The sense of smell is carried through the olfactory nerve, which is distributed on the nasal mucous membrane and covers the upper-fourth of the turbinate bones, septum nasi, nasal meatuses and ethmoidal cells called olfactory area. This part of the mucous membrane is pale-yellow in colour and is lined by epithelial cells.
- The epithelium is pseudostratified columnar and is considerably thicker than that of the respiratory region. The surface cells are of two kinds
- Sustentacular cells
- Olfactory cells.
- The sustentacular cells are more numerous cells, each consists of three parts:
- A superficial portion, which is broad and cylindrical and contains pigment and granules, arranged in longitudinal rows. The cells have well marked striated thickened borders, which unite to form the so called membrana limitans olfactoria.
- A middle portion, which contains an oval nucleus. As the nucleus of these cells all lie in the same plane, they form a distinct narrow band, which is known as the zone of oval nuclei.
- A thin filamentous process, which extends from the nuclear portion down between the cells of the deeper layers. This process is irregular and pitted by the pressure of the surrounding cells. It usually forks and apparently anastomoses with processes of other cells to form a sort of protoplasmic reticulum.
- The olfactory cells lie between the sustentacular cells. Their nuclei are spherical lie at different levels and most of them are more deeply placed than the sustentacular cells. From the nuclear portion of each cell a delicate process extends to the surface, where it ends in several minute hair like processes. From the opposite pole of the cell a longer process extends centrally which has a centripetal nerve fibre of one of the olfactory bulb. The olfactory cell is thus seen to be the bipolar nerve cell with a short peripheral and a long central process and is not a neuroepithelial cell but an analogue of spiral ganglion cell being only example of peripherally placed ganglion cell found in certain lower animals.
- The basal cells are placed between the nuclear parts of the olfactory cells and the basement membrane.
- These are small nucleated elements with irregular branching cytoplasm of which anastomoses with that of the neighboring basal cells and of the sustentacular cells to form the peculiar protoplasmic reticulum already mentioned.
- The basement membrane is not well developed.
- The stroma consists of loosely arranged collagenous fibres, delicate elastic fibres and connective tissue cells. Embedded in this stroma are numerous simple branched tubular glands, the glands of Bowman. Each tubule consists of a duct, a body and a fundus.
- The secretary cells are larger and irregular and contain a yellowish pigment, which with that of the sustentacular cells is responsible for the colour of the olfactory mucosa.
