288 A. 0. V. Tymms: 



1. Supporting; framework. 



In place of the simple cartilaginous tracheal rings and bronchial 

 semi-rings, there has been much modification at the tracheo-bron- 

 cliial junction, in the formation of the sujiporting framework of the 

 syrinx, so that we may distinguish the following elements : — 



(a) The rod-like portion of the pessulus. 



(b) Dorsal triangular plate of the pessulus, 



(c) V'^entral triangulai- plate of the pessulus, 



(d) Last six tracheal rings, 



(e) First two bronchial semi-rings. 



(a) Pessulus. — The pessulus is situated just below the junctiou 

 of the bronchi, and it passes from the ventral to the dorsal surface 

 of the tracheal extremity. It is rod-like in shape, and in the adult 

 partly osseous in structure. Ventrally, it expands into the body 

 of a large median triangular plate, with a cranially-directed apex. 

 Dorsally, it is continued into a similar but smaller plate, the whole 

 structure (rod and two plates) lesembling somewhat a double- 

 headed bolt. Along the cranial margin of the pessulus thei'e is a 

 thin fold of raucous membrane, termed the mendjrana semilunaris. 



(b) Ventral Triangular Plate. — As already stated, this is situ- 

 ated on the ventral surface of the tracheal extremity. Its apex 

 extends as far cranially as to lie just liehind the fouith last tracheal 

 ring. Its basal angles articulate with the exti-emities of the first 

 bronihial semi-rings. Its lateral edges are in close i-elation to the 

 last tliree tracheal rings, fusion taking place usually only with the 

 last ring. Its body is partly calcareous, and may be even osseouis 

 in structure. 



(c) Dorsal triangular plate. — It is situated on the dorsal surfa/e 

 of the tracheal extremity, and its a))ex extends cranially between th-.; 

 doi'sal ends of the last two tracheal rings, but does not (piite leach 

 the ante-penultimate ring. The dorsal ends of thr first broni'hial 

 semi-i-ings articulate with the lateral angles of the plate, and with 

 its sides the ends of the last two tracheal rings are in close relation, 

 but do not fu.se. 



(d) Last six tracheal rings. — (i.) Last tiacheal ring. Of the 

 six rings, this is the most developed. It is not flattened, but 

 rounded. Ventrally, it widens considt rnbly, and fuses with the 

 triangular plate, but dorsally. it ]>res.ses closely against the ]>late, 

 without fusing. 



(ii.) Penultimate ring. This second-last ring is band-like, .•iiid 

 7 ather wider than the others. Its ventral extiemitit-s pi-ess closely 

 against the sides of the ventral triangular ]date. while doisally the 

 ends lie on each side of the dorsal plate. 



