Morphological classification of the masticatory muscles in dogs based on their innervation

Positional relationships between the masticatory muscles and their innervating nerves with special reference to the lateral pterygoid and the midmedial and discotemporal muscle bundles of temporalis

“…Based on findings of the human fetus, MeridaVelasco et al (1993) and O $ g, u$ tcen-Toller & Juniper (1993) believed that the lateral pterygoid is divided into 2 or 3 main parts. In contrast, according to morphological studies including the present study, this muscle is not clearly divided into distinct parts, although it does show various patterns of 1 to 3 heads in the anterior half (Eisler, 1912 ;Troiano, 1967 ;Schumacher et al 1976 ;Naito, 1979 ;Terada & Sato, 1982 ;Naohara, 1989 ;Tomo, 1990 ;Abe, 1992 ;Birou et al 1992 ;Foucart et al 1998). The insertion area of the lateral pterygoid is broadly spread between the disc of the temporomandibular joint and the condyle of the mandible, and there is no clear division between the heads near their posterior halves.…”

“…However, the muscle is not clearly separated into 2 independent muscles based on innervation findings near its insertion (Eisler, 1912 ;Sicher & DuBrul, 1970 ;Schumacher et al 1976 ;Naito, 1979 ;Terada & Sato, 1982 ;Tomo, 1990). The muscle frequently shows not only a 2-head pattern but also the 1-head or a 3-head pattern (Troiano, 1967 ;Naohara, 1989 ;Abe, 1992 ;Birou et al 1992).…”

“…Various muscle bundles are observed in the main part of the muscle. The anteromedial bundle which originates from the infratemporal crest of the sphenoid bone has been given various names : the deep sphenoid part of temporalis (Eisler, 1912), the medial portion of temporalis (Zenker, 1954), the second layer of zygomaticomandibularis (Yoshikawa & Suzuki, 1962), the accessory head of temporalis (Tomo, 1990) and sphenomandibularis (Dunn et al 1996). The anterolateral bundle of temporalis which is located on the lateral surface of the aponeurosis of the main part of the muscle has also been variously named : the sphenofrontal part of temporalis (Eisler, 1912), the first layer of zygomaticomandibularis (Yoshikawa & Suzuki, 1962) and the pre-anterior belly of temporalis (Shankland et al 1996).…”

“…However, other studies have classified the muscles into lateral and medial groups according to their positional relationship with the main trunk of the mandibular nerve, i.e. the masseter, temporalis and lateral pterygoid laterally and the medial pterygoid medially (Gegenbaur, 1903 ;Edgeworth, 1914 ;Lubosch, 1938 ;Terada & Sato, 1982 ;Tomo, 1990 ;Tomo et al 1993). From comparative embryological studies, Edgeworth (1914) reported that the common anlage of the masticatory muscles is first divided into medial and lateral parts ; the medial part of the anlage becomes the medial pterygoid.…”

The Laminar Structure of the Common Opossum Masseter (Didelphis marsupialis)

“…However, the reverse laminar theory is not an accepted theory at present. Later, Tomo (1990), Watanabe (1997) and Nakajima et al (2000) attempted to separate the masseter into the layers from the innervations. However, no new theory superior to the laminar structure theory has as yet been reported.…”