The Bird's Crop

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The crop is also sometimes known as the ingluvies, and is the dilatation of the oesophagus before its entrance into the thorax. The walls of the crop contain mucous glands. The crop is used as a receptacle for the food, which therein is softened and acted upon by water and the saliva and warmth of the bird.

Between a narrow, temporarily-dilated oesophagus and a permanent crop-like dilatation many intermediate stages exist.  A distinct sac-like crop is present in most seed-eating birds, as in the Gallinae, Columbae, Pteroclidae and in Opisthocomus.  This also occurs in  Thinocorys, Attagis, Psittaci, and, among the Passeres plus many of the Fringillidae and the Drepanididae.

The crop is less marked or only temporary in the Birds-of-Prey, the Cassowary, the Humming-birds, in Mormon, Pedionomus, and Panurus; and is represented by a slight but permanent dilatation in the Cormorant, various Ducks and Storks, and in the Flamingo. It is absent in all other birds. It reaches its highest development in the Pigeons, consisting of a right and a left globular half which are united by an unpaired portion; the inner walls possess numerous irregular ridges, and show during the breeding-season an extraordinary activity, the cells of the mucous membrane proliferating and peeling off as a cheesy matter, with which both sexes feed their nestlings for a considerable time.

The most peculiarly constructed crop is that of Opisthocomus whose oesophagus is much widened and forms a long doubled loop, which rests upon the great pectoral muscles, and almost suppresses the anterior part of the keel of the breastbone. The walls are extremely muscular, and are inside furnished with numerous furrows and ridges, to enable the HOACTZIN to squeeze out the juicy leaves of the tree, Arum arborescens, upon which it feeds.

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