Sternum

MANDIBLE: osteology, myology.

Bone: mandible.


Features

• Also called mandibular, mandibular, jaw bone.
• Unpaired
• Bone of lower jaw.
• Bears lower teeth .
• Consist of U shaped body and two rami projects upward from the posterior part of the body.

Osteology

Body

• It has external and internal surface; upper and lower border.

External surface

• The anterior part of the external surface present a faint ridge (this indicate the position of foetal symphysis meni).
• Near the inferior border this ridge encloses a triangular prominence called mental protuberance.
• At the two lateral ends of the triangle are the mental tubercles.
• Mental foramen : lies on the either side of the mental tubercles , lies vertically below the second premolar tooth. Mental nerves and vessels emerge from this foramen.
• Faint external oblique line ascends posterosuperiorly from the mental tubercle and continues into the anterior border of the ramus.
• Incisive fossa: fossa on the surface below the incisor  teeth.

Internal surface

• Contain mylohyoid line that runs down anterior inferiorly from behind the third molar tooth to the mental symphysis.
• The line is sharp and clear posteriorly and full faint anteriorly.
• A shallow mandibular fossa seen above the line .
• A sublingual fossa seen above the line anteriorly.
• Mental spine:
• Genial tubercle:
• Genial foramen: found above mental spine.
• Mylohyoid groove: starts near the mandibular foramen on the internal aspect of the ramus, runs down below the mylohyoid line.
• The internal surface may sometimes exhibit a rounded prominence medial to the molar teeth above the mylohyoid line called torus mandibularis.

Upper border

• Bears 16 alveoli for the roots of the teeth.

Lower border

• Also called base.
• Digastric fossa present on either  side of the mandible in lower border.
• Behind the fossa the base is thick and shows a girth , anteroposterior convexity .
• As it approaches the ramus , this convexity turns into an external concavity giving the base a sinus shape.

Ramus

• Contain posterior border , anterior border  inferior border.
• Posterior border continues with the base of the body.
• The posterior and inferior border meet at the angle of the mandible.
• The anterior border is sharp and continues downward and forward on the lateral surface of the body as the oblique line.
• Upper part if the ramus contain two process coronoid and condylar process.

Coronoid process

• Lies anteriorly  at the upper part of the ramus.
• Flat
• Triangular in shape.

Condylar process

• Lies posteriorly at the upper part of the ramus.
• Separated from coronoid process by mandibular notch.
• Upper end expand to form head of the mandible.
• Head is elongated transversely and is convex both transversely and anteroposterior.
• It bears a smooth articular surface that articulates with mandibular fossa of the temporal bone to form the temporomandibular joint.

Neck

• Constricted part immediately below the head.
• Pterygoid fossa is the rough depression on the anterior surface of the neck.
• Mandibular foramen : located little above the center of the medial surface of the ramus extended to mandibular canal.
• The medial margin of the foramen formed by a projection called the lingula.
• Mylohyoid groove runs below mylohyoid line.

Myology

Muscular insertions

• Masseter :  inserted into the lateral surface to the ramus and of the angles.
• Temporalis: inserted into the medial surface of the coronoid process including it's apex and it's anterior and posterior border.
• Lateral pterygoid: inserted into the fovea on the anterior aspect of the neck.
• Medial pterygoid: inserted into the medial surface of the angle and the adjoining part of the ramus.

Muscular origins

• Buccinator: arises from the outer surface of the body just below the molar teeth.
• Anterior belly of digastric:: arises from the digastric fossa.
• Mylohyoid: arises from the mylohyoid line.

Other attachments

• Capsule of temporomandibular joint: attached around the margin of the articular surface.
• The mylohyoid nerve and vessels run forward in the mylohyoid groove.

Relations

• Facial artery:deep to the ramus near the ankle (origin)◆ deep to ramus ◆ lower border of the body◆ superficial to the body of the mandible.
• Lingual nerve: related to the medial aspect of the body of the mandible just above the posterior end of the mylohyoid line.
• Sublingual gland lies over sublingual fossa.
• Submandibular gland lies over submandibular fossa.
• Parotid gland related to the upper part of the posterior border of the ramus.