398 J. B. JOHNSTON 
The limits of the formatio olfactoria can be determined more 
accurately by a study of the form, size and grouping of the cells. 
Throughout all the area where fila olfactoria are visible, the wall 
of the bulb consists of a thick layer of granule cells adjacent to 
the ventricle, and a fiber layer of varying thickness in the midst 
of which are scattered large and small mitral cells. In the areas 
not covered by bundles of fila olfactoria the mitral cells are less 
numerous and sometimes absent. (figs. 28, 29, 30). 
The deep granule cells are very numerous, closely crowded 
and sometimes arranged in irregular concentric layers. Golgi 
sections show that they possess two or more long slender den- 
drites radially placed and that the dendrites interlace with those 
of the mitral cells in the glomeruli. These cells are therefore 
true olfactory receptive cells, comparable to the deep cells of 
the olfactory formation of fishes (Johnston ’01). The granule 
cells form a dense thick layer surrounding the ventricle through- 
out the whole extent of the bulb (figs. 28, 29, 30) and constitute 
the greater part of its volume. The groove or constriction 
which marks the olfactory peduncle follows accurately the 
caudal border of this granule cell layer. In the lateral wall the 
pyriform lobe and the olfactory tubercle push forward some- 
what and the peduncular groove presents a V-shaped bend for- 
ward (fig. 5), but everywhere the groove faithfully indicates the 
boundary between primary or bulbar structure and the second- 
ary centers or olfactory lobe. 
The layer of mitral cells and of small brush cells is not as 
extensive as that of granule cells. In the rostral half of the bulb 
this layer is continuous around the periphery of the section, but 
the cells are less numerous in the lateral and medial angles where 
the fila olfactoria are not evident. In the caudal part of the 
bulb the corresponding areas become quite free from mitral cells 
for some distance (fig. 29, 31). The distribution of the mitral 
cells is determined by their relation to the fila olfactoria. Golgi 
preparations show that the dendrites of mitral cells and of the 
small brush cells penetrate the bundles of fila and ramify richly, 
thus transforming the bundles into elongated glomeruli or series 
of glomeruli. A brush often lies on the surface of one of these 
