_Insertion._—The cartilaginous prolongations of the turbinal bones and the inner wall of the false nostril. _Action._—To rotate the turbinal outward and dilate the nostril and the vestibule of the nasal cavity. _Structure._—Fleshy. A division into two or more parts may be recognized. A small part posteriorly is inserted into the cartilage of the dorsal turbinal bone, while the bulk of the muscle is inserted into the cartilage of the ventral turbinal bone. A few fibers also pass between the cornu of the alar cartilage and the outer wing of the nostril. _Relations._—Superficially, the levator labii superioris proprius, the levator nasolabialis, and the lateral nasal artery; deeply, the maxilla, the premaxilla, and the anterior nasal branch of the infraorbital nerve. _Blood-supply._—Facial artery. _Nerve-supply._—Facial nerve. MUSCLES OF THE EYELIDS 1. =Orbicularis oculi.=—This is a flat, elliptical, sphincter muscle, situated in and around the eyelids, the portion in the upper lid being much broader than that in the lower. The chief attachment is to the skin of the lids, but some bundles are attached to the palpebral ligament at the inner canthus and to the lacrimal bone. Its action is to close the lids. 2. =Corrugator supercilii.=—This is a very thin, small muscle, which arises over the root of the supraorbital process and spreads out in the upper eyelid, blending with the orbicularis. Its action is to assist in raising the upper lid or, especially in pathological conditions, to wrinkle the skin. 3. =Malaris.=—This is a very thin muscle, which varies much in different subjects. It extends from the fascia in front of the orbit to the lower lid. Its action is to depress the lower lid. The foregoing muscles receive their blood-supply from the facial, transverse facial, supraorbital, and infraorbital arteries; the nerve-supply is derived from the facial nerve. 4. =Levator palpebræ superioris.=—This slender, flat muscle is almost entirely within the orbital cavity. It arises on the pterygoid crest, passes forward above the rectus oculi superior and below the lacrimal gland, and terminates in a thin tendon in the upper lid. _Action._—To elevate the upper lid. _Blood-supply._—Ophthalmic artery. _Nerve-supply._—Oculomotor nerve. MUSCLES OF MASTICATION The muscles of this group are six in number in the horse. They arise chiefly from the upper jaw and the base of the cranium, and are all inserted into the mandible. 1. =Masseter.=—This muscle extends from the zygomatic arch and facial crest over the broad part of the mandibular ramus. It is semi-elliptical in outline. _Origin._—By a strong tendon from the zygomatic arch and the facial crest. _Insertion._—The outer surface of the broad part of the ramus of the mandible. _Action._—Its action is to bring the jaws together. Acting singly, it also carries the lower jaw toward the side of the contracting muscle.