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.