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Show 414 DR. J. MURIE ON THE EMU. [Apr. 11, the breeding-season, this seems a more reasonable one, inasmuch as there are many instances of appearances and habits being temporarily assumed in birds during the breeding-time-e. g. the wattles of the Tragopan, and gular pouch of the Bustard &c. It agrees also with the fact that inflation of the sac is more often noticed then, and the peculiar drumming-noise is very frequent under sexual excitement. The phenomena accompanying distention of this tracheal pouch in the birds in the Society's Gardens, as I have observed them, are as follows:-The head and the neck are slightly raised, the latter somewhat bent; then there appears a swelling in the lower part of the neck, and the bird with partly opened mouth gives utterance to a series of grunting-like sounds, during which there is observable a tremulous-like motion in the distended lower part of the neck, while a certain compressed state of the glottis appears to take place. The physiological action seems to be first a deep inspiration, followed by expiration of a forced kind with total or partial closure of the glottis, the tracheal pouch then fills, the aperture is more or less dilated, and as the glottis is temporarily and successively relaxed the air rushes from the lungs towards the open mouth, and, passing the orifice of the sac in jets, gives rise to the peculiar hollow metallic sounds, as does air when blown over the open bunghole of a cask. When an organ or appendage admits of so many ingenious suppositions as to its function as have been assigned to the one here treated of, it is not surprising that the precise one should remain uncertainly known. Interpreting the nature of the tracheal pouch from structure and position, as well as the mode and times of its being called into action, with the fact that the sac increases from the young to the adult stage, and that its maximum of size and apparent use correspond with the period of the procreative faculty, the following seems to be its use:-that it serves as an organ of sound employed by the bird under the phenomena incident to the passion of lust, and hence is more powerfully brought into play during the breeding-season. Homology.-It is not m y intention in the present communication to enter largely into the homology of this tracheal sac. Its wider relations in a homological sense evidently possess as much of interest as does the more limited study of its ceconomical function in the Emu. I shall therefore only advert to the probable direction in which its homology is to be studied, by referring to a note by the French translators of Meckel's ' Vergleichende Anatomie,' " Ce sac peut etre compare' aux sacs laryngiens des sauriens, a raison de l'influence qu'exerce la volonte de l'animal sur la distention de cette poche par l'air"*. Among Reptilians, however, the Chameleon, as is well known, possesses a dilatable tracheal sac, in many respects closely resembling that existing in the Emu. In a specimen of this animal which I examined this pouch was of considerable size, and in its textural structure quite like that of the E m u . Instead, however, of being simple and ovoid or globular when distended, as in the Emu, it was pyriform, and with a partially constricted sacculus at * Edition already cited, tome x. p. 405. |