a, cartilage of the median septum; b, “wings”; c, ridge formed by internal portion of wing.
The nares or cranial openings of the nasal cavity are supported by a number of cartilages which form the framework of the snout or external nose. The cartilaginous continuation of the lamina perpendicularis extends some distance craniad of the tips of the nasal and premaxillary bones, forming the septum of the external nose ([Fig. 103], a). From the dorsal edge of this projecting cartilaginous septum, two thin cartilaginous wings (b) extend laterad, forming the dorsal wall of the narial opening. Each then turns ventrad to form the lateral wall of the opening, but does not form the ventral floor; instead it curves mediad and finally dorsad, thus extending from the floor of the opening as a prominent ridge (c) within the nares. The cartilaginous wing is thus rolled into a sort of spiral, ending with a free edge within the narial opening. From the ventral edge of the median cartilaginous septum there are likewise lateral extensions, which form part of the ventral boundary of the nares; these do not quite reach the ventral parts of the dorsal wings, however, so that a small part of the narial opening is not bounded by cartilage. A section of the narial cartilages is shown in [Fig. 103]. The wings from the dorsal edge of the septum do not extend quite to the cranial tip of the nose, so that a notch is formed on the ventrolateral side of the nares; a section in this region would therefore differ from that figured.
The framework of the external nose thus formed is covered externally by thick hairless skin, containing many glands. From the ventral end of the internarial septum a groove passes ventrad, partly dividing the upper lip.
The inner surfaces of the cartilages are covered by the mucosa, which forms a number of ridges. The narial opening is almost completely divided by the prominent ridge which is supported by the free edge (c) of the cartilaginous wing above [described]. This free edge is covered by a thick layer of mucosa, and the entire ridge so formed is continuous caudad with the inferior nasal concha. On the medial side of the partial partition formed by this ridge and near the ventral side there begins a slight distance caudad of the outer opening another ridge, supported by the ventrally incurved portion of the cartilaginous ring. This soon becomes a thick swelling; beneath it opens the lachrymal canal, and that part of the passageway that lies ventromediad of it is the beginning of the inferior meatus of the nose. About one or two centimeters caudad of the external opening a third ridge projects from the dorsolateral wall of the cavity toward the large ridge first described; dorsomediad of it is a narrow passage which is the entrance to the superior meatus of the nose.
The olfactory mucosa, or that part to which the olfactory nerve is distributed, and which therefore acts as the sensory surface, is confined to the dorsocaudal parts of the nasal cavity, in the region occupied by the cells of the ethmoid. The air penetrates to this region probably only by a definite act of snuffing, the inferior meatus serving as the usual passageway of air to the lungs.
The nasopharynx and pharynx are considered under the alimentary canal ([page 231]).
2. The Larynx.
The larynx is the enlarged upper end of the air-passage which leads from the pharynx to the lungs. It is a box composed of pieces of cartilage connected by ligaments and moved by muscles, and it is lined by mucous membrane. At the root of the tongue about one centimeter caudad of the body of the hyoid bone is the triangular leaf-like epiglottis ([Fig. 95], e, [page 227]). When food is taken this closes the opening into the larynx, and the food passes over it into the œsophagus. It is so curved that its apex is directed craniad. A fold of mucous membrane, the plica glossoepiglottica or frenulum of the epiglottis, extends from the middle of its cranial surface to the root of the tongue, and on each side of this fold is a depression.
From each side of the base of the epiglottis a fold, plica aryepiglottica ([Fig. 95], f), extends caudad to the base of the arytenoid cartilage (g). Dorsad of this fold and separated from it by a depression is a ridge which marks the position of the caudal hyoid cornu. The plicæ aryepiglotticæ (f) and the epiglottis (e) form the boundaries of the aditus laryngis, or opening into the larynx.
The cavity of the larynx is divided into three portions. The upper one of these is the vestibule of the larynx. It is bounded caudad by two folds of mucosa (i) that stretch from the caudal surface of the epiglottis near its base to the tips of the arytenoid cartilages. These folds are the false vocal cords (i). Their vibration is said to produce purring. Caudad of the false vocal cords two folds of the mucosa stretch from the apices of the arytenoid cartilages to the thyroid cartilages, near the base of the epiglottis. These folds are nearer the median plane than the false vocal cords. They are the true vocal cords (j). The middle portion of the laryngeal cavity is that between the true and the false vocal cords. It is produced laterally into a very small pouch or pocket on each side, the ventriculus. The narrow slit between the true vocal cords is the glottis (h). It can be narrowed and widened by the action of muscles. The vocal cords, which bound it, are set vibrating by currents of air transmitted from the lungs, and the voice-sounds are thus produced. The caudal portion (inferior portion) of the laryngeal cavity is that between the glottis and the first tracheal cartilage. It is narrowed near the glottis.