• Insertion: Into the lips as it surrounds the mouth. Dog: Also into the nasal cartilage on the side of the nose.
• Action: Closes the mouth by pressing and tightening the lips. Dog: Fibers to the nasal cartilage pull the nose downward and enlarge the nostril opening.
• Structure: The orbicularis oris surrounds the mouth, lying in the upper and lower lips. It is continuous with the muscle of the other side (in both the upper and lower lips) in the horse; absent in the front of the upper lip in the ox, and separated slightly in the upper and lower lips in the dog. The uppermost fibers reach the nose in the dog. In the horse it consists of two parts—a wide portion surrounding the lips, and a narrow portion in the margin of the lips. This allows for greater control of lip movements in the horse. The orbicularis oris is well developed in the horse and ox, which use the lips for grazing, and less developed (less mobile) in the dog and feline.
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