RESPIRATORY SYSTEM 65 



narrow interval from the last tracheal ring. The second and 

 third rings are similar, but are more slender and lengthy ; 

 they are convex downwards, but very slightly so ; hence the 

 interannular intervals are slight here also. Their anterior 

 ends are very slight, inturned, impinging but to a small 

 extent on the membrana tympaniformis, which completes 

 the bronchial tubes internally, and, in consequence of the 

 absence of any three-way piece, passes continuously from 

 one bronchus to another, so closing the tracheal tube in- 

 feriorly. The fourth, fifth, and succeeding bronchial rings 

 are similar in character, but their ends, which tend to be 

 dilated posteriorly, are successively more and more incurved 

 to about the tenth. Nowhere are the bronchial rings com- 

 plete. There is at most only a trace of a membrana semi- 

 lunaris, in the form of a feeble, scarcely raised antero-pos- 

 teriorly directed fold of mucous membrane.' 



This syrinx, therefore, differs from the more typical 

 tracheo-bronchial syrinx in, at any rate, three essentials — 

 (1) the absence of tracheo-bronchial muscles ; (2) in the 

 slight amount of specialisation of the last rings of the trachea ; 

 and (3) in the absence of a pessulus. The only distinguish- 

 ing feature of the syrinx which is present is the membrana 

 tympaniformis. But the presence of this, and of the rudi- 

 mentary membrana semilunaris, fully justified Foebes in 

 contradicting the assertion, prevalent at the time when he 

 wrote, that the Struthiones had no ' lower larynx,' an assertion, 

 indeed, which could not possibly be made with the syrinx of 

 Bhea, a quite typically tracheo-bronchial one, in existence. 

 Still, it is undoubted that the syrinx of the ostrich is in a 

 very simple condition, and hardly deserves the name. In 

 the stork tribe we have a series of stages in the degeneration 

 of the syrinx. In Abdvmia sphenorhyncha, as in other 

 storks, there are no intrinsic syringeal muscles, but the 

 membrana tympaniformis is well developed and of consider- 

 able extent. In Xenorhynchus the membrana tympaniformis 

 is almost, but not quite, obliterated, and, finally, in Ciconia 

 the bronchial rings are rings, and not semi-rings ; there is in 

 them no trace of a membrana tympaniformis ; but in all the 



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