SENSE OF SMELL. 



531 



brane was named, it was thought that the secretion it forms came 

 from the brain : he demonstrated that it came from the membrane 

 itself. It is covered by epithelium, which, near the orifice of the 

 nostril in the vestibule, is pavement, but elsewhere, except on the 

 olfactory membrane, this epithelium is columnar and ciliated. 

 In man the olfactory membrane is soft, vascular, and of a yellow 

 color. It is covered by epithelium composed of two varieties of cells, 

 with a superficial lamina through which the ends of the cells pro- 

 ject. One variety, olfactory or olfactorial cells, is spindle-shaped 

 and bipolar (Fig. 309), having a nu- 

 cleus. One of the poles extends to the 

 surface, its extremity passing through 

 the lamina, and in amphibia, reptiles, 

 and birds terminates in fine, hair-like 

 filaments. It is uncertain whether 

 these filaments exist in mammals. 

 The other pole extends downward 

 and is connected with one of the 

 fibrils of the olfactory nerve, and 

 passes through the cribriform plate 

 of the ethmoid bone, and arborizes 

 within one of the olfactory glomeruli 

 (Fig. 310). The other variety of cells 

 is the columnar or sustentacular cell. 

 These are columnar epithelium-cells 

 with nucleated cell-bodies at the free 

 surface of the mucous membranes and 

 branching processes extending down- 

 ward : they are supporting cells. The 

 corium of the olfactory membrane 

 is very thick and vascular, with 

 bundles of olfactory nerve-fibers and 

 a large number of serous glands, Bow- 

 man's glands, whose ducts open upon 

 the surface of the membrane between 

 the epithelial cells. 



Olfactory Nerves. These are about 

 twenty in number, non-medullated, and are given off from the 

 olfactory bulb. ' They pass through the cribriform plate of the 

 ethmoid bone and are distributed to the olfactory membrane. 



Olfactory Bulb. This is in reality a portion of the hemispheres, 

 and is described as " a cap superimposed upon a conical process 

 of the cerebrum." It consists of gray and white matter, and is 

 thus described by Schafer : " Dorsally there is a flattened ring 

 of longitudinal white bundles enclosing neuroglia, as in the 

 olfactory tract ; but below this ring several layers are recognized, 

 as follows : 



FIG. 309. Olfactory mucous 

 membrane : a, sustentacular 

 cells ; 6, olfactory cells ; c, basal 

 cells; d, submucous fibrous tis- 

 sue ; e, glands of Bowman ; I, 

 nerve-fibers (Leroy). 



