SENSE OF SMELL. 



533 



medullated, and are continued from the olfactory fibers of the 

 Schneiderian or olfactory mucous membrane of the nasal fossae. 

 In this mucous membrane they take origin from the bipolar 

 olfactory cells which are characteristic of the membrane, and they 

 end in arborizations within the olfactory glomeruli, where they 

 come in contact with the arborizations of the mitral cells." 



Olfactory Tract. This structure is an outgrowth from the brain, 

 which is in man at one period of development hollow, but later 

 the cavity is filled with neuroglia, outside of which are bundles of 

 white fibers, and still more external is a layer of neuroglia. There 



Mitral cells. 



f Large 

 nerve- 



Molecu- j cell, 

 lar layer, j Small 

 nerve- 

 cell. 



Layer of olfactory 

 glomeruli. 



Peripheral nerve- 

 fibers. 



FIG. 311. The olfactory bulb, after Golgi and Ramon y Cajal. The granular layer 



is not shown. 



are no nerve-cells in the tract. It lies in the olfactory sulcus, a 

 depression in the under surface of the frontal lobe of the cere- 

 brum, and terminates in the olfactory bulb. Traced backward, 

 the tract is found to be made up of two roots, external and in- 

 ternal ; between these is the trigonum olfactorium, which is some- 

 times called the middle or gray root, but which is in reality cortical 

 gray matter. The external root is connected with the end of the 

 hippocampal gyrus, and the internal with the beginning of the 

 gyrus fornicatus. 



Functions of the Olfactory Nerves. The olfactory nerves are 

 beyond all doubt the channels by which olfactory impressions 

 reach the brain (Fig. 312). They are nerves of the special sense 



