• Origin: Bones of the side of the face (in front of the eye in the horse; lower, at the end of the facial crest in the ox; above the large molar in the dog).
• Insertion: Horse and ox: Skin on the front of the upper lip, by common tendon with the same muscle of the other side. Dog: Front end of the upper lip and the side of the nostril.
• Action: Horse and ox: Muscles of both sides: Lift the front of the upper lip; by continued action, evert the lips, exposing the front teeth. One side only: Lifts and pulls the upper lip slightly to that side. Dog: Lifts the upper lip and widens the nostril opening.
• Structure: In the horse, the levator labii maxillaris is a long, teardrop-shaped muscle. It begins wide and thin, then narrows and thickens, develops a round tendon, meets the tendon of the same muscle of the other side, expands into a wide tendinous sheet, and finally inserts into the skin of the upper lip on the front of the snout. The belly and the tendon can be seen on the surface and are directed upward, inward, and forward. In the ox, it is a flattened muscle that passes between the two divisions of the levator nasolabialis and develops several tendons. It lies lower on the face than in the horse but still passes inward, upward, and forward over the nose to meet the tendons of the other side before expanding into the wide central tendon and inserting. In the dog, the levator labii maxillaris lies deep to the levator nasolabialis. In the feline, descriptions of this muscle vary among authors:it is described either as lying parallel and above the caninus, or as the rear portion of the nasolabialis, as described in this text.
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