Floor of Mouth
The term floor of the mouth is used differently by different authors, but in all cases it is applied to the floor of the oral cavity proper and does not include the vestibule. It is sometimes used to mean the structures that actually serve as boundaries of the cavity inferiorly. In this sense, the structures that form it would be the superior and lateral surfaces of the anterior part of the tongue and the mucous membrane that is reflected from the side of the tongue to the inner aspect of the mandible. Other authors have used the term to mean the muscular and other structures that fill the interval bounded by the mandible and the hyoid bone. This would mean primarily the mylohyoid muscle, which is then thought of as the boundary between the mouth above and the submandibular triangle of the neck below the muscle.
The right and left mylohyoid muscles form a diaphragm that is stretched between the two mylohyoid lines of the mandible and the body of the hyoid bone. The posterior fibers of each muscle insert on the body of the hyoid bone, and from there forward to the symphysis of the mandible the right and left muscles meet each other in a midline raphe. The mylohyoid muscle is supplied by the mylohyoid nerve, which is a branch of the inferior alveolar nerve, which itself is a branch of the mandibular division of the trigeminal nerve.
Slightly off of the midline, the anterior belly of the digastric muscle lies along the inferior surface of the mylohyoid muscle. Anteriorly it attaches to the digastric fossa of the mandible, and posteriorly it ends in the intermediate tendon, by means of which it is continuous with the posterior belly of the digastric muscle, which attaches to the mastoid notch of the temporal bone. The intermediate tendon is anchored to the hyoid bone by a fascial loop. The anterior belly is also supplied by the mylohyoid nerve and the posterior belly by a branch from the facial nerve.
Closely related to the posterior belly of the digastric muscle, the stylohyoid muscle extends from near the root of the styloid process to the greater horn of the hyoid bone. It usually attaches to the hyoid by two slips, between which the posterior belly and intermediate tendon of the digastric muscle pass. The stylohyoid is supplied by a branch of the facial nerve.
The right and left geniohyoid muscles, one on each side of the midline, rest on the superior surface of the mylohyoid muscle. They are attached anteriorly to the mental spines and posteriorly to the body of the hyoid bone. The geniohyoid muscle is supplied by fibers from the first cervical nerve that accompanies the hypoglossal nerve.
With the foregoing description of the related muscles in mind, the hyoid bone can be thought of as held in a muscular sling hung between the mandible and the stylomastoid region of the temporal bone, thus making the floor of the mouth quite mobile. All of these muscles can help in the elevation of the hyoid bone and the floor of the mouth. The geniohyoid and stylohyoid muscles determine the anteroposterior position of the hyoid bone, lengthening and shortening the floor of the mouth. The infrahyoid (strap) muscles (omohyoid, sternohyoid, sternothyroid, and thyrohyoid) pull the hyoid bone and floor of the mouth inferiorly.
A usage of the term floor of the mouth which is less technical than the two previously given is to think of the structure as the mucous membrane that is reflected from the side of the tongue to the mandible. The attachment of the mucous membrane of this area to the mandible, where it is continuous with the gum, is along a line drawn from the posterior end of the mylohyoid line to a point just above the mental spine.