THE SENSES 



cribriform plate to form a glomerular basket with the branches of the mitral 

 cells of the olfactory bulb. 



The olfactory bulb must be studied in relation with the nerve fibers and 

 olfactory cells with which it is connected. These parts together form a sen- 

 sory end-organ which resembles in many respects the retina. The discovery 



of its true structure has thrown a flood of light on 

 the architecture of the nerve centers as a whole. 



The olfactory bulb is not a nerve, but a modi- 

 fication of the brain cortex. A transection shows 

 it to be made up of four layers: i. Peripheral 

 fibers. 2. Olfactory glomerules. 3. Layer of 

 mitral cells. 4. Layer of granular cells and deep 

 nerve fibers. 



The first and external layer is composed of 

 the fine nerve fibrils of the olfactory nerves. 

 They pass through the cribriform plate of the 

 ethmoid, arising from the olfactory cells of which 

 they are processes. 



The glomerular layer contains numbers of 

 small round bodies whose structure shows that 

 they are made up of the interlocking expansions 

 of the olfactory fibers, on the one hand, and of 

 the branches of the " mitral" cells, on the other. 

 These are mingled in a close network, but do not 

 anastomose. It was by the study of these bodies 

 in part that the fact of the non-continuity of the 

 neurones was demonstrated, figure 429. This 

 layer also contains small fusiform cells with 

 branching dendrites that extend outward to the 

 glomeruli. Each has an axis-cylinder process 

 which passes inward to join the fibers of the 

 internal olfactory nerves. 



The layer of mitral cells contains large cells, 

 some of them triangular and some in the shape of a miter. They have 

 numerous dendrites, one of which passes into a glomerulus and then breaks 

 up in a fine arborization. An axis-cylinder process passes off from the 

 inner surface and is continued as an internal olfactory nerve fiber in the 

 olfactory tract. 



The layer of granules and central fibers contains a large number of very 

 small nerve cells, which are peculiar in that they have no axis- cylinder. 

 Their dendrites extend chiefly into the layer of mitral cells. They resemble 

 the spongioblasts of the retina and probably have commissural functions. 

 This layer has also some small star-shaped cells whose dendrites end in the 



FIG. 428. Bipolar Olfac- 

 tory Cells from the Nasal 

 Fossae of the Rat (Full-term 

 Fetus). A y Epithelium of the 

 olfactory mucosa; e y epithelial 

 cells;/,/, nerve cells; i, nerve 

 fibers terminating freely on 

 the epithelial surface; h, 

 olfactory nerve fibers; g, sen- 

 sory nerve derived from the 

 trigeminus. (Cajal.) 



