Sjjrin.r of Foui. 289 



(iii.) Ante-penultiniatu lin^-. Tliu third-last ring is more re- 

 (hued. It is made up ut' two straight, hiteral, baud-like rudiuicnts. 

 its ventral ends do uul fuse, but lie on each side, agaiust the apex 

 of the ventral triaugvdar plate, its dorsal ends taper considerably, 

 and come to lie close to each other, without fusing, and just beyond 

 die apex of the dorsal triangular plate. 



(iv.) Fourth-last ring. This ring is so reduced and incomplete 

 as to be represented by a mere thread-like band, situated a little 

 beyond the apex of the ventral triangular plate. The left side is 

 thinner, and does not go much furtlier than half-way towards the 

 dorsal surface, but generally fuses with the third-last ring. The 

 right side is wider, and nearly reaches to the dorsal surface. Ven- 

 trally, the ends come very close together. 



(v.) Fifth-last ring. This ring differs from a normal ring in 

 l)eing slightly incomplete dorsally, where its ends are turned down- 

 wards, towards the apex of the triangular plate. It is flattened, and 

 purely cartilaginous in structure. 



(vi.) Sixth-last ring. This is the only complete ring of the 

 syrinx, and differs from those above the rings of the syrinx in not 

 being osseous in structure. 



(e) Bronchial semi-rings. — The first two semi-rings only are 

 uspecially modified in connection with the syi'inx. The first is 

 characterised by its large size, thickness and marked cui-vature, 

 the concavity being directed cranially. It is not flattened, but 

 round, and it articulates very intimately with the basal angles .A 

 Ixitli ventral and dorsal triangular plates. 



The second semi-ring has its ventral ends fused with those of 

 the first, but dorsally they are widely separated. 



2. Syringeal mendnanes. — As already stated, the syrinx has a 

 general membraneous appearance, and on closer examination several 

 distinct membranes mav be recognised. As a whole, they are 

 characterised by being set or stretched between an air space on 

 either side. As will be seen, the wall of each mend)rane has histo- 

 logically three layers. The following membranes may be dis- 

 tinguished : — 



(a) Membrauae exteniae. 



(b) Membranae iiittanae, 



(c) Membranae traclieales. 



(d) Mend)rana semilunaris, 



(e) Broncliidesmu.s. 



(a) Membrauae externae. — These membranes are situated one 

 on Ciach side of the tracheal lower extremity. They are very thin 



14 



