OCR Text |
Show 782 MR. W. A. FORBES ON THE [June 21, examined by me, and all agree closely in their tracheal structure with A. mantelli. M y pecimens of A. australis and A. oweni (two) agree together in having the last three tracheal rings incomplete posteriorly; whilst in A. mantelli and A. haasti, of which I have seen only single specimens, the last ring alone is incomplete. In the Gasuariida we meet with peculiarities in the structure of the bifurcating trachea not existing in the other " Ratitae." In Casuarius galeatus (figs. 5, 6, p. 783) the trachea is somewhat dilated for the terminal inch or so of its extent, tranversely and also posteriorly. The last tracheal rings (for a number varying in different specimens, in the specimen in question 12, in another 23) are incomplete in the middle line behind, though the posterior ends are closely approximated together **. These rings are tolerably uniform in breadth posteriorly, tapering only somewhat at their extremities; the interannular intervals are mere chinks. Anteriorly, however, the rings being dilated in the median line and attenuated laterally, theintervals are better developed. The last 5 or 6 rings are more and more curved downwards anteriorly, whilst their posterior moieties are somewhat dilated, their ends gradually receding more and more from each other in the middle line. There is no trace of a pessulus. The first bronchial semirings much resemble the last tracheal rings, their anterior extremities being closely approximated together, and their posterior ones dilated, and somewhat pointed, terminally. The second, third, and fourth bronchial semirings are simple cartilaginous hoops, tolerably deep, separated only by narrow intervals, and, as usual, completed internally by a memBrana tympani-formis. The fifth, sixth, and seventh are similar but longer semirings, the sixth and seventh being dilated anteriorly. The succeeding rings are similar but quite simple hoops, never forming anywhere complete circles. There is no trace of any intrinsic muscles. The stemo-tracheales are inserted on the trachea at about the twelfth ring from the last. Anteriorly they expand on the tracheal wall, and are in contact with each other over the middle line, as is also the case in Dromaus, though not in the other three genera. A small part of each muscle runs to be inserted into the posterior wall of the trachea near the margin. The lateral muscle of the trachea passes between these two portions of the sterno-trachealis of its side, but does not pass down further than the commencement of the tracheal tympanum. 1 Between the extremities of each imperfect tracheal ring runs a short band of connective and elastic tissue, with the fibres running transversely. These extend the whole length of tbe trachea, and when well developed have the appearance of a longitudinal band running along the middle line of the tube posteriorly. By the contraction of these fibres, the ends of the tracheal rings, where these are incomplete, or their more slender middle portions where perfect, are drawn together, and pressed into the interior of the tube, so forming what at first sight looks very much like a longitudinal, though incomplete tracheal septum, such as is found in some Procellariidse and other birds Inconsequence of this structure, a transverse section of the tracheal tympanum posteriorly presents two strong convexities separated by a median concavity. |