OLFACTORY ORGAN. 



379 



nucleus is round, with a distinct nucleolus, and the protoplasm 

 forms a spindle-shaped cell sending out two processes. The 

 upper one, which reaches to the free surface of the epithelial 

 layer, is very short. It bears on its free end a number (six 

 to eight) tof firm short hairs. The lower thinner process passes 

 over to form the axis cylinder of a centripetal nerve fibre, 

 which runs to the bulbus olfactorius (Fig. 274). 



The supporting cells (Fig. 274) are in many ways com- 

 parable with the Miiller's fibres of the retina, They are epi- 

 thelial cells of a cylindrical form, which become smaller at the 



FIG. 275. 



Epithelial 

 layer 



Duct 



Branch of 

 olfactory nerve 



Tunica propria 



Olfactory gland- \j- 



Cross-section 

 of nerve 



Few- 



Vertical section through the mucous membrane of the regio olfactoria of a rabbit. X 360. 



lower end. Small depressions in the lateral surfaces are often 

 seen. These are filled up by the bodies of the olfactory cells. 

 The basal ends of these cells are often forked, so that they 

 touch the basal membrane with two or more parts. The oval 

 nuclei lie at the same level in the thicker part of the cell. 

 The protoplasm contains yellowish pigment, which gives to this 

 part of the mucous membrane a characteristic color. The sup- 

 porting cells possess a fine cuticular border. The borders of 

 all the cells stand in such close connection with one another 

 that they form a membrane, the membrana limitans olfactoria, 



