18G7.] DR. J. MURIE ON THE EMU. 409 



breadth about the middle of the sac, or its rough diameter, was 

 between 3 and 4 inches. 



My 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 mem- 

 branous white fibro-elastic tissue, overlaid, however, in part with 

 some very delicate and widely scattered transversely striped mus- 

 cular 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 tra- 

 cheal 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. Wede- 

 meyer gives 2g 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 oc- 

 casioned by the lowermost or fifth ring of the right side bifurcating 

 as it approaches the open interspace. In the first male bird exa- 

 mined by me, in a similar manner, there were seven rings on the 

 right side and six on the left. The photograph exhibited demon- 

 strated 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 emargi- 

 nated. 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. \,f.b.). 



* Anat. of Vertebrates, 18G6, vol. ii. p. 110. 



