258 NORMAL HISTOLOGY. 



The olfactory bulb may be divided into five layers : first, the 

 layer of peripheral nerves, containing the neurites of the olfactory 

 nerve ; second, the layer containing the olfactory glomeruli ; third, 

 the molecular layer ; fourth, the layer of the mitral cells ; fifth, the 

 granular layer. 



The first layer is, as already stated, occupied by the neurites from 

 the nervous cells in the olfactory mucous membrane. These neurites 

 constitute the axis-cylinders of the olfactory nerve. 



The glomeruli of the second layer are small globular masses 

 formed by the closely associated teleneurites of the olfactory nerves 

 and teledendrites from the mitral ceils of the fourth layer, the den- 

 drites from which pass through the third or molecular layer. A 

 few cells of neurogliar nature may be associated with these nervous 

 terminations, but the chief mass of each glomerulus is composed of 

 interwoven teleneurites and teledendrites. 



The third, or molecular, layer contains small spindle-shaped 

 nerve-cells, which send dendrites to the glomeruli of the second 

 layer and neurites into the granular (fifth) layer, where they turn 

 and take a centripetal direction toward the cerebrum. 



The fourth layer is characterized by the presence of large tri- 

 angular nerve-cells, the mitral cells, the dendrites from which pass 

 through the molecular layer, to end in teledendrites within the 

 glomeruli. A single mitral cell sends dendrites to more than one 

 glomerulus. The neurites from these cells pass, centripetally, to 

 the olfactory centre of the cerebrum. 



The fifth, granular, layer contains the centripetal neurites of the 

 mitral cells, and also centrifugal neurites from the cerebrum. The 

 latter are distributed in teleneurites within the granular layer 

 itself. This layer also contains small polygonal nerve-cells of two 

 sorts : first, cells resembling those of the third type represented in 

 Fig. 217, the processes from which are distributed in the granular 

 and molecular layers. They are probably association-cells. Second, 

 cells (Fig. 241) with dendrites in the granular layer and teleneurites 

 in the molecular layer. These cells would distribute impulses re- 

 ceived from the centrifugal fibres, which end in the granular layer, 

 among the teledendrites in the molecular layer. 



The sense of smell, then, is aroused by stimulations of the distal 

 ends of the nervous cells in the olfactory mucous membrane (Fig. 

 241), which are transmitted to the glomeruli, where they leave the 

 first neuron, being communicated to the second, represented by the 



