CORTEX OF THE PALLIUM.
Based on the arrangement of the nerve-cells, the following six strata or layers are distinguished.
i. The molecular layer. This constitutes the most superficial stratum and is a dense feltwork, composed principally of fibres running mostly parallel to the surface ; hence, it is also designated as the layer of tangential fibres or the tangential In addition to numerous neuroglia cells, this layer contains the terminal ramifications of the dendrites of the more deeply lying pyramidal cells and the end-arborizations of the nerve-fibres coming from the white substance and ending in the cortex. Further, it con tains certain cells, including medium sized polygonal elements, with from four to six dendrites and a nerve-process arborizing within the molecular layer, and fusiform or tri angular cells, with few more or less horizontally coursing dendrites and two or several nerve-processes, that also run horizontally and end within the molecular layer. The ele ments with several neurites encountered within the tangential fibre-layer, are known as Cajal cells.
2. The outer granule layer, a stratum of small pyramidal cells.
3. The layer of small and medium sized pyramidal cells.
4. The inner granule layer, a stratum of small pyramidal cells.
5. The layer of large pyramidal cells. The cell-body of the pyramidal cells is pyramidal in form, the base presenting towards the white substance and the apex directed towards the molecular layer. The apex is prolonged into a robust protoplasmic process, the primordial branch, which gives off lateral twigs at right angles, runs toward the molecular layer and there ends after repeated division. The basal dendrites pass off from the base of the cell-body, radiating laterally, or towards the white matter. The nerve process springs from the base of the cell, or from a basal dendrite close to the cell-body, and runs towards the white substance ; during its course through the gray substance, the nerve-process gives off fine collaterals, that run horizontally or obliquely and end after a number of branchings.
6. The layer of polymorphic cells. Here 'are found cells, ovoid, fusiform, triangular or polygonal in form, which often exhibit a robust protoplasmic process, directed towards the molecular layer. Each cell sends off a nerve-process that passes to the white substance, after giving off a number of collaterals. Additional cells, with short nerve-processes or of Golgi's II type, are encountered, not only in this layer, but also within the strata of small and large pyramidal cells. Finally, the so-called cells of Martinotti occur as fusiform or triangular elements, whose distinguishing characteristic consists therein, that the nerve-process traverses the layer of the pyramidal cells to reach the molecular stratum, where it ends.
Regarding the disposition of the thicker or thinner parallel bundles of fibres enter the cortex from the white substance, proceed towards the periph ery, and, gradually diminishing in thickness, towards the layer of the small pyramidal cells resolve into their component fibres. These bundles are known as the rays or radii and consist of the nerve-processes of the pyramidal and of the polymorphic cells, which are passing from the cortex, and of the nerve-fibres, which enter from the white substance and end within the cortex; these last are also called the terminal fibres. Between the individual medullary rays are narrow interspaces containing delicate horizon tally coursing fibres, which form the interradial feltwork. The latter are somewhat denser where the medullary radii break up into their individual fibres and thereby pro duce the stripe of Baillarger. The fibres of the interradial feltwork are the collaterals of the nerve-processes of the pyramidal cells. Towards the periphery, beyond the inter radial feltwork where the radii resolve into their component fibres, lies the supraradial feltwork, which marks the ending of the terminal fibres and, farther outward, joins the layer of tangential fibres.
The cerebral cortex does not, however, present the same structure in all regions. Local variations occur, in relation to the arrangement of the several cell-layers, as well as in regard to the behavior of the fibre-layers. There exists a tyto- and a myclo architectonic differentiation, the recent investigations of Brodmann and of Vogt having shown that the entire cerebral cortex may be subdivided into numerous histologically different cortical fields. While it is impracticable here to discuss in detail the differ ences, Fig. 113 presents these relations, so far as the make-up of the cell-layers is con cerned, in the precentral and postcentral convolutions and in the auditory and visual cortical areas, according to the earlier studies of Ramon y Cajal. The preponderance of the large and giant pyramidal cells in the precentral convolution is to be noted in contrast to the peculiar structure of the visual cortex, in which the original six-layered type is transformed into one of nine layers, by the introduction of special layers of stellate cells (Fig. 113, D 4 and 5).