PDF (Lo-Res) - Smithsonian Institution Libraries

268 SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY
compression
FIGURE 1.—Lateral view depicting the combination of forces in the upper bill
of a bird (Struthio sp.) that produce resistance against the push from the mandible
when grasping food. Lateral view. F'=useful force applied to object,
R=rectractory (active muscular) force, N=push transferred to the braincase via
dorsal upper-bill stalk.
traction (backward shift) of the palate. Therefore, it is called
the retracting or retractory force. The ancestors of birds apparently
had no obvious source of such a force. Birds, however,
have evolved the following three sources of retracting force.
1. M. retractor palatini (Figure 2) is the ventromedial part of
the pterygoid muscle (of Moller, 1930, 1931), which is rather
large in tinamous (Figure 2 A,B; Dzerzhinsky, 1983; Elzanowski,
1987), ostriches (Figure 2 D,E), and especially in Apteryx
(Figure 2 C). In paleognaths (sensu Pycraft, 1900) this muscle
usually originates on the pterygoid and on the rear end of the
vomer. Its caudal attachment is not situated at the midline on
the base of the braincase, as in many neognaths (sensu Pycraft,
1900), but more laterally, near the caudal attachment of the occipito-mandibular
ligament, i.e., on the medial part of ala tym-
FlGURE 2.—Retractor palatini muscle in ventral view: A,B, tinamou, Eudromia elegans; c, Kiwi, Apteryx sp.
(scale=5 mm); D,E, Ostrich, Struthio camelus. aca=aponeurosis of insertion of M. pterygoideus caudalis;
ad'=aponeurose of insertion of M. depressor mandibulae that is related to M. retractor palatini; art=aponeurosis
of insertion of M. retractor palatini; Pt=pterygoid; Rtp=M. retractor palatini; Vbm=vomer (A,C,D, superficial
layer; B,E, deeper layers) (A,B, after Dzerzhinsky, 1983.)