SENSATIONS OF SMELL AND TASTE 



643 



pressions. Another factor which diminishes the importance of olfactory 

 sensations in man is the ease with which the sense organ becomes fatigued. 

 It often happens that the inmates of a room are perfectly comfortable 

 and may perceive no fault in the ventilation, although a newcomer from 

 the outside at once remarks that the air is foul. 



THE ORGAN OF SMELL is situated at the upper part of the nasal cavi- 

 ties. Here the mucous membrane covering the superior and middle turbinate 

 bones and the corresponding part of the septum is different from that 

 covering the rest of the nasal passages. Over the lower parts of the nasal 

 cavities the mucous membrane is of the ordinary respiratory type, and 

 is composed of ciliated columnar epithelium containing a number of goblet- 

 cells. In the olfactory part the epithelium is much thicker, of a yellow 

 colour, and apparently composed of a layer of columnar cells resting on 

 several layers of nuclei. These nuclei belong to the olfactory cells proper, 

 true spindle-shaped nerve cells with one process extending towards the 

 mucus covering the free surface, while the other is continued along channels 

 in the bone, and through the cribriform plate as one of the non-medullated' 

 olfactory nerve fibres. These. nerve fibres dip into the olfactory lobes, 



FIG. 319. An tero -poster! or section through the nasal fossal. The arrows show 

 the direction of the air currents during inspiration. 



where they terminate by a much-branched arborisation or end basket in 

 the so-called olfactory glomeruli, in close connection with a similar!}' 

 branched dendrite of the large t mitral ' cells of the olfactory lobe. The 

 axons from these latter carry the olfactory impulse towards the rest of the 

 brain. In the connective tissue basis (dermis) of the mucous membrane 

 are a number of small mucous or serous glands (Bowman's glands) whose 

 office it is to keep the surface of the membrane constantly moist. 



