the medial bron- 

 chial walls. The cephahc portions of these attachments show a 

 modification. As these portions of the ends of the bronchides- 

 mus extend to the attachment stated, they expand, dorso- 

 ventrally, and find further attachment as far ventral as the 

 ventral third of the membranous medial walls of the bronchi 

 and as far cephalad as the level of the third bronchial half-rings. 

 This last attachment may be said to mark the caudal limits of 

 the internal tympanic membranes. The caudal border of the 

 bronchidesmus is broader than its cephalic border and presents 

 two oval openings, one on each side of the median plane, which 

 lead into two smaller irregular pockets within the bronchidesmus. 



