1867.] DR. J. MURIE ON THE EMU. 413 



these two considerations involves the idea of the sac's possessing a 

 strncture analogous to those found in the respiratory surfaces gene- 

 rally. But so far as tissue is taken into account, its examination, by 

 former observers as well as myself, does not show its texture to be 

 sufficiently rich in vascular network to perform the function of 

 aerification, at least such aerification as might be supposed to be 

 required in a warm-blooded vertebrate. 



No doubt this tracheal pouch might correspond to the thoracic 

 and other pneumatic cavities which are present in this as in many 

 other birds, these, as some maintain (an opinion I am inclined to 

 adopt), subserving the purposes of respiration rather than that of 

 buoyancy in flight, which formerly was considered their function. 

 But as the act of filling the tracheal sac, unlike the other pneumatic 

 chambers, involves unusual and highly increased muscular efforts, 

 and as the sac does not, so far as has been observed in the living 

 bird, retain regularly or dilate synchronously with ordinary respi- 

 ration, it would follow that, in a mere occasional dilatation, so little 

 advantage would accrue that one can hardly believe it possible that 

 the physiological changes would be of the same nature as those 

 occurring in the bone-cavities &c. 



Concerning the second use, namely, as a swimming-bladder, the 

 reasoning seems far-fetched. It is true that the Emu alone of all 

 the struthious birds is known to take readily to the water ; but the 

 amount of buoyancy which such an air-sac would confer hardly 

 seems equal to the occasion. If the bird depended on such an appa- 

 ratus alone, muscular effort on its own part would far outbalance 

 such a structure ; and, moreover, in swimming-birds generally no 

 such appliance is required, they remaining floating about for a long 

 period, while the Emu, at most, w^ould, in the event of crossing a 

 swollen current, dash right through the watery element. 



In considering the third suggested use, for running-purposes, if 

 we found its presence in all the species of Strnthionidce, or such a 

 structure in other animals noted for their speed of foot, we might 

 the more readily believe it a necessity and adjunct to fleetness of 

 ])rogressiou. But this tracheal pouch, be it remembered, is strictly 

 confined to Dromceus novce-hoUandicB, and is found in no other spe- 

 cies of the group, and, as I shall hereafter show, it occurs in a well- 

 developed manner in slow-moving Reptilians, invalidating the idea 

 of its being an accessory to celerity of motion. Besides the pheno- 

 menon of inflation has not been observed to take place in rapid 

 movement, but rather at quieter times and at the will of the bird. 



As for the fourth use, simple vocal resonance, this indeed may be 

 augmented by the chamber in question ; but that it represents a 

 special organ of voice is, to say the least of it, doubtful. The com- 

 parison between the tracheal enlargement of Ducks and this sac 

 does not strengthen the argument, as it is remarkable that the females 

 of such birds often have the loudest voice, though no tracheal en- 

 largement is present ; whereas its presence in the males is not neces- 

 sarily conjoined with voluminous vocal powers. 



Regarding the fifth assigned use, the sexual organ of sound durins: 



