For a general account of the digestive organs, see ALIMENTARY CANAL.
Here only a few peculiar features may be mentioned.
The young pigeons are fed by both parents with a peculiar stuff, the
product of the strongly proliferating epithelial cells of the crop,
which cells undergo a cheese-like fatty degeneration, and mixed with
mucus, perhaps also with the proventricular juice, make up a milk-like
fluid. Should the young die or be removed during this period, the
parents are liable to die, suffering severely from the turgid
congestion of the hypertrophied walls of the crop.
The male of the hornbills, _Bucerotinae_, feeds his mate, which is
imprisoned, or walled-up in a hollow tree, during the whole time of
incubation, by regorging his food. This bolus is surrounded, as by a
bag, by the cast-up lining of the gizzard. Since this process is
repeated for many days the habitual reaction of the stomach well-nigh
exhausts the male. A graphic account of this is given in Livingstone's
travels.
The hoactzin, _Opisthocomus_, feeds to a great extent upon the leaves
of the aroid _Montrichardia_ or _Caladium arborescens_. The crop is
modified into a large and very rugose triturating apparatus, while the
gizzard, thereby relieved of its function, is reduced to the utmost.
The large and heavy crop has caused a unique modification of the
sternal apparatus. The keel is pushed back to the distal third of the
sternum, whilst the original anterior margin of the keel is
correspondingly elongated, and the furcula fused with the rostral
portion.
In the ostrich, _Struthio_, the craze of overloading the stomach with
pebbles which, when triturated into sand, are not voided, has brought
about a dislocation, so that the enormously widened and stretched
space between proventriculus and gizzard forms a bag, directed
downwards, whilst the gizzard itself with part of the duodenum is
rotated round its axis to more than 100 deg.. A similar rotation and
dislocation occurs in various petrels, in correlation with the
indigestible sepia-bills, &c., which these birds swallow in great
quantities. In _Plotus_, the snakebird, the pyloric chamber of the
stomach is beset with a mass of hair-like stiff filaments which permit
nothing but fluid to pass into the duodenum. The gizzard of various
birds which are addicted to eating hairy caterpillars, e.g. _Cuculus
canorus_ and trogons, is often lined with the broken-off hairs of
these caterpillars, which, penetrating the cuticle, assume a regular
spiral arrangement, due to the rotatory motion of the muscles of the
gizzard.