7. The fusiform layer (Figs. 81, 82, 83 and 84) is found every where in the cerebral cortex. It presents very little topograph ical variation. Its spindle-shaped cell-bodies lie in the deep part of the radiary zone and in the felt-work of Kaes. The long axes of the spindles are perpendicular to the surface in the crown of a gyrus but are parallel with it in the fissural walls and floor. From these cell-bodies one axone and several dendrites are given off; the dendrites arborize toward the surface, the axones are corticifugal. The function of the spindle cell is probably association. The felt-work of Kaes is a rich plexus of fibers in which the white and gray substance meet. It is pro duced by the intermingling of the association, commissural and projection fibers.
Atypical Neurones.—We have noticed two atypical neurones scattered among the typical cells of the various layers, viz.: (I) several sizes and varieties of second-type neurones, including the double-brush cells of Cajal, and (2) the inverted pyramids of Martinotti, all associative in function. The dendrites of the Martinotti cells are short and simple. The axones run toward the stratum zonale, giving off collaterals at different levels; they terminate in T-like branches within the plexiform or some deeper layer.
Axons of the Cortex.—There are three systems of axones in the cortex: the projection, the association and the corn missural. The projection system includes corticifugal and corticipetal fibers. The first are axones of the pyramids in the cortex which descend to nuclei at lower levels in the cerebro spinal axis, bearing impulses away from the cortex. The second rise in nuclei of lower levels and terminate in the cortex; they carry afferent impulses to the cortex. Both sets of axones enter into the cortical radiations (of Meynert) and, by their collaterals, help to form the tangential fibers of the zonal and Baillargic lines. The association fibers of the cortex are (r) the dendrites of the pyramidal, stellate and fusifom cells and (2) the axones and dendrites of the Golgi, Cajal and Marti notti cells. These two groups associate the different laminw and the closely adjacent cells of the same lamina. They com prise many fibers of the stratum zonale and of the lines of Baillarger. (3) The axones of pyramids and fusiform cells that descend into the white substance and run to some other part of the cortex in the same hemisphere constitute the longer association fibers. The commissural fibers are also axones of the pyramids (and perhaps of the fusiform cells). They pass from the cortex of one hemisphere to that of the other side.
Radiations.—The various fibers leaving and entering the cortex are the chief constituents of the cortical radiations (of Meynert); the Martinotti cells and the apical dendrites of the deep pyramids and fusiform cells assist somewhat. In temporal cortex where the lines of Baillarger are very faint or absent and the radiations very long, the apical dendrites of nearly all the pyramids enter into the radiations: here, the radiary zone extends to the stratum zonale.
Atypical Cortex.—The decided variations from the typical cortex are found in the visual and olfactory areas.
Visual Receptive Center (Fig. 83).—The cortex in the cuneus and gyms lingualis presents three marked variations from typical cortex: i. The greatly accentuated outer line of Baillarger, which may be seen with the naked eye dividing the cortex into two gray layers. This line was first seen and described in the visual area by Gennari (1776) and, ten years later, by Vicq d'Azyr. It is present only in the visuo-sensory cortex; elsewhere, there is a very faint outer line Baillarger. 2. The external layer of large pyramids is replaced by a layer of stellate cells 25A in diameter. These stellate cells give off three or four strong processes which appear to arborize in the outer line of Baillarger just beneath it. They are found in the receptive visual areas but not elsewhere, in the cerebral cortex. Beneath the deep stellate cells (layer 5) is the inner line of Baillarger (Bolton). 3. The internal layer of large pyramids is replaced by a conglomerate made up of three varieties of pyramids. From without inward there are: First, small in verted pyramids, cells of Martinotti, which extend their axones out toward the surface. Second, the scattered giant pyramids, 25-3og in diameter, arranged in a single row and called the solitary cells of Meynert. The axones of the giant pyramids probably pass into the optic radiation. Third, a layer of medium-sized pyramids.
Olfactory Cortex.—All the divisions of the rhinencephalon are here mentioned, though only a part of them need be de scribed: the olfactory bulb, triangle, parolfactory area, anterior perforated substance, septum pellucidum, gyrus subcallosus, gyrus supracallosus (longitudinal stria), fasciola cinerea, gyrus subsplenialis, fascia dentata, hippocampus, subiculum and uncus. The posterior inferior part of the hippocampal gyrus and the gyrus cinguli are ordinarily included in the rhinen cephalon; but, according to Elliot Smith, they belong to the neopallium (Figs. 84, 85 and 86).