Subiculum Hippocampi, the Lower Wall of Hippocampal Sulcus (Fig. 85).—This is known as the subiculum. It is especially distinguished for its remarkable stratum zonale, which is visible to the naked eye, and for its long radiations, which reach the zonal layer and give the cortex a striated appearance. (I) The plexiform layer is almost wholly occupied by the stratum zonale, called here the external medullary lamina. (2) The layer of olfactory islets. The islets are closely packed nests of minute triangular cells, 5 1.1 in diameter, resembling those in the uncus. (3) The stratum radiatum occupies about three-fourths of the depth of this cortex. In its deep part (the stratum lucidum) there are several layers of medium-sized pyramids, arranged in columns. The prominent apical processes of these pyramids collect in bundles and proceed outward to the stratum zonale, separating the columns of pyramids and producing the striations above mentioned. As the apical dendrites approach the olfac tory islets they branch richly. The axones of the pyramids run straight to the white core of the gyrus or into the alveus. The pyramids continue without interruption through the hippo campus into the nucleus of the dentate fascia. The alveus, which forms the ventricular surface of the hippocampus, is made up largely of the axones of these pyramids; from the alveus they proceed into the crus of the fornix. (4) A few fusi form or stellate cells lie next the alveus. They belong to the type of Golgi, the axone being wonderfully branched. In function they are associative. It is in the region of these associative neurones that the axones of the pyramids bend and adjust themselves so as to enter the alveus nearly parallel with its surface, hence the name stratum oriens applied to it by Edinger.
The fascia dentata (Fig. 85) is a free lip of cortex facing inward anterior to the hippocampal sulcus. It presents a type of struc ture, which is continued forward through the pars transversa into the reflected part of the uncus; and which extends back ward through the fasciola cinerea and gyrus subsplenialis into the gyrus supracallosus. It is similar in structure to the sub iculum, the first and the third layers only present a marked vari ation. The stratum zonale is not so prominent as in the sub iculum; and the stratum radiatum is entirely replaced by the nucleus fascim dentatx. The nucleus is composed of pyramids Qf polymorphous and fusiform cells and their branches. Their dendrites radiate toward the stratum zonale, their axones pro ceed into the crus of the fornix. The dentate fascia is absent in anosmatic animals (A. Hill) (Fig. 41).
Cortex Tractus Olfactorii, Trigonum Olfactorium, Gyrus Sub callosus, Septum Pellucidum, and Substantia Perforata Anterior (Figs. 31 and 34).—These are the parts into which run the fibers of the olfactory tract and out of which grow the olfactory striae. They are more conspicuous in the embryo than in the adult human brain. The cortex of this whole region is so retro
gressive as to require but brief description. The plexiform layer may be identified. The whole gray substance beneath that is occupied by scattered pyramids of medium size, separated by strands of fibers belonging to the olfactory tract and strim and, perhaps, to the cingulum.
The cortex of the gyrus cinguli (Fig. 34) is characterized by an entire absence of large fibers and large cells, by an oblique and irregular direction of the pyramids and by a most remarkable color affinity possessed by the deep cells. There are only four cell layers. (r) The plexiform presents a faint stratum zonale but nothing characteristic. (2) The layer of small pyramids is ill defined. (3) A layer of medium-sized pyramids placed at various angles occupies the place of the third, fourth, fifth and sixth layers of typical cortex. (4) The layer of spindle cells. In the spindle-cell layer are found the remarkable chromophilous cells. They are triangular or pyramidal in shape and have greater affinity for stains than the cells of any other part of the cerebral cortex.
The claustrum (Figs. 38 and 54) is a sheet of peculiar gray substance which, according to Meynert, may be classed as cortical. In structure it resembles the seventh layer of typical cortex, being made up of fusiform cell-bodies. The claustrum is a vertical antero-posterior sheet placed medial to the island, and lateral to the external capthile. The surface in contact with the external capsule is smooth, but the external surface is convoluted to coincide with the gyri insulae. At its lower border it joins the lentiform nucleus and anterior perforated substance.
The histogenesis of the cerebral cortex is not entirely clear. It develops from within outward. It evolves earliest in the anterior and posterior central gyri, the motor and sensory areas. Its development in the special sense regions follows, being later in the psychic sensory than in the receptive sensory; while in the prefrontal region (higher psychic) the cortex is differentiated last of all.
The lamince are laid down in the central gyri in the i8 weeks' fcetus studied by J. S. Bolton; but all neurones are embryonic except the Betz-cells. Laminae may also be seen, with some difficulty, in the anterior part of the visuo-sensory cortex, where the outer line of Baillarger (line of Gennari) divides the cortex into two zones of very embryonic cells. At the same time (i8 weeks) the prefrontal region (higher psychic) is crowded with undeveloped neuroblasts diffusely arranged; no lamination is evident there until the sixth month and the cortex is one half the normal thickness (Brain, Vol. 35).
According to J. S. Bolton the cortex evolves in three primary cell-layers; an inner, a middle and an outer and two fiber layers; an inner fiber layer separating the inner and middle cell-layers, and an outer fiber layer, the stratum zonale which lies next the surface (Brain, Vol. 33).