Muscles of the oral diaphragm

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The oral diaphragm (also called the diaphragma oris) is the muscular floor of the oral cavity formed by two principal muscles: the mylohyoid and the geniohyoid. Additional muscles associated with the floor (the suprahyoid group) are described below.

Primary Muscles of the Oral Diaphragm

1. Mylohyoid (the true "diaphragm")

The paired mylohyoid muscles are the structural foundation of the oral diaphragm. Each is a flat, triangular muscle that together form a muscular sheet across the floor of the mouth — the diaphragma oris.

Mylohyoid and Geniohyoid muscles – inferior and lateral views

Fig. 8.255 – (A) Mylohyoid muscles from below, (B) with Geniohyoid, (C) lateral view. — Gray's Anatomy for Students

Feature	Detail
Origin	Mylohyoid line on the medial surface of the body of the mandible
Insertion	Median fibrous raphe (midline) running from mandibular symphysis to hyoid bone; posterior fibers attach directly to the body of the hyoid
Innervation	Nerve to mylohyoid — a branch of the inferior alveolar nerve (from mandibular nerve, V₃)
Actions	(1) Supports and elevates floor of oral cavity; (2) elevates and pulls hyoid forward during swallowing; (3) depresses mandible when hyoid is fixed

The posterior margin of each mylohyoid is free, and this free border — together with the superior and middle pharyngeal constrictors — forms the triangular oropharyngeal aperture, the major gateway through which vessels, nerves, and muscles (e.g., lingual artery, hypoglossal nerve, hyoglossus) pass between the floor of the mouth and the neck.

Gateway into the floor of the oral cavity — the oropharyngeal triangle

Fig. 8.256 — Gateway into the Floor of the Oral Cavity. — Gray's Anatomy for Students

2. Geniohyoid

The geniohyoid muscles lie immediately superior to the mylohyoid (on its upper surface), running as paired cord-like muscles on either side of the midline.

Feature	Detail
Origin	Inferior mental spines (genial tubercles) on the posterior surface of the mandibular symphysis
Insertion	Anterior surface of the body of the hyoid bone
Innervation	C1 fibers that hitchhike along the hypoglossal nerve (CN XII)
Actions	(1) Pulls hyoid forward and upward during swallowing; (2) depresses mandible when hyoid is fixed

Associated Suprahyoid Muscles

All muscles above the hyoid bone contribute to the functional oral diaphragm complex. The complete suprahyoid group includes:

Muscle	Origin	Insertion	Nerve	Action
Mylohyoid	Mylohyoid line of mandible	Median raphe + hyoid	Nerve to mylohyoid (V₃)	Forms floor; elevates hyoid; depresses mandible
Geniohyoid	Inferior mental spines of mandible	Body of hyoid	C1 via CN XII	Pulls hyoid forward; depresses mandible
Digastric (anterior belly)	Digastric fossa of mandible	Intermediate tendon at hyoid	Nerve to mylohyoid (V₃)	Depresses mandible; elevates hyoid
Digastric (posterior belly)	Mastoid notch of temporal bone	Intermediate tendon at hyoid	Facial nerve (CN VII)	Elevates and retracts hyoid
Stylohyoid	Styloid process	Body/greater horn of hyoid	Facial nerve (CN VII)	Elevates and retracts hyoid
Mylohyoid (as above)	—	—	—	—

Key Functional Points

The mylohyoid is the structural diaphragm; it divides the floor into a sublingual space above and a submental/submandibular space below.
During swallowing, the mylohyoid and geniohyoid elevate and advance the hyoid, raising the larynx and initiating the pharyngeal phase.
The submandibular gland is "hooked" around the free posterior margin of the mylohyoid — its deep part lies above it (sublingual space) and its superficial part below (submandibular space).
Ludwig's angina (oral cellulitis) is a life-threatening infection that spreads through these spaces, exploiting the anatomical boundaries defined by the mylohyoid diaphragm.

Sources: Gray's Anatomy for Students, p. 1232–1234; Color Atlas of Human Anatomy Vol. 2, p. 258–259

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