306 THE ORGANS 



columnar type, and is considerably thicker than that of the respira- 

 tory region. The surface cells are of two kinds: (i) sustentacular 

 cells, and (2) olfactory cells. 



(i) The sustentacular cells are the more numerous. Each cell 

 consists of three parts: {a) A superficial portion, which is broad and 

 cyhndrical, and contains pigment, and granules arranged in longi- 

 tudinal rows. The cells have well-marked, striated, thickened free 

 borders, which unite to form the so-called memhrana limitans oljactoria. 

 (b) A middle portion which contains an oval nucleus. As the nuclei 

 of these cells all lie in the same plane, they form a distinct narrow 

 band, which is known as the zone of oval nuclei, (c) A thin filament- 

 ous process which extends from the nuclear portion down between the 

 cells of the deeper layers. This process is irregular and pitted by 

 pressure of surrounding cells. It usually forks and apparently 

 anastomoses with processes of other cells to form a sort of proto- 

 plasmic reticulum. 



(2) The olfactory cells lie between the sustentacular cells. Their 

 nuclei are spherical, he at different levels, and are most of them more 

 deeply placed than those of the sustentacular cells. They thus form 

 a broad band, the zone of round nuclei. From the nuclear portion of 

 the cell a delicate process extends to the surface, where it ends in sev- 

 eral minute hair-like processes. From the opposite pole of the cell a 

 longer process extends centrally as a centripetal nerve fibre. The 

 olfactory cell is thus seen to be of the nature of a ganglion cell (see 

 also page 432). 



Between the nuclear parts of the olfactory cells and the basement 

 membrane are the basal cells. These are small nucleated elements, 

 the irregular branching protoplasm of which anastomoses with that 

 of 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 white fibres, dehcate elastic 

 fibres, and connective-tissue cells. Embedded in the stroma are 

 large numbers of simple branched tubular glands, the glands of Bow- 

 man. Each tubule consists of a duct, a body, and a fundus. The 

 secreting cells are large and irregular and contain a yellowish pigment, 

 which with that of the sustentacular cells is responsible for the peculiar 

 color of the olfactory mucosa. These glands were long described as 

 serous, but are now believed to be mucous in character. They fre- 

 quently extend beyond the limits of the olfactory region. 



