~_M. pterygoideus ventralis._~--Contraction of this muscle retracts the
upper mandible by moving the palatine posteriorly, and simultaneously
adducts the lower mandible.
~_M. pterygoideus dorsalis._~--This muscle functions in essentially the
same manner as _M. pterygoideus ventralis_. The result of having a part
of its origin on the pterygoid as well as on the palatine is to
facilitate retraction of the upper mandible.
~_M. adductor mandibulae._~--This is the chief adductor of the lower
mandible and the muscle functions solely in that capacity. In birds
having great crushing ability, this muscle is much larger and more
powerful and the skull is reinforced behind the quadrate in order to
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._~--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_.