408 



AN AMERICA X TEXT-BOOK OE PHYSIOLOGY. 



F. Smell. 



The complex paired cavity of the nose is divisible into a lower respiratory 

 and an upper olfactory tract, the mucous membrane over each of which is 

 distinctive. The covering of the respiratory tract is known as the Schneider- 

 ian or pituitary membrane; its surface is overlaid with cylindrical ciliated 

 epithelium, the ciliary current of which is directed posteriorly toward the 

 pharynx. 



The Schneiderian membrane lines the lower two-thirds of the septum, the 

 middle and inferior turbinated bodies, and the bony sinuses which communi- 

 cate with the nasal chamber. The mem- 

 brane upon the turbinated bodies and 

 the lower part of the septum is composed 

 largely of erectile tissue. 



The function of the respiratory tract 

 is threefold : it restrains the passage 

 of solid particles into the lungs; it 

 warms the air inspired to approximately 



Fn.i. 203. -Section of olfactory mucous mem- 

 brane (after V. Brunn) : the olfactory cells are in 

 black. 



Fig. 204.— Cells of the olfactory region (after V. 

 Brunn): a, olfactory cells; 6, epithelial cells; n, 

 central process prolonged as an olfactory nerve- 

 lilpril; /. uucleua; c, knob-like clear termination 

 of peripheral process ; //, bunch of olfactory hairs. 



the temperature of the body ; and it gives up moisture sufficient nearly to 

 saturate the air. 



The olfactory mucous membrane, which alone is the peripheral organ for 

 smell, is seated in the upper part of the nasal chamber, away from the line 

 of the direct current of inspired air. The membrane is thick and is covered 

 by an epithelium composed of two kinds of cells, columnar and rod cells. 

 The latter are the true olfactory cells (Figs. 203, 204), with which the fibres 

 of the olfactory nerve are known to be connected. These olfactory cells, in 

 fact, are comparable to nerve-cells in that the fibres connected with them, the 

 fibres composing the olfactory nerve, are direct outgrowths from the cells 

 ( Fig. 205), essentially similar in every way to the nerve-fibre processes springing 

 from nerve-cells in the nerve-centres. In this respect the olfactory cells differ 

 from the sensory cells in other organs of special sense. The membrane 



