Dilator naris apicalis muscle

The dilator naris apicalis muscle corresponds to the alar part of the nasal muscle in humans. In domestic mammals, it is only clearly discernible in ungulates.

Conformation: Due to the transverse widening of the tip of the nose and the orientation of the nostrils, it is located dorsally. It thus joins its counterpart on the opposite side along the median plane, forming a single muscle with transverse fibers. A strong median raphe marks this dual origin in cattle. The fleshy bundles start from each side, within the thickness of the muzzle, and spread into the medial perimeter of the nostrils. In equids, the muscle is fully unified, thick, and composed of short bundles extending from the alar cartilage of one nostril to that of the opposite nostril. In pigs, it cannot be isolated, but it can be considered, along with the lateral nasal muscle, to form the fleshy relief surrounding the snout. In carnivores, it is generally absent.

Insertions: This muscle inserts on the dorsal surface of the alar cartilages, to a greater or lesser extent depending on the species. Its insertions are particularly broad in equids, where they extend to the horn of each cartilage. Some bundles also insert on the tips of the nasal bones.

Relations: Partly covered in ungulates by the tendon of the upper lip elevator muscles, this muscle adheres to the skin on the rest of its surface, except in cattle, where a thick layer of muzzle glands separates it from the skin. It shares bundles with the lateral nasal muscle and with the orbicularis oris muscle. It covers much of the nasal cartilages.

Functions: The dilator naris apicalis muscle is a powerful dilator of the nostrils, raising the lateral commissure.

Vessels and nerves: The dilator naris apicalis muscle is vascularized by the superior labial artery and, in equids, by the major palatine artery. It is innervated by the dorsal buccal branch of the facial nerve.

Variations:

• In equines, the dilator naris apicalis muscle is highly developed. It is single, short, and irregularly quadrilateral, entirely formed by parallel fleshy bundles. These bundles cover the lamina and horn of the alar cartilages, inserting along their full length. The ventral end merges with the orbicularis oris muscle. Contraction of this muscle significantly dilates the nostrils by reducing the curvature of the alar cartilage, the two ends of which are then separated from each other.

• In cattle, the dilator naris apicalis muscle is paired, short, and thick. It originates from a strong median raphe attached to the end of the nasal septum and the body of the incisive bone. From there, its bundles radiate into the medial border and commissures of the nostrils, with the most ventral bundles slightly covering the marginal part of the orbicularis oris muscle.

• In pigs, the dilator naris apicalis muscle is fused with the lateral nasal muscle, forming a muscular bundle in the snout.

• In carnivores, the dilator naris apicalis muscle is absent. In dogs, it is at most represented by a few scattered fibers deep within the nose.

• In rabbits, the dilator naris apicalis is fused, on each side, with the corresponding part of the orbicularis oris muscle.