In Latin a bird's bill or beak is known as the
Rostrum. This consists of an upper, chiefly premaxillary
and maxillary, and of a lower, or mandibular, half. The horny covering
is to a certain extent moulded after the shape of the supporting bones.
The soft cutaneous portion of the skin is frequently restricted to a thin
layer between the periosteum and the Malpighian layer of the epiderm; in
it run numerous blood- vessels and nerves, the latter occasionally penetrating
the horny layer, and ending in tactile or sensory corpuscles.
On the other hand, in very stout beaks, the cutaneous
layer forms conical elongations which project into the thick horny parts,
especially into the ends of the upper and lower bill. In the broad edge
of the mandible of Parrots such projections are particularly numerous and
long; when they calcify, as cutaneous structures are liable to do, they
bear in horizontal sections a superficial resemblance to the germs of teeth,
and have been mistaken as such by various anatomists (see TEETH).
The horny sheath, or rhamphotheca, is produced
by the outer layers of the Malpighian cells, and resembles in structure
other horny parts, as claws, nails, and spurs. The horny sheath of the
bill sometimes consists of a number of pieces more or less separate. In
the Ostriches and Tinamous there is a lateral pair and an unpaired piece
for each jaw; in the Tubinares on the upper jaw at least one pair of lateral
or maxillary pieces, an unpaired piece which covers the culmen and is continued
into the prolonged nasal tubes, and an apical hook, strongly curved and
pointed: each half of the under jaw is covered by one ventral, one dorsal,
and one terminal piece, the latter partly fusing with that of the other
side into a strong scoop.
Indications of such a compound rhamphotheca are,
however, found in other birds, especially in the Steganopodes, in some
Herons, like Nycticorax and Scopus, and in Penguins; the culminar or dorsal
unpaired piece being more or less separated from the lateral pieces. In
the majority of birds the horny covering forms one coherent sheath.
Sometimes, as in the Anseres,
the greater portion of the outer sheath of the bill is soft, and only the
tip of the bill is transformed into a thick horny "neb," which contains
numerous tactile organs.