OCR Text |
Show 786 MR. W. A. FORBES O N T H E [June 21, The exact number of tracheal rings which are fused to form this box varies in different specimens from four to six ; in some cases it is, apparently, formed by four rings on one side and five on another. The lowest tracheal ring is strongly concave downwards, but in front in the middle line is transversely truncated. Posteriorly the tympanic box is deeply and widely notched. There is a distinct, narrow, cartilaginous pessulus, which runs from behind forwards, connecting the anterior and posterior walls of this box, and interrupting, in the mid line, the continuity of the memBrana tympanifornis, which completes the bronchial walls internally. The first bronchial semiring is nearly straight, and of cylindrical form. It is closely connected at its extremities with the last tracheal ring ; but between these points is a pretty wide, lunate, interannular interval. The anterior ends of these first semirings are inturned considerably, but do not meet each other, or the pessulus ; posteriorly they do not extend inwards, by some way, as far as the preceding or succeeding rings. The second bronchial semiring is similar in form, but dilated slightly behind ; it is closely approximated to the preceding ring in the greater part of its extent, but is anteriorly strongly curved downwards (in a somewhat sinuous way), so that here a considerable space is left between the two semirings in question. The third and fourth rings are considerably dilated, and produced inwards, posteriorly; anteriorly, in the particular specimen figured, they are fused into a compaiatively narrow ring. The fifth, sixth, and succeeding semirings are quite simple incomplete hoops of cartilage, which become smaller and less complete internally as they approach the lungs. Between the pessulus in the middle line and the tracheal box and first four bronchial semirings, is spread a membrane, completing the tracheal and bronchial walls at the bifurcation of the tube. This memhrana tympaniformis is thinnest posteriorly; but in the middle part of its extent, over a triangular area-the base of the triangle resting on the pessulus, whilst the apex is at the posterior end of the fourth bronchial semiring-it is thickened and of a fibro-cartilaginous consistency. From the sides of about the last ten tracheal rings a single thin but broad band of muscle arises, the fibres of which, running downwards and forwards, are inserted into the anterior halves of the first five bronchial semirings and the membranes between them, extending, in some specimens, almost to the anterior tips of the first two semirings. This muscle is therefore in all respects a true intrinsic syringeal one. The lateral tracheal muscle stops anteriorly to the origin of the intrinsic one. Internally, there is a well-marked, sharp-bordered memBrana semilunaris, resting on the pessulus, developed between the bronchi. In the space corresponding to the interval between the last tracheal and first bronchial rings, an accumulation of the fibrous and elastic tissues of the living membrane of the bronchus gives rise to a well-developed vocal cord. Bhea macrorhyncha closely resembles, in all respects, B. americana |