996 ORGANS OF THE SENSES AND THE COMMON INTEGUMENT 



and increasing that of the other; ridges or spurs of bone growing into one or other 

 cavity from the septum are also sometimes present. Immediately over the incisive 

 canal at the lower edge of the cartilage of the septum a depression, the nasopalatine 

 recess, is seen. In the septum close to this recess a minute orifice may be discerned; 

 it leads backward into a blind pouch, the rudimentary vomeronasal organ of Jacobson, 

 which is supported by a strip of cartilage, the vomeronasal cartilage. This organ 

 is well-developed in many of the lower animals, where it apparently plays a part 

 in the sense of smell, since it is supplied by twigs of the olfactory nerve and lined 

 by epithelium similar to that in the olfactory region of the nose. 



The roof of the nasal cavity is narrow from side to side, except at its posterior 

 part, and may be divided, from behind forward, into sphenoidal, ethmoidal, and 

 frontonasal parts, after the bones which form it. 



The floor is concave from side to side and almost horizontal antero-posteriorly; 

 its anterior three-fourths are formed by the palatine process of the maxilla, its 

 posterior fourth by the horizontal process of the palatine bone. In its antero- 

 medial part, directly over the incisive foramen, a small depression, the nasopalatine 

 recess, is sometimes seen; it points downward and forward and occupies the 

 position of a canal which connected the nasal with the buccal cavity in early 

 fetal life. 



The Mucous Membrane (membrana mucosa mm). The nasal mucous membrane 

 lines the nasal cavities, and is intimately adherent to the periosteum or perichon- 

 drium. It is continuous with the skin through the nares, and with the mucous 

 membrane of the nasal part of the pharynx through the choanse. From the nasal 

 cavity its continuity with the conjunctiva may be traced, through the nasolacrimal 

 and lacrimal ducts; and with the frontal, ethmoidal, sphenoidal, and maxillary 

 sinuses, through the several openings in the meatuses. The mucous membrane 

 is thickest, and most vascular, over the nasal conchae. It is also thick over the 

 septum; but it is very thin in the meatuses on the floor of the nasal cavities, and in 

 the various sinuses. 



Owing to the thickness of the greater part of this membrane, the nasal cavities 

 are much narrower, and the middle and inferior nasal conchse appear larger and 

 more prominent than in the skeleton; also the various apertures communicating 

 with the meatuses are considerably narrowed. 



Structure of the Mucous Membrane (Fig. 857). The epithelium covering the mucous mem- 

 brane differs in its character according to the functions of the part of the nose in which it is found. 

 In the respiratory region it is columnar and ciliated. Interspersed among the columnar cells 

 are goblet or mucin cells, while between their bases are found smaller pyramidal cells. Beneath 

 the epithelium and its basement membrane is a fibrous layer infiltrated with lymph corpuscles, 

 so as to form in many parts a diffuse adenoid tissue, and under this a nearly continuous layer 

 of small and larger glands, some mucous and some serous, the ducts of which open upon the 

 surface. In the olfactory region the mucous membrane is yellowish in color and the epithelial 

 cells are columnar and non-ciliated; they are of two kinds, supporting cells and olfactory cells. 

 The supporting cells contain oval nuclei, which are situated in the deeper parts of the cells and 

 constitute the zone of oval nuclei; the superficial part of each cell is columnar, and contains 

 granules of yellow pigment, while its deep part is prolonged as a delicate process which ramifies 

 and communicates with similar processes from neighboring cells, so as to form a net-work in the 

 mucous membrane. Lying between the deep processes of the supporting cells are a number of 

 bipolar nerve cells, the olfactory cells, each consisting of a small amount of granular protoplasm 

 with a large spherical nucleus, and possessing two processes a superficial one which runs between 

 the columnar epithelial cells, and projects on the surface of the mucous membrane as a fine, 

 hair-like process, the olfactory hair ; the other or deep process runs inward, is frequently beaded, 

 and is continued as the axon of an olfactory nerve fiber. Beneath the epithelium, and extending 

 through the thickness of the mucous membrane, is a layer of tubular, often branched, glands, 

 the glands of Bowman, identical in structure with serous glands. The epithelial cells of the nose, 

 fauces and respiratory passages play an important role in the maintenance of an equable tempera- 

 ture, by the moisture with which they keep the surface always slightly lubricated. 



Vessels and Nerves. The arteries of the nasal cavities are the anterior and posterior eth- 

 moidal branches of the ophthalmic, which supply the ethmoidal cells, frontal sinuses, and roof 



