OCR Text |
Show 1868.] DR. J. MURIE ON THE GULAR POUCH OF OTIS. 477 pressed at the will of the bird, as evidently is the case in the organ in question. Besides this, the sac presents a wide external aperture (i. e. into the mouth) with natural and healthy-looking walls lined with mucous membrane. As has been demonstrated, the enlarged tracheal pouch of the E m u is not a sudden or accidental circumstance, but the further development of a rudimentary structure found in the young; so in an analogical manner should we expect to find rudiments of the gular pouch in the young male Bustard. If the gular pouch is, so to speak, sporadic, irregularly dispersed among individual specimens, merely the product of inconstant fortuitous circumstances, and produced in old birds in what must seem an incredibly short space of time, then in it we have a most extraordinary physiological fact, and such as does not tally with our present knowledge of the laws of development. 7. Observations respecting the development of this appendage, and especially such as point out the precise period and manner of growth, are yet a desideratum. As I have already hinted in m y introductory remarks, some anomalies are certainly difficult of explanation. 8. Finally, with regard to the mechanism of inflation, the first thing is how the air gets there. Now, according to the laws of pneumatics applied physiologically, the pressure of the surrounding atmosphere ordinarily would not be sufficient to overcome the resistance and tonicity of the living tissues, such as to produce complete distention. Neither is it likely that sufflation is the result of a vacuum. A lengthened inspiration m a y aid, but I believe cannot directly and fully dilate the cavity; that is to say, the tongue being raised and the aperture into the gular pouch unobstructed, the air drawn into the lungs during the inspiratory effort would not equally rush in and fill the gular sac to repletion, as necessarily it does the pulmonary cells and pneumatic cavities. The lungs and subsidiary air-passages once full, however, and expiration naturally taking place, the mouth and posterior nasal passages require only to be partially closed for the thoracic muscular contraction to drive the air into the sac. In other words, muscular power is as requisite to inflate it as to empty it. A familiar illustration might be given in the blowing out of a bladder. Judging from the actions of the living Australian Bustard, the above explanation holds good, inasmuch as previous to expansion of the gular pouch it does not gape, but inspires quietly. W h e n the pouch is blown out and the bird utters the cooing snapping sounds, the mouth is then more or less open. The cooing noise may be laryngeal. If from the gular pouch, compression of the muscular and fibro-elastic tissues of the neck must drive the air out, which, the fibres at the neck of the sac resisting, cause it to escape in jets. By relaxation of the mandibular fibres and contraction of those of the inferior part of the neck, emptiness of the pouch results, and the neck assumes its usual proportions. |