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).
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.