Musculus pterygoideus (Baumel, 1993)

Musculus pterygoideus

The M. pterygoideus may exhibit various divisions depending on species ( Baumel, 1993). The large M. pterygoideus has different fibre arrangements and a clear cleft in both quail and zebra finch, which may divide the muscle into various portions. The cleft is previously observed in development of this muscle in quail; however, whether the cleft corresponds to specific divisions of this muscle is not described ( McClearn & Noden, 1988). We describe and discuss the M. pterygoideus in both quail and zebra finch as a single large muscle in relation to pneumatic sinus patterning, with the exception of its relationship to the suborbital diverticulum, antorbital sinus and antorbital cavity.

Quail: The M. pterygoideus originates from the ventral and lateral surface of the pars lateralis and lateral, ventral and medial surfaces of the pterygoid. The muscle inserts on the mandible across its ventromedial, medial and dorsomedial surfaces over the medial mandibular process, and a small portion over the medial surface of the mandible immediately anterior to the process ( Figs 10A–C View Figure 10 , 11A–C View Figure 11 ). Two portions of the M. pterygoideus are associated with the paranasal sinus system. From the origination of the anterior portion of the M. pterygoideus ( M. pterygoideus ventralis) that is posteroventral to the ostium of the suborbital diverticulum, the muscle is adjacent to the suborbital diverticulum and does not reside within the antorbital cavity ( Fig. 3F View Figure 3 ). The posterior portion of the suborbital diverticulum is immediately dorsal to the M. pterygoideus dorsalis medialis, which in turn occupies the region dorsal to the M. pterygoideus ventralis and lateral to the pterygoid (see suborbital diverticulum section).

Zebra finch: The M. pterygoideus originates from the entire dorsal surface of the palatine (with the exception of the lamella dorsalis), ventral surfaces of the pars lateralis (fossa ventralis) and angulus posterolateralis for small portions, ventral surface of the maxillary process, and the medial, ventral and lateral surfaces of the pterygoid ( Figs 10E–G View Figure 10 , 11E–G View Figure 11 ). Unlike quail, a distinct additional insertion is present on the braincase ( M. pterygoideus retractor) in the zebra finch. This muscle originates from the posterior portion of the pes pterygoid and lateral surface of the parasphenoid, and inserts on the anteroventral surface of the braincase. Distally, the M. pterygoideus complex, except for the retractor inserting on the braincase, inserts on the medial mandibular process across its ventromedial, medial and dorsomedial surfaces, and on the medial surface of the mandible immediately anterior to the process. Four portions of the M. pterygoideus complex are associated with the paranasal sinus system. The association of the M. pterygoideus complex with the antorbital cavity in the zebra finch differs from quail, which has no muscle associated with the cavity. In the zebra finch, due to the origination of the anterior portion of the M. pterygoideus complex ( M. pterygoideus dorsalis lateralis) covering the majority of the dorsal surface of the pars lateralis immediately dorsal to approximately midlength of the antorbital sinus, the M. pterygoideus dorsalis lateralis resides within the ventral portion of the antorbital cavity. The M. pterygoideus dorsalis lateralis travels further posteriorly by exiting the antorbital cavity and passes immediately ventral to the suborbital diverticulum ( Fig. 3O–Q View Figure 3 ). Posteriorly, the suborbital diverticulum is associated with the other three portions of M. pterygoideus complex ( M. pterygoideus dorsalis medialis, M. pterygoideus dorsalis internus and M. pterygoideus retractor), M. pseudotemporalis profundus and M. adductor mandibulae externus (see suborbital diverticulum section, Fig. 3Q View Figure 3 ).

In summary, the two soft tissue components (nasal gland duct and the neurovasculature) occupy the dorsal portion of the antorbital cavity in quail and zebra finches, whereas the nasolacrimal duct occupies the dorsal margin of the antorbital fenestra in both taxa. The M. pterygoideus dorsalis lateralis in the zebra finch resides in the ventral portion of the antorbital cavity but no M. pterygoideus is associated with the cavity in quail. The topographical positions of those soft tissues relative to the antorbital cavity and fenestra, and paranasal sinus are consistent with previous studies ( Witmer, 1990, 1995, 1997a; Holliday & Witmer, 2007; Witmer & Ridgely, 2008; and references therein for detailed discussion). The dissociation between the M. pterygoideus and antorbital cavity in quail is not an exception, although the frequency of this dissociation among birds is not well known (e.g. Witmer, 1995).

PARATYMPANIC SINUS

The paratympanic sinus is derived from the Eustachian tube in extant birds. This sinus expands proximally as the tympanic sinus bounded by the inner ear medially and tympanic membrane laterally. The tympanic sinus gives rise to the two pneumatic systems, composed of the periotic (anterior, posterior and dorsal tympanic diverticula) and mandibular arch (quadrate and siphoneal diverticula) pneumatic systems. The periotic pneumatic system, surrounding the braincase and inner ear, is highly elaborated and similar between quail and zebra finch, as in many other birds ( Fig. 9 View Figure 9 ). Likewise, distal extensions into the quadrate and mandible, as the mandibular arch pneumatic system, are also similar between quail and zebra finch, as in many other birds ( Fig. 9 View Figure 9 ). We streamline the presentation of these pneumatic regions below and emphasize the major differences we did find – the proximal extension of the siphoneal diverticulum and siphonium in these taxa and the presence of a marginal sinus only in the zebra finch.