2. =Dilatator naris lateralis= (M. caninus).—This thin, triangular muscle lies on the lateral nasal region, and passes between the two branches of the levator nasolabialis. _Origin._—The maxilla, close to the anterior extremity of the facial crest. _Insertion._—The outer wing of the nostril. [Illustration: FIG. 174.—NASAL AND SUPERIOR LABIAL MUSCLES OF HORSE. _a_, _a′_, Dilatator naris transversus; _b_, levator labii superioris proprius; _b′_, tendon of _b_; _b″_, common tendon of two levatores labii superioris proprii; _c_, _c′_, dilatator naris inferior; _d_, _e_, dilatator naris superior; _f_, orbicularis oris; _g_, levator nasolabialis, a portion of which is removed; _h_, dilatator naris lateralis (cut); _i_, cornu of alar cartilage; _k_, nostril; _k′_, false nostril; _l_, nasal diverticulum; _m_, nasal bone. (After Ellenberger-Baum, Top. Anat. d. Pferdes.) ] _Action._—To dilate the nostril. _Structure._—The muscle has a flat tendon of origin, passes between the two branches of the levator nasolabialis, and spreads out in the external wing of the nostril. The lower fibers blend with the orbicularis oris. _Relations._—Superficially, the skin, fascia, and the labial branch of the levator nasolabialis; deeply, the maxilla and the nasal branch of the levator nasolabialis. _Blood-supply._—Facial artery. _Nerve-supply._—Facial nerve. 3. =Dilatator naris transversus= (M. transversus nasi).—This is an unpaired, quadrilateral muscle, which lies between the nostrils. It consists of two layers. _Attachments._—Superficial layer, the superficial faces of the laminæ of the alar cartilages; deep layer, the convex edges of the cornua of the same. _Action._—To dilate the nostrils. _Structure._—It is composed of transverse fleshy fibers, which blend below with the orbicularis. _Relations._—Superficially, the skin, fascia, and tendinous expansion of the levator labii superioris proprius; deeply, the alar cartilages, the extremity of the septum nasi, and the palato-labial artery. _Blood-supply._—Palato-labial artery. _Nerve-supply._—Facial nerve. 4. =Dilatator naris superior= (Pars dorsalis m. lateralis nasi).—This very thin muscle occupies the angle between the nasal process of the premaxilla and the nasal bone. _Origin._—The lateral border of the nasal bone. _Insertion._—The inner wall of the false nostril and the parietal lamina of the septal cartilage. _Action._—To dilate the vestibule of the nasal cavity. _Structure._—Fleshy. _Relations._—Superficially, the skin, fascia, and levator labii superioris proprius; deeply, the parietal cartilage and false nostril. _Blood-supply._—Facial artery. _Nerve-supply._—Facial nerve. 5. =Dilatator naris inferior= (Pars ventralis m. lateralis nasi).—This is a similar but thicker muscle, which lies on the nasal process of the premaxilla. _Origin._—The maxilla and the nasal process of the premaxilla.