240 ON THE TRACHEA IN THE " EAT1TE " BIRDS. 



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 tympaniformis, 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 con- 

 siderably, 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 comparatively 

 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 membrana 

 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 speci- 

 mens, 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 semilu- 

 naris, 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 

 lining membrane of the bronchus gives rise to a well -developed vocal 

 cord. 



