Individual Muscles Attachments, Action, & Structure

The muscles of the head consist of the chewing muscles (temporalis, masseter, and digastric) and the facial muscles (zygomaticus, orbicularis oris, etc.). The chewing muscles are thick and volumetric, and they originate and insert on bone. They open and close the lower jaw, with the action taking place at the jaw joint (temporomandibular joint). The facial muscles are thin. They originate either from the skull or from the surface of other muscles, and they generally insert into other facial muscles or into the skin. When they contract, they move the features of the face (eyes, nose, mouth, ears). As they pull the facial features, they often gather the skin into folds and wrinkles that lie perpendicular to the direction of their muscular fibers (perpendicular to the direction of pull). The mouth region receives the most muscles; therefore, it is the most mobile part of the face. Some facial muscles are so thin that they do not create any direct form on the surface (caninus, malaris, orbicularis oculi), whereas other facial muscles or their tendons may create surface form directly (buccinator, levator labii maxillaris, zygomaticus, and depressor labii mandibularis). Facial muscles are generally more visible on the surface in the horse and the ox than in the dog and feline. The facial muscles, as they move the eyes, nose, mouth, and ears, generate whatever facial expressions animals are capable of producing. . . . • Attachment: A short ligament at the inner corner of the eye, whose inner end attaches to the skull. . . . . . . • Action: Eyelid portion: closes eyelids (blinking), primarily by depressing the upper eyelid. Outer portion: tightens and compresses the skin surrounding the eye, protecting the eyeball. . . . . . . • Structure: The orbicularis oculi is a flat, elliptical muscle consisting of two portions. The eyelid portion lies in the upper and lower eyelids, and the outer portion surrounds the eye and lies on the skull. . . .