Proceedings of the Scientific Meetings of the Zoological Society of London 1867

Page 460

1867.] DR. J. MURIE ON THE EMU. 409 breadth about the middle of the sac, or its rough diameter, was between 3 and 4 inches. M y examination of the structure of the wall of the sac agrees with Wedemeyer and Macartney's observations, and not with Knox's, who describes it as a muscular bag. I find it to be composed of mem­branous white fibro-elastic tissue, overlaid, however, in part with some very delicate and widely scattered transversely striped mus­cular fibre, the remnants or representative of the platysma myoides. (Professor Owen* names this the "constrictor colli;" and remarks, " this muscle is well developed in the Emeu, and acts when the drum-like dilatation of the windpipe is sounded.") The skin and subcutaneous tissues form the anterior boundary of the sac; while posteriorly it lies upon the vessels and muscles of the ventral surface of the neck, some of the former appearing as raised cords on its inner surface. Its fibrous envelope furthermore closely invests the anterior surface of the trachea, and at the fissure, presently to be described, appears almost to be continuous with the mucous tissue of the lining walls of the trachea itself. Traced in this way it seems like a kind of hernia of the lining membrane of the trachea, strengthened by the surrounding fibrous and other tissues. But at the same time it must be noted there is a visible though faint line of demarcation around the opening, at 0*2 inch from its edge. The opening from the trachea into the sac (fig. 1, ap.), caused by deficiency of the tracheal rings in front, is 2 | inches long; and the slit has an average breadth of 0*3 inch in the ordinary retracted condition, but when pulled apart it becomes of a very much wider oval figure. In the female bird in question its position was between the fifty-fourth and fifty-ninth rings; in the young male between the fifty-eighth and sixty-third. Knox asserts there are about thirteen tra­cheal rings deficient, and that it commences at the fifty-second. Fremery's observation agrees more nearly with mine, as he notices its occurrence between the fifty-third and sixty-second rings. Wede­meyer gives 2\ inches as the length of the opening. As shown in fig. 1, on the right side there are six cartilaginous rings which do not meet, but on the left side only five ; this is oc­casioned by the lowermost or fifth ring of the right side bifurcating as it approaches the open interspace. In the first male bird exa­mined by me, in a similar manner, there were seven rings on the right side and six on the left. The photograph exhibited demon­strated this peculiarity. The edges of the opening are not perfectly straight, but wavy, from the ring-cartilages projecting further than their fibrous interspaces. The edge of the rings beneath and above the opening forming the superior and inferior boundary of the vacuity are slightly emargi­nated. Between the second and third upper cartilages of the fissure on either side is a small fibrous band-like duplication of the wall of the sac, which stretches outwards and upwards, and seems to bind or form a stay and partial septum to the otherwise yielding wall of the sac (fig. 1,/. bi). * Anat. of Vertebrates, 1866, vol. ii. p. 110.