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Show 1867.] DR. J. MURIE ON THE EMU. 413 these two considerations involves the idea of the sac's possessing a structure analogous to those found in the respiratory surfaces generally. 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 respiration, 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 E m u 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 apparatus 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, would, 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 Struthionidee, 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 progression. But this tracheal pouch, be it remembered, is strictly confined to Dromceus novee-hollandice, and is found in no other species 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 phenomenon 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 comparison 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 enlargement is present; whereas its presence in the males is not necessarily conjoined with voluminous vocal powers. Regarding the fifth assigned use, the sexual organ of sound during |