Description of the Modifications of certain Organs. 63T 



■without a break over the ends of the split rings. The outer lining 

 of the pouch is coutinuous with the elastic and common connective 

 tissue, which envelopes the tracheal rings, and it receives numerous 

 tibres from the broad although ihiu muscular bands, which accompany 

 the trachea as Mm, tracheales longi and then, below the pouch leave 

 the trachea, to be inserted into the sternal apparatus as Mm. tracheo- 

 sternales. Outside the pouch follows of course the thinned out com- 

 mon skin of the neck, the fibres of the M. constrictor colli forming 

 a thin network over the lateral and ventral sides. 



The whole organ is highly elastic, and, when blowu out by the 

 bird at will, is capable of assuming large dimensions. In Murie's 

 adult specimen the length of the slit was about 7 cm ; the length of 

 the sac about 38 and its width 7—10 cm. In an adult specimen in 

 the Cambridge Museum the length of the slit is 4,5 cm, the width of 

 the slit 0,6 cm; width of trachea 3,4 cm, depth of trachea 1,5 cm. 



Although the Emus possess no syringeal muscles, they have on 

 each bronchus an internal tympaniform membraue, and are capable 

 of emitting a deep, hollow and grunting sound. The pouch seems 

 to act as a sort of resoundiug organ , and is most frequently dilated 

 during the breeding seasou. 



In a young male, dissected by Murie ^), about one third of the 

 adult size, the pouch was still very small (Fig. 8), it did hardly 

 Cover the flattened trachea in width and its length extended only over 

 a few inches beyond the slit. 



In a very young specimen, probably only three or four days old 

 (Fig. 9), I found no iudication of a pouch, but as many as 8 rings 

 deficient in the middle line; the öß"" and 65"" rings being complete. 



The pouch is consequently a hernia of the tracheal walls, r en- 

 dered possible by a medioveutral deficiency of some cartilaginous 

 rings, p r o d u c e d by the wilful action of the bird through the pressure 

 of the air within the respiratory tract, and increasing in size from 

 youth to age. That such a hernia is etfected through mechanical 

 strain cannot be doubted, but the question is how the deficiency in 

 the rings arose. It either began as an accidental defect in the thick- 

 ness of the cartilage, and the bulging out of the membrane was a 

 subsequent feature, or the frequent distention of the trachea was the 

 primary agent and produced the thinning and ultimate rupture of 



1) J. Murie, On the tracheal pouch of the Emu (Dromaeus Novae 

 Hollandiae), in: Proc. Zool. Soc, 1867, p. 405—415. 



