The cortex of the olfactory bulb (Fig. 86) is divided into five layers as pictured by Barker. These five layers are as follows, named from the surface toward the center: (1) The stratum nervosum, composed of the T-branched fibers from the olfactory nerve and their collaterals. These fibers run nearly parallel with the surface for some distance, then bend centrally and break up into their end-tufts in the second layer. (2) The stratum glomerulosum is made up of round bodies, called glom eruli, which are composed of the end-tufts of olfactory nerve fibers and of brush-like dendrites from the spindle and mitral cells of the third and fourth layers. The glomeruli constitute the synapses between the first and second olfactory neurones.
(3) The stratum reticulare. This is a network of mitral den drites interwoven with arborizing processes from the granules in the fifth layer and the branches of a few endogenous spindle cells, called the brush cells. The mitral dendrites are on their way to the glomeruli in the second layer. The spindle cells likewise, both large and small, throw their dendritic processes down into the stratum glomerulosum, where they end in rich tufts or brushes; and their axones penetrate the fourth and fifth layers, enter into the white sheath of the bulb and thence are continued into the olfactory tract. (4) The stratum cellulare, or layer of mitral cell-bodies. The mitral cells have large pyramidal bodies with one axone and rich dendritic processes. The latter arborize through the reticular layer to the glomeruli of the second layer, where they terminate in the form of end brushes. The axones of the mitral cells run centrally through the granular layer, to which they give off collaterals, and then turn backward in the white sheath and constitute most of the olfactory tract. The white sheath incloses a mass of cells derived from the ependymal lining of the ventricle in the em byro. (5) The stratum granulosum is composed of a thick layer of small cell-bodies, "granUles," whose processes arborize richly in the granular, cellular and reticular layers. Imbedded in the granular layer are the medullated axones coursing toward the white sheath and the olfactory tract. The function of the granular layer is not understood. The mitral and spindle cells of the olfactory bulb, it should be carefully noted, form the terminal nucleus of the olfactory nerves; the points of contact between them are established in the glomeruli; and the axones of the nucleus constitute the olfactory tract and terminate in four nuclei—the cortex of the olfactory tract, the olfactory triangle, anterior perforated substance and septum pellucidum.
In these nuclei lie the bodies of the third order olfactory neu rones, whose axones form the olfactory striae: the medial, the intermediate and the lateral olfactory striae (Fig. 86). The lateral stria of the olfactory tract runs directly to the uncus, hence we shall study that region next.
The uncus of the hippocampal gyros (Figs. 75, 84 and 85) probably represents the greater part of the lobus pyriformis of osmatic mammals. It constitutes the chief cortical center of smell. However, it is probable that the subiculum, hippo campus, fascia dentata, the subsplenial and callosal gyri belong in the cortical area of smell, as all showed arrested development in two cases of congenital absence of the olfactory bulbs (Zuckerkandl). The fascia dentata is of first importance ac cording to Alexander Hill. He calls attention to the fact that the narwhal, which has no sense of smell, possesses every part of the hippocampal region excepting the dentate fascia (Camp bell). The uncus comprises the whole anterior part of the gyrus hippocampi. In structure the crown of the hippocampal gyrus and the uncus are nealy identical. They have only five layers of cells. (I) As already pointed out the plexiform layer is thick and possesses a dense stratum zonale, only second to that of the subiculum. (2) The place of the small pyramids is usurped by the "olfactory islets" (Calleja) which are curious nests of large stellate cells (28 ,u) interspersed with small nests of very minute pyramidal cells. (3) The tassel cells of Cajal. Peculiar pyramidal cells, with such rich dendritic arborizations hanging from the bases as to resemble tassels, are seen in the place of the medium-sized pyramids. At the line of Baillarger there are no cells; the fourth layer of typical cortex is entirely wanting. (4) The stellate layer and internal layer of large pyramids are replaced by a layer of intermixed fusiform and triangular cells heavy with Nissl bodies. (5) The fusiform-cell layer is nearly typical.
Nucleus Amygdalce (Fig. 39).—In the anterior wall of the inferior horn of the lateral ventricle, near the temporal pole and dorsal to, but continuous with, the uncus hippocampi, is the amygdala, a nucleus of doubtful classification. The amygdala is in part continuous with the corpus striatum and, according to Campbell, appears on the surface of the uncus as the gyrus semilunaris.