THE SENSES. 



517 



Sense of Smell. 



The distinguishing character of this sense is that it appreciates the 

 quality of gaseous or vaporous substances. It can therefore detect 

 odoriferous matters at a distance, and when concealed from sight. It 

 differs, furthermore, from the sense of taste in being more distinctly 

 localized ; since it is confined to the upper portion of the nasal passages 

 and depends on the filaments of a single pair of nerves. 



The mucous membrane covering the superior and middle turbinated 

 bones and the upper part of the septum nasi, which is alone capable 

 of receiving odorous impressions, is known as the olfactory membrane. 

 It is distinguishable from that lining the rest of the nasal passages : 

 1st, by its color, which in man, the sheep, and the calf is yellow, but 

 in most other mammalia has a brownish tinge ; 2dly, by its softer 

 consistency; 3dly, in the greater thickness of the whole membrane, 

 and especially of its epithelial layer. According to Kolliker, the epi- 

 thelium of the olfactory membrane, in the sheep and rabbit, is about 

 sixty per cent, thicker than that of the remaining nasal membrane. 

 In most quadrupeds the epithelium of the nasal mucous membrane 

 generally is covered with vibrating cilia, which are absent in the 

 olfactory portion ; but in man cilia are also found in the olfactory 

 portion. This difference is probably connected with the inferior acute- 

 ness of smell in man, as compared 

 with the lower animals. 



The nasal passages are pro- 

 vided with nerves from three 

 sources. 



I. The first and most important 

 of these are the olfactory nerves 

 (Fig. 129, j). They are derived 

 from the olfactory bulbs, resting 

 on the cribriform plate of the 

 ethmoid bone, through which 

 their filaments penetrate the up- 

 per part of the nasal passages. 

 They contain only pale, flattened, 

 nucleated nerve fibres, destitute 

 of a medullary layer. Their 



branches divide and Subdivide, DISTRIBUTION OF NERVES is THE NASAL PAS- 



FIG. 129. 



microscopic plexuses in SAGES.-I. Olfactory bulb, with its nerves. 2. 



Nasal branch of the fifth pair. 3. Spheno-pala- 



tne substance ot the. olfactory tine ganglion. 



membrane ; and the finest ner- 



vous ramifications have been followed nearly to the epithelial surface 



of the membrane. 



There is no doubt that the filaments given off from the olfactory 

 bulbs are the special agents for communicating olfactory impressions, 

 and that they are the only ones endowed with this kind of sensibility. 



