ACTION OF JAW MUSCLES
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.