THE OLFACTORY APPARATUS. 



181 



known as granule cells and probably represent the greater number 

 of spindle and stellate cells of lower forms. It is supposed that 

 hi the course of evolution the mitral cells became predominant 

 while the smaller and less efficient cells lost their function and 

 now exist only as vestigeal structures, the granule cells. By 

 some authors, however, the granule cells are described as of two 

 kinds; one possessed of short neurites ending within the bulb, 

 the other giving rise to excessively fine neurites which run hi the 

 olfactory tract to the forebrain nuclei. Mention must be made 



Olfactory fibers. 



I 





Mitral cell dendrite. 

 Dendrites of deep cells. 



FIG. 97. An olfactory glomerulus from the brain of the sturgeon to show the 

 part taken by the dendrites of deep 'cells in forming the glomeruli. 



of centrifugal fibers which come forward from the forebrain 

 in the olfactory tract and end in the olfactory bulb. It is probable 

 that these are commissural fibers coming from the bulb of the other 

 side. 



In all vertebrates the cells whose dendrites help to form the 

 glomeruli send their neurites inward toward the ventricle and 

 backward toward the forebrain, and there is no evidence that 

 the neurites from the different types of cells have any different 

 destinations or behavior. The whole number of fibers arising 



