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Defining the Boundaries: The Lay of the Land
To appreciate the structures within the floor of the mouth, it’s helpful to first visualize its boundaries. The entire region is somewhat horseshoe-shaped, mirroring the curvature of the mandible, or lower jaw bone, which forms its lateral and anterior confines. Looking from within the mouth, the superior boundary is naturally the undersurface of the mobile part of the tongue. Inferiorly, the muscular sheet known as the mylohyoid muscle forms the primary support, creating a dynamic diaphragm that separates the oral cavity proper from the deeper structures of the neck, specifically the submental and submandibular triangles. Posteriorly, the floor of the mouth transitions towards the base of the tongue and the oropharynx. The entire surface visible within the oral cavity is lined by a delicate mucous membrane, which plays its own role in the region’s function.The Muscular Foundation: Support and Movement
The structural integrity and dynamism of the floor of the mouth are largely attributable to a pair of key muscles that form a sort of hammock or sling. These muscles are not only pivotal for supporting the overlying structures but also participate actively in complex movements like swallowing and depressing the mandible.The Mylohyoid Muscle: The Oral Diaphragm
The mylohyoid muscle is the star player in forming the floor. It is a flat, trough-like or paired triangular muscle that arises from the entire length of the mylohyoid line, an oblique ridge on the inner surface of each side of the mandible, extending from the mandibular symphysis (the midline fusion point of the jaw) anteriorly to the last molar tooth posteriorly. From this broad origin, its fibers course inferomedially – downwards and towards the midline. The posterior fibers insert into the anterior surface of the body of the hyoid bone, a U-shaped bone located in the anterior neck, below the mandible and above the larynx. The majority of the anterior and middle fibers from each side, however, interdigitate and fuse in the midline with their counterparts from the opposite side, forming a median fibrous raphe that extends from the mandibular symphysis to the hyoid bone. This arrangement creates a strong, yet flexible, muscular sheet that effectively constitutes the true floor of the oral cavity. When the mylohyoid muscles contract, they can elevate the hyoid bone and, with it, the tongue and the floor of the mouth, which is crucial during the initial phases of swallowing. If the hyoid bone is fixed by other muscles, the mylohyoid can assist in depressing the mandible, helping to open the mouth.The Geniohyoid Muscle: A Superior Assistant
Located superior to the mylohyoid, and running parallel to the anterior belly of the digastric muscle (though the digastric is more of a suprahyoid neck muscle), is the geniohyoid muscle. This is a narrower, shorter, paired muscle that sits closer to the midline. Each geniohyoid muscle originates from the inferior mental spine (or genial tubercle) on the posterior aspect of the mandibular symphysis. From this pointed origin, its fibers run posteriorly and slightly inferiorly to insert onto the anterior surface of the body of the hyoid bone. The geniohyoid muscles lie directly on top of the mylohyoid. Their action is to pull the hyoid bone anterosuperiorly – forward and upward – which consequently shortens the floor of the mouth and widens the pharynx during swallowing. Similar to the mylohyoid, if the hyoid is stabilized, the geniohyoid can also assist in depressing the mandible and retracting it.Glandular Inhabitants: The Saliva Production Crew
The floor of the mouth is also a critical site for saliva production and delivery, thanks to the presence of major salivary glands and their ductal systems. Saliva is indispensable for lubricating food, initiating digestion, and maintaining oral hygiene.The Sublingual Glands
The sublingual glands are the smallest of the three major paired salivary glands (the others being the parotid and submandibular glands). As their name suggests, they are located just beneath the mucous membrane of the floor of the mouth, superior to the mylohyoid muscle and lateral to the genioglossus muscle and the submandibular duct. Each almond-shaped gland is not a single discrete unit but rather an aggregation of smaller glandular lobules. Unlike the other major salivary glands that have one large duct, the sublingual gland typically drains its predominantly mucous secretions via a series of 8 to 20 small ducts, known as the ducts of Rivinus. These tiny ducts open individually along the crest of the sublingual fold (plica sublingualis), a raised fold of mucosa on either side of the lingual frenulum. Sometimes, a few of these anterior ducts may coalesce to form a slightly larger duct, termed Bartholin’s duct, which might join the submandibular duct or open independently near its orifice at the sublingual caruncle.The Submandibular Gland Duct (Wharton’s Duct)
While the bulk of the submandibular gland itself lies inferior and posterior to the mylohyoid muscle in the submandibular triangle of the neck, its duct, known as Wharton’s duct, takes a significant journey through the floor of the mouth to deliver saliva. This relatively long duct, about 5 cm in length, arises from the deep part of the submandibular gland, which hooks around the posterior free edge of the mylohyoid muscle. From there, Wharton’s duct courses anteriorly and superiorly, running in the space between the mylohyoid muscle inferiorly and the hyoglossus and genioglossus muscles medially. It travels just deep to the sublingual gland and is famously crossed twice by the lingual nerve. Finally, Wharton’s duct opens into the oral cavity via a small, papilla-like opening called the sublingual caruncle (or sublingual papilla), which is a visible elevation located on either side of the base of the lingual frenulum in the anterior part of the floor of the mouth. It primarily delivers mixed serous and mucous saliva.Navigating the Network: Nerves and Blood Vessels
A complex network of nerves and blood vessels traverses the sublingual region, providing sensation, motor control, and vital nourishment to its various components.The Lingual Nerve: Sensation and More
The lingual nerve, a major branch of the mandibular division of the trigeminal nerve (CN V3), is a key sensory nerve for the anterior two-thirds of the tongue, the floor of the mouth, and the lingual gingiva (gums). It enters the floor of the mouth from a posterolateral position, passing between the medial pterygoid muscle and the ramus of the mandible. As it courses anteriorly and medially, it initially runs lateral and superior to the submandibular duct. It then famously loops beneath Wharton’s duct, crossing it inferiorly, and then ascends on its medial side as it continues towards the tip of the tongue. The lingual nerve carries not only general somatic afferent fibers for touch, pain, and temperature but also special visceral afferent fibers for taste from the anterior two-thirds of the tongue (fibers it acquired from the chorda tympani, a branch of the facial nerve). Furthermore, it conveys preganglionic parasympathetic fibers (also from the chorda tympani) to the submandibular ganglion, from which postganglionic fibers arise to innervate the submandibular and sublingual salivary glands, stimulating saliva secretion.The Hypoglossal Nerve (Cranial Nerve XII)
The hypoglossal nerve (CN XII) is the primary motor nerve to the intrinsic and extrinsic muscles of the tongue (except for the palatoglossus, which is supplied by the vagus nerve). While its main target is the tongue musculature, it passes in close proximity to the structures of the floor of the mouth. After exiting the skull, it arches anteriorly, deep to the posterior belly of the digastric and stylohyoid muscles, and enters the submandibular region. It then runs superior to the hyoid bone, superficial to the hyoglossus muscle, and deep to the mylohyoid muscle, accompanying Wharton’s duct and the lingual nerve for part of its course before branching extensively to supply the tongue muscles. Its integrity is crucial for all tongue movements involved in speech, chewing, and swallowing.Blood Supply and Drainage
The principal arterial supply to the floor of the mouth comes from the sublingual artery, which is a branch of the lingual artery (itself a branch of the external carotid artery). The lingual artery arises deep in the neck and passes deep to the hyoglossus muscle. The sublingual artery specifically arises at the anterior border of the hyoglossus muscle and runs forward between the genioglossus muscle and the mylohyoid muscle, supplying the sublingual gland, the mylohyoid muscle, and the mucous membrane of the floor of the mouth and lingual gingiva. Smaller branches from the facial artery (like the submental artery) may also contribute. Venous drainage largely mirrors the arterial supply, with the sublingual vein accompanying the sublingual artery and typically draining into the lingual vein, which in turn empties into the internal jugular vein. There are often connections with the facial vein as well.The Surface Lining: Mucosa and Special Features
The innermost lining of the floor of the mouth, the part directly in contact with the oral environment, is a specialized mucous membrane.Sublingual Mucosa Characteristics
The mucosa covering the floor of the mouth is notably thin, smooth, and highly vascularized. This high degree of vascularity, visible as a rich network of underlying blood vessels, makes the sublingual area an effective site for rapid absorption of certain lipid-soluble medications directly into the systemic circulation, bypassing the first-pass metabolism in the liver. This route is often exploited for drugs like nitroglycerin. The mucosa is non-keratinized stratified squamous epithelium, suited for an environment that is constantly moist and subject to movement.Key Surface Landmarks
Several distinct features are visible on the surface of the floor of the mouth:- Lingual Frenulum (Frenulum Linguae): This is a prominent midline fold of mucous membrane that connects the undersurface of the anterior part of the tongue to the floor of the mouth. It can vary in length and thickness, and if unusually short or tight (ankyloglossia or “tongue-tie”), it can restrict tongue movement and potentially affect speech or feeding.
- Sublingual Folds (Plicae Sublinguales): On either side of the lingual frenulum, extending posterolaterally, are elevated crests or folds of mucosa. These are the sublingual folds, and they are raised by the underlying superior aspect of the sublingual glands and the submandibular ducts. The multiple small ducts of the sublingual gland (ducts of Rivinus) open along the summit of these folds.
- Sublingual Caruncles (Sublingual Papillae): Situated at the anterior end of each sublingual fold, on either side of the base of the lingual frenulum, are small, fleshy prominences called the sublingual caruncles. These are important landmarks as they mark the openings of the submandibular gland ducts (Wharton’s ducts) into the oral cavity.
The floor of the mouth, despite its relatively concealed position, is a highly organized and dynamic anatomical region. Its intricate arrangement of muscles provides essential support and mobility, while its glands ensure adequate lubrication and initial digestive processes. The rich neurovascular network not only sustains these structures but also contributes to complex sensory and motor functions vital for everyday life.In essence, the sublingual area is far more than just the space beneath the tongue. It is a carefully constructed compartment where muscles provide a dynamic stage, glands offer vital secretions, and nerves and vessels ensure everything functions smoothly and stays nourished. Its unique mucosal properties further add to its significance, highlighting how even seemingly simple anatomical areas play multifaceted roles in the overall economy of the human body. A deeper appreciation of this region underscores the elegance and efficiency of biological design.
