Five mandibular muscles connect the beaks to each other and to the buccal mass sheath, a connective tissue sheet that encapsulates the muscular part of the buccal mass
Schematic diagram showing the beaks and musculature of Octopus bimaculoides. (A) The beaks and musculature of a complete buccal mass. (B) The upper beak and lower beak separated to show the underlying muscles. (C) The beaks and musculature separated to show the shape of the beaks and the shape of the individual muscles. AMM, anterior mandibular muscle (yellow); LMM, lateral mandibular muscle (purple);PMM, posterior mandibular muscle (blue); SMM, superior mandibular muscle (green) (source: 1)
These are the upper and lower beak. Both beaks are U-shaped in cross section and the anterior end is folded over itself to form the biting surfaces (i.e. the rostrum and the jaw angles). The upper beak has enlarged lateral walls that fit within, but do not contact the lower beak. The lower beak has enlarged wings and a reduced hood relative to the upper beak.
The upper beak is inverted relative to the lower beak. The rounded dorsal surface of the upper beak and the nalogous ventral surface of the lower beak are termed crests. The left and right sides of the beaks are referred to as the lateral walls. The fold that forms the rostrum and hood of the lower beak also has enlarged dorsal extensions, termed the left and right wings. The upper beak fits within the lower beak such that the lateral walls overlap, but they do not contact each other within the joint. Only the biting surfaces (i.e. the rostra and jaw angles)contact.
The fold that forms the rostrum and hood of the lower beak has enlarged dorsal extensions, termed the left and right wings.
The posterior mandibular muscle is the smallest and thinnest of the mandibular muscles. It is a thin sheet of muscle that originates on the posterior region of the lower beak crest and extends directly to an insertion on the lateral walls of the upper beak below its crest. As the posterior edges of the trough-like beaks are open, the posterior mandibular muscle, along with the overlying buccal mass sheath, forms the posterior wall of the buccal cavity and serves to contain and secure the buccal complex within the buccal cavity.
The lateral mandibular muscles are robust, cylindrical, and symmetrically paired muscles originating on a large area of the left and right lateral walls of the upper beak. The muscle extends laterally and has a somewhat smaller insertion on the buccal mass sheath. Three different orientations of muscle fibers are observed. The first group of fibers originates on the lateral walls of the upper beak and extends parallel to the long axis of the muscle to insert on the buccal mass sheath. The other two groups of muscle fibers are perpendicular to the orientation of the first as well as to each other, one group oriented dorsoventrally and the other antero-posteriorly.
The anterior mandibular muscle is relatively thin and originates on the anterior portion of the lower beak crest and overlying buccal mass sheath and follows the curve of the crest dorsally to insert on the lateral walls of the upper beak just below the level of the upper beak crest. Its muscle fibers follow a direct course from origin to insertion.
The superior mandibular muscle is a robust dorsal muscle with three divisions. This muscle originates along the crest of the upper beak and includes a central division and left and right divisions that extend anteroventrally to insert on the enlarged wings of the lower beak. These robust left and right divisions constitute the bulk of the superior muscle and include fibers oriented parallel to the line from origin to insertion.