Origin from the alveolar border of the mandible, between the molar tooth and the canine. The fibres pass dorsad into the lower lip, where they intermingle with those of the orbicularis oris. The muscle is perhaps not constant.
Action.—Depressor of the lower lip.
Sometimes a few fibres arising from the cranial portion of the alveolar border of the mandibula pass mediad, seeming to join corresponding fibres from the opposite side. Such fibres would constitute a M. transversus menti.
B. Deep Muscles of the Head.
(a) Muscles of Mastication.
M. digastricus ([Fig. 65], b).—The digastric is a thick prismatic muscle lying mediad of the angle of the lower jaw, connecting it with the base of the skull.
Origin by fleshy fibres from the outer surface of the jugular process of the occipital bone, and by a thin tendon from the tip of the mastoid process and from the ridge between the mastoid and the jugular processes. The muscle passes craniad, becoming at the same time broader and more voluminous.
Insertion.—The ventral border of the mandible craniad of the caudal border of the molar tooth. The insertion area extends slightly onto both surfaces of the bone and is triangular, with its apex craniad.
Relations.—Outer (ventral) surface with the following, beginning at the caudal end: the rectus capitis lateralis, the cleidomastoid (h), the submaxillary gland (2), the stylohyoid (d), a large lymph-gland (3), and the integument. Inner (dorsal) surface with the jugulohyoid, the tympanic bulla, the masseter (a), the hyoglossus, and the mylohyoid (c).
Action.—Depressor of the lower jaw.