

THE SENSE OF SMELL. 



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arborizations about the dendrons of the mitral cells of the olfactory 

 glomeruli. These bipolar cells greatly resemble the cells of a gan- 

 glion of a posterior root of the spinal cord, one neuraxon going to the 

 olfactory mucous membrane and the central neuraxon going to the 

 olfactory bulb. 



The olfactory bulb from without inward consists of four layers : 



1. The nerve-fibers. 



2. Stratum glomerulosum. 



3. Stratum gelatinosum. 



4. Layer of central nerve-fibers. 



In the first layer each fibril is a central neuraxon of a rod-shaped 

 nerve-cell from the olfactory mucous membrane. The fibers of the 

 olfactory nerves pass into the glomeruli lying beneath. Within the 

 glomerulus the endings of the olfactory fibrils come in contact with 

 an olfactory end-brush of an apical dendron of a mitral cell. 



In the stratum glomerulosum each glomerulus consists of the 

 terminal arborizations of an olfactory nerve-fiber, together with the 

 olfactory end-brushes from the apical dendrons of the mitral cells. 



The stratum gelatinosum in its inner part contains two chief 

 forms of cells: the deep and superficial layers of mitral cells which 

 correspond to the pyramidal cells of the cerebral cortex. 



The fourth layer in its outer part has a large number of very 

 small granular cells between which pass the descending neuraxons of 

 the mitral cells. The nerve-fibers of the olfactory bulbs collect at 

 their posterior extremities into two bundles: the olfactory tracts. 

 The outer root-fibers of the olfactory tract come into relation with 

 the gyrus hippocampus, the uncus, and cornu ammonis. The inner 

 root-fibers pass into the gyrus fornicatus. 



Olfactory Sensations. The student, in order to obtain clear-cut 

 ideas as to the mechanism of the special sense of smell, should bear 

 in mind the principle of the arrangement of the olfactory nerve- 

 terminations. It is recalled that within the mucous membrane lie 

 the olfactory cells. From the peripheral end of each cell project 

 seven or eight ciliumlike processes. These not only project to the 

 surface of the mucous membrane, but even to the surface of the serous 

 fluid moistening the membrane. Thus, the terminal filaments are 

 placed in an exposed position so that they may very readily respond 

 to any irritant. 



The proper stimulus for olfactory-nerve filaments are odorous 

 substances which reach the regio olfactoria through the air and must 

 be in a volatile state. Hence^ olfactory sensations are produced by 



