-Like _M. adductor mandibulae_,
this muscle performs only the one function of adducting the lower
mandible, and like _M. pseudotemporalis profundus_ it is a synergist of
that muscle.

~_M. adductor mandibulae posterior._~--Although this muscle undoubtedly
acts as an adductor of the lower mandible, I believe that, because of
its disadvantageous insertion so near the articulation, its main
function must be concerned with firming the mandible against the
quadrate. This is to say that its function is partially that of a
ligament.

~_M. protractor quadrati._~--When _M. protractor quadrati_ contracts,
the quadrate bone is swung medially. This action, as mentioned
previously, results in protraction of the upper jaw, and, as a
consequence, its action supplements the action of _M. protractor
pterygoidei_.

CRANIAL OSTEOLOGY

The ability of most birds to protract the upper mandible, and the
structure of the skull which enables them to do so are responsible for
common reference to the skull as "kinetic" (Beecher, 1951a:412; Fisher,
1955:175). The movement is effected by muscular action on a series of
movable bones that exert their forward force on the upper mandible,
which in turn swings on a horizontal hinge, the "naso-frontal hinge,"
at the base of the beak. The bone initiating the movement is the
quadrate, which is hinged posteriorly by its otic process and which
ordinarily swings up or down depending on the muscle or muscles being
contracted at any given moment. The upward swing of the quadrate pushes
the jugal bar, which is attached to its lateral tip, along its
longitudinal axis, in an anterodorsal direction, and the force is
transferred to the upper mandible, which is thereby elevated. A
synergetic mechanism is simultaneously initiated by the same bone--the
quadrate. Since the quadrate body articulates with the pterygoid, the
upward movement forces the pterygoid to slide along a ridge in the
ventral midline of the cranium, the sphenoidal rostrum, thus pushing
the palatine forward and exerting an upward push on the upper mandible.

In the columbids the quadrate has a bifurcated otic process that
functions as the hinge. The posterior tips of the forks are situated
almost vertically (one above the other) and the movement of the
quadrate is not so much up and down, or vertical, as it is horizontal
(fig. 12). When the quadrate moves medially the upper mandible is
protracted; a lateral movement results in retraction. The