BULBUS OLFACTORIUS.
The bulbus olfactorius exhibits the following layers : 1. The layer of the superficial nerve-fibres. This, the fibre-layer, is formed by the nerve-fibres coming from the olfactory epithelium (Fig. i16). Within the epithelium of the olfactory mucous membrane, the bipolar nerve-cells lie among the sus tentacular cells. They are elongated narrow fusiform or irregular elements, with a thick peripherally directed process, that ends within the epithelium, and a delicate centrally di rected process, beset with varicosities, that traverses the tunics propria undivided. United into small bundles, the fila olfactoria, the central fibres pass through the apertures of the lamina cribrosa, enter the bulbus olfactorius and there form a close feltwork of crossing fibres, the fibre-layer.
2. The glomerular lay er. Joining the stratum of nerve-fibres, the layer of glom eruli olfactorii follows. Here the end-arborizations of the fibres from the fibre-layer meet those of the dendrites of certain cells, namely, the brush and mitral cells. In consequence of the close intermingling of these delicate terminal twigs, small round or ovoid structures, the glomeruli olfaclorii, are formed. The olfactory fibres com posing the fibre-layer divide often into two, or indeed into three, twigs, which enter the glomeruli ; in this manner, such ramifications may penetrate into two different glomeruli.
3. The molecular layer. This layer, also known as the stratum gelatinosum of Clarke, forms a stratum comparable to the layer of small pyramidal cells of the cerebral cortex. Within it, along with traversing and branching fibres, are found large and small brush-cells.
4. The layer of mitral cells. When compared with the cerebral cortex, it rep resents the layer of the large pyramidal cells. The component mitral cells are of quite characteristic form. The cell-body is large, exhibits the form of a triangle or a mitre, and resembles that of the Purkinje cells of the cerebellar cortex. The protoplasmic processes are of two kinds, the ordinary dendritic stems and the so-called olfactory brushes, the penicilli olfactorii. The former pass obliquely from the cells, then run more horizontally, branch once, and end, usually after a long course, free, forming a feltwork within the deepest part of the molecular layer. The olfactory brushes traverse the molecular layer and assist in forming the glomeruli with their elaborate varicose end arborizations. The nerve-processes of the mitral cells extend towards the granule-layer, bend sagittally at various levels and continue within the tractus olfactorius. During their
course they give off collaterals, which end in free branches within the superficial and deep strata of the molecular layer.
The are often spindle-form in shape and horizontally placed. The larger cells are found within the molecular layer, external to the mitral cells, which latter they in general resemble in giving off the two kinds of dendrites and in sending their nerve processes to the granule-layer. The small brush-cells, also known as the peripheral brush cells, lie close beneath and between the glomeruli. They likewise send one dendrite to the glomerulus, the nerve-processes behaving like those of the large brush-cells.
5. The Within this stratum are nd the granule cells or granula, peculiar small elements with long processes. These granula also penetrate between the mitral cells and, beyond these, into the molecular layer as far as the glom eruli. The granule cells are triangular, resembling the pyramidal cells, or fusiform or pear-shaped, all being placed vertically. An outwardly directed process, mostly single but rarely double, divides repeatedly after a longer or shorter course, usually close beneath the mitral cells, to form a brush-like terminal arborization, that ends within the most superficial region of the molecular layer at the glomeruli in delicate filaments. In the opposite direction, that is inward, the granules exhibit several processes, which are usually smooth and slightly branched and end free after a short course. As yet, a nerve-process has not been discovered. In addition to the granules, this layer contains cells of Golgi's II type—multipolar elements with fusiform or polygonal cell-body and a nerve-process that breaks up within the granule-layer. The nerve-fibres running within the granule-layer are partly the nerve-processes of the mitral and brush cells ; further, fibres enter the bulbus to end some within the granule-layer, and some within the molec ular layer in the vicinity of the glomeruli, after having penetrated the layer of mitral cells.
The nerve-processes of the mitral and brush cells, that pass to the tractus olfacto rius, end within the cortex of the tractus and of the tuberculum olfactorium and within the olfactory area of the substantia perforata anterior and the adjoining parts of the sep tum pellucidum. These end-stations exhibit the structure of a 'modified cerebral cortex.