OCR Text |
Show 780 MR. W. A. FORBES ON THE [June 21, rings are here, as elsewhere, entire simple rings, of an average depth of about *15 inch, and are separated only by very slight interannular intervals. The trachea is slightly compressed and posteriorly carinated for about the last 7 ring's. The last ring but four is somewhat produced downwards in the middle line, both anteriorly and posteriorly ; it is, in consequence, narrower laterally than elsewhere. The antepenultimate ring presents the same features more strongly developed. In two of the four specimens examined it sends down a small pessuliform process of cartilage in the middle line behind, filling the chink left between the posterior extremities of the two next (incomplete) rings. The penultimate ring is narrower and more cylindrical than its predecessors ; it is also wider transversely, and incomplete behind in the middle line, its extremities, however, being closely approximated to each other. The last tracheal ring is still wider transversely, and more cylindrical; and it too is incomplete posteriorly, to a greater extent than its predecessor; viewed from the side it is convex upwards, as are its few immediate predecessors in a less degree. The interannular intervals between all these rings are, when undisturbed, mere chinks filled up by dense fibrous and elastic tissues. There is no trace of a pessulus, though the last tracheal ring is slightly produced downwards in front. The first bronchial semiring, on each side, is narrow and cylindrical, strongest anteriorly, and somewhat attenuated posteriorly. It is separated only by a 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 small here also. Their anterior ends are very slightly 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 the other, so closing the tracheal tube inferiorly. 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 complete. There is, at most, only a trace of a memBrana semilunaris, in the form of a very feeble, scarcely raised, antero-posteriorly directed fold of mucous membrane. Internally, the mucous membrane of the interior is greatly thickened, forming a vocal cord, in the region of the last three tracheal rings and first two bronchial semirings (vide fig. 1, b). There is no trace of any intrinsic voice-muscle ; and the lateral tracheal muscles stop at the point of insertion of the stemo-tracheales. The genus Apteryx, in the simplicity of the structure of its lower larynx, stands on the same level as Struthio. In Apteryx mantelli (figs. 3, 4) about the last dozen tracheal rings are quite simple in form, with narrow interannular intervals, and no anterior and posterior notching. The penultimate ring is produced slightly downwards, in a triangular way, both anteriorly and posteriorly. The last ring is also produced downwards anteriorly, but is incom- |