300 



THE SPECIAL SENSES. 



cells are essentially nerve cells, and in this respect resemble the 

 sense cells in the retina, the rods and cones, rather than those of the 

 ear or of the organs of taste. The distribution of the olfactory 

 cells, according to v. Brunn, is confined to the nasal septum and a 

 portion of the upper turbinate bone. The area covered in each nos- 

 tril corresponds to about 250 square millimeters. The epithelium 

 of the lower and middle turbinates and the floor of the nostrils is 

 composed of the usual ciliated cells found in the respiratory passages, 

 while the so-called vestibular region of the nose, the part roofed in 



by the cartilage, is covered 

 by a stratified pavement 

 epithelium corresponding in 

 structure with that of the 

 skin. These latter portions 

 of the nose are supplied 

 with sensory fibers derived 

 from the fifth or trigeminal 

 nerve. We must consider 

 the 500 sq. mm. of olfac- 

 tory epithelium as the 

 olfactory sense organ com- 

 parable physiologically and 

 perhaps anatomically to the 

 rod and cone layer of the 

 retina. The connections of 

 these cells with the central 

 nervous system have al- 

 ready been described (p. 

 217). It will be remem- 

 bered that the fine, non- 

 medullated fibers springing 

 from the basal end of the 

 sense cells enter the olfac- 

 tory bulb and end in ter- 

 minal arborizations in the olfactory glomeruli, where they make con- 

 nections by contact with the dendrites of the mitral cells of the 

 bulb. Through the axons of these mitral cells the impulses are con- 

 ducted along the olfactory tract to their various terminations in the 

 olfactory lobe itself, either of the same or of the opposite side, and 

 eventually also in the cortical region, the uncinate gyrus of the 

 hippocampal lobe. As regards the olfactory sense cells, the nerve 

 cells in the olfactory bulb might be compared with the nerve gan- 

 glion layer of the retina, and the nerve fibers of the olfactory tract 

 with the fibers of the optic nerve. 



The Mechanism of Smelling. Odoriferous substances to 



Fig. 122. Cells of the olfactory region (after 

 v. Brunn): a, a, Olfactory cells; 6, 6, epithelial 

 cells; n, n, central process prolonged as an olfac- 

 tory nerve fibril; I, Z, nucleus; c, knob-like clear 

 termination of peripheral process; h, h, bunch of 

 olfactory hairs. 



