THE OLFACTORY Al'PAltATUS 



611 



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 modification 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 den- 

 drites, 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 

 mitral-cell layer. Among these cells run numerous fibers, chiefly from the 

 mitral cells and the fusiform cells of the glomerular layer. The general ar- 

 rangement is shown in figure 429. 



FIG. 428. Bipolar Olfactory 

 Cells from the Nasal Fossae of 

 the Rat (Full-term Fetus). A, 

 Epithelium of the olfactory 

 mucosa; e, epithelial cells; /, f, 

 nerve cells; i, nerve fibers ter- 

 minating freely on the epithelial 

 surface; h, olfactory nerve fibers; 

 g, sensory nerve derived from the 

 trigeminus. (Cajal.) 



