2. The layer of small pyramids (Figs. 8o and 82), as well as the third layer, is situated in the supraradiary zone. It is com posed chiefly of small closely packed cell-bodies, pyramidal in shape. They measure eight to ten microns in diameter. Their apices point toward the surface. From the apices, surfaces and lateral angles, dendrites are given off which ramify in the stratum zonale of the first layer. The axone issues from the base of the pyramid and runs down through the subjacent layers. Among the small pyramids are a few polymorphous cells. There are a few large and small-second type cells and Martinotti cells.
The axones of the small second-type cells form very rich arbori zations which extend into the stratum zonale. The T-branched axones of Martinotti's cells also ramify in this stratum (see below).
3. The layer of medium-sized pyramids (Fig. 8o) is a nearly pure layer; and, like the overlying layer, is nearly uniform throughout the cerebral cortex. In arrangement of cell-bodies and processes it is like the second layer. The pyramids get farther apart and become larger in size as the layer is descended. They measure ten to fifteen microns in their vertical diameter. Besides the pyramids, there are second-type cells with exceed ingly rich dendritic and axonic processes, the double-brush cells of Cajal, whose branches associate the cells of the first four layers of cortex. Basket cells are also present. Their axones run tangentially and give off collaterals that form pericellular baskets around the external large pyramids. The faint super radiary line (J. S. Bolton) is at the level of the medium-sized pyramids. Layers "two" and "three" might be combined in one as was formerly the custom.
4. External Layer of Large Pyramids (Figs. 8o and 8i).—This layer coincides in position with the outer line of Baillarger. The pyramids are larger and farther apart than in the above layer, and show a considerable accession of Nissl bodies as compared with the smaller pyramids. They measure 15 to 20µ by 25 to 30 and form "one of the most important criteria in dividing the brain surface into different histological territories" (Camp bell). The apical processes appear to reach the first layer and ramify in the stratum zonale; the lateral and basal dendrites arborize within the outer line of Baillarger; the axone runs in ward to the white substance. The external large pyramids are found in nearly all parts of the cortex. Golgi, Cajal and Martinotti cells are intermingled with them. There are some peculiar Golgi cells among these pyramids whose axones run horizontally and give off collaterals that form pericellular bas kets around the pyramids (Johnston). They are associative in function. In the motor cortex, the external large pyramids show Nissl degeneration and later complete destruction in amyo trophic lateral sclerosis. They lie within the outer line of Baillarger. In visual cortex, along the calcarine fissure, the superficial large pyramids are replaced by large stellate cells, and the double zone of stellate cells (fourth and fifth layers) incloses the very thick outer line of Baillarger. Bolton locates
the inner line inferior to layer 5. There are two lines of Bail larger well shown in the superior parietal gyrus and in many other places; one in the fourth and the other in the sixth layer. The lines of Baillarger are very faint in motor and common sen sory cortex.
5. The layer of stellate or polymorphous cells presents great variation in different regions and is a valuable guide for cortical localization. It is a characteristic of all sensory cortex except olfactory. It is best marked in the calcarine area (Fig. 83) where it and the fourth layer are associated with the greatly thickened outer Baillargic line (Gennari). It is well marked in the auditory cortex. In the motor area it is hardly distinguish able, but it appears suddenly at the bottom of the central sulcus and forms a distinct feature in the common sensory region of the posterior central gyrus (Fig. 82). The cell-bodies of the stellate layer measure five to eight microns in diameter and are of many shapes, some being pyramidal. They are richly branched. The dendrites arborize at once near the cell-body. The axones of a considerable number of cells extend horizontally within the layer, while others run toward the surface and end in the overlying lamine.
6. Internal Layer of Large Pyramids (Fig. 80 .—This is the most important layer of the cortex for localization. It lies in the radiary zone of fibers and is present in almost every part of the cerebral cortex, though distinguished by definite variations in the different regions (Figs. 82, 83 and 84). Its great pyra mids are intermingled with irregular cell-bodies of the Golgi type and with Martinotti cells. They are loaded with Nissl bodies and give off one axone and many dendrites. The lateral and basal dendrites ramify in the inner line of Baillarger; the dendrite of the summit runs straight out toward the surface and, like the same process from other pyramids, arborizes in the stratum zonale of the plexiform layer; the axone enters the medullary substance and becomes a projection, association or commissural fiber. The giant pyramids (Betz), or ganglionic cells (Bevin Lewis), of the anterior central gyrus are "pyriform" in shape. The giant pyramids characterize the motor cortex. Nowhere else are they so large. Neither do they have in any other region the "pyriform" shape or the definite nest-like grouping seen in the anterior central gyrus. They measure 25 by 6o microns in the leg area of that gyrus; 20 by 45 microns in the arm area; and in the head area 17 by 35 microns (Bevin Lewis). In cases of amyotrophic lateral sclerosis studied by Campbell, 87.5 per cent. of these cells in the affected area were entirely destroyed and those remaining showed signs of degeneration. According to Holmes and May, the pyriform pyramidal cells in the sixth layer give entire origin to the pyra midal tracts (the cerebrospinal fasciculi) and their axones enter no other systems (Brain, Vol. 32).