Muscles of Mastication

1. Muscles of Mastication • The muscles that are responsible for mastication are in general strong and showed marked species-specific variations due to the different anatomy of the whole masticatory apparatus, including the skeletal components, the teeth and the temporo-mandibular joint.

2. The masseter muscle • The masseter muscle is a broad multipemnnate (bilateral symmetrical) muscle with mutipletendinous insertions. It originates from the ventral border of the zygomatic arch and the facial crest and inserts the lateral aspect of the mandble, extending from the facial notch to the temporomandibular joint.

3. Mandibular muscles of the dog

4. Mandibular muscles of the horse

7. Layers of masseter muscle • The masseter muscles of carnivores is seperated into three layers (superficial, middle, deep) by tendinous sheets (Fig. 2-4). • The superficial portion is strongest and originates from the rostral half of the zygomatic arch, passes over the ramus of mandible caudoventrally and partly inserts on the ventrolateral surface of the mandible.

8. Layers of masseter muscle • The rest of the muscle passes around the ventral border of the mandible and the angular process to insert on the ventromedial side, where it covers the digastric muscle. • The middle layer, the weakest part of the masseter muscle, originates from the ventral border of the zygomatic arch, medial to the superficial layer and inserts on the lateral surface of the mandible.

9. Layers of masseter muscle • It is not possible to isolate the rostral origin of the deep layer, since it is fused to the temporalismuscl; caudally it originates from the medial surface of the zygomatic arch. • In the pig the three layers are firmly fused and difficult to dissect. • In the ox, the tendinous intersections are pronounced forming five distinct parts.

10. The change of the fibre direction between each portion increases the masticatoryforceof the muscle. The superficial portion extends from the facial tubercurosity to the caudal border of the mandible. • The deep layer originates from the facial crest and zygomatic arch, passes caudoventrally and inserts on the lateral surface of the mandibularramus.

11. The masseter muscle of the horse shows up to fifteen tendinousintermuscular strands, which are originated sagitally and divide the muscle into multiple layers. • The superficial layers arise from the facial crest, pass caudoventrally and insert on the ventral and caudal borders of the mandible.

12. The deep layers originate from the zygomatic arch, pass over the ramus of mandible in a horizontal direction and unites with the superficial portions, with which they insert on the lateral surface of the ramus of mandible. • If the masseter muscles of both sides act together, they force the upper and lower jaw together; if acting singly, they move the mandible to the side of the contracting muscle, which is essential for the grinding process of herbivores.

13. The Pterygoid muscles • The pterygoid muscles pass on the palatine, pterygoid and sphenoid bones to the medial aspect of the mandible (Fig.2-5). • The lateral pterygoid muscle is the smaller of the two. It originates from the pterygoid process of the basisphenoid bone, passes caudoventrally and inserts on the medial surface of the ramus of mandible near the condylar process.

14. The much larger medial pterygoid muscle occupies a position of the medial surface of the mandible similar to that of the masseter laterally. • It extends from the basisphenoid and palatine bones to the ventral border of the mandible and the medial surface of the ramus of the mandible.

15. The Temporal Muscle • The temporal muscle occupies the temporal fossa, its size varying in the different species depending on the size of the fossa(Fig. 2-2and2-3). • It originates from the temporal crest which forms the edge of the temporal fossa, and from the temporal fascia. • From there it extends downward, covered by the auricular muscles, and inserts on the coronoid process of the mandible.

16. Temporal muscle • It is the strongest muscle of the head in carnivores. • The margins of its origin are the temporal line, the nuchal crest, the zygomatic process of the temporal bone and the medial surface of the temporal fossa(Fig. 2-4). • From its extensive origin the muscle bundles curve cranioventerally beneath the zygomatic arch and the orbital ligament and pass around the coronoid process of the madible, to which it inserts.

17. Temporal muscle • A tendinous branch fuses with the deep layer of the masseter muscle. • In dolichocephalic (having a relatively long skull) dogs, the temporal muscle meets the corresponding muscle of the opposite side in the midline and forms a midline sulcus(a groove or furrow) • In brachycephalic (having a relatively broad, short skull) dogs, the two muscle do not meet and therefore no sulcus is visible, except for a small indentation (a deep recess or notch on the edge or surface of something) between the interparietal bones in some breeds of dog.

18. Temporal muscle • While the temporal muscle is indistinct in ruminants, it is visible under the skin in the horse. • However, even in the horse, the temporal muscle is not well developedcompared to the other masticatory muscles. • It originates from the borders of the temporal fossa, the temporal line, external sagittal crest, the nuchal crest and the pterygoid crest and the surface of the temporal fossa which it occupies completely.

19. Temporal muscle • It partly fuses with the masseter and inserts to the coronoid process of the mandible. • It raises the mandible, acting together with the other masticatory muscles.

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