o
withstand the pressure of the lower mandible against the quadrate
during adduction (Sims, 1955:374 and Bowman, 1961:219-222).

~_M. pseudotemporalis profundus._~--With origin and insertion on highly
movable bones, this muscle, when it contracts, retracts the upper
mandible and adducts the lower mandible. Like the pterygoid muscles,
this muscle, by itself, would allow the bird to grasp objects by means
of its mandibles. However, _M. pseudotemporalis profundus_ could
produce a more powerful grip because it takes origin farther anteriorly
on the lower mandible.

~_M. protractor pterygoidei._~--Contraction of _M. protractor
pterygoidei_ pulls the pterygoid anteromedially and causes it to slide
forward along the sphenoidal rostrum. This action aids in protraction
of the upper mandible.

~_M. depressor mandibulae._~--The depressor of the lower mandible is
the sole muscle other than _M. geniohyoideus_ involved in the function
of abducting the lower jaw of doves. Its size can be correlated
especially well with feeding habits of the bird. Other birds that force
their closed mandibles into fruit, wood or the earth and then forcibly
open them, belong to groups possessing enlarged depressors. Contraction
of the muscle pulls the postarticular (retroarticular) process upward
with the resultant downward movement of that part of the mandible which
is anterior to the articulation. Since there is no "gaping" in doves
the muscle is only large enough to overcome the inherent tone of the
relaxed adductor muscles.

In some non-passerine species as well as in certain passerines the
muscle also serves to raise the upper jaw by acting on the quadrate,
which is capable of rotating vertically on its otic process. Especially
in the gapers, where resistance is offered near the tip of the lower
mandible, contraction of the muscle pulls the entire mandible dorsad
thus forcing the jugal and palatal struts forward (Zusi, 1959:537-539).
The action supplements that of _Mm. protractor pterygoidei et quadrati_
and is enhanced by anterior migration of the origin of _M. depressor
mandibulae_.

There is no lifting action involved in contraction of the depressor
muscle in doves for two reasons--(A) the origin of the muscle is
situated much too far posteriorly on the skull, and, more important,
(B) the quadrate is not hinged for vertical movement. As will be
discussed later, it moves only in a horizontal plane.

~_M. pseudotemporalis superficialis._~-