GRAY MATTER OF THE CORTEX. 



679 



Molecular layer 



Layer of small 

 pyramidal cells. 



Layer of large 

 pyramidal cells. 



Nerve-cells. According to Cajal, "whose description is now generally accepted, 

 the nerve-cells are arranged in four layers, named from the surface inward as 

 follows : (1) the molecular layer ; (2) the layer of small pyramidal cells ; (3) the 

 layer of large pyramidal cells ; (4) the layer of polymorphous cells. 



The Molecular Layer. In this layer the cells are polygonal, triangular, or fusi- 

 form in shape. Each polygonal cell gives off some four or five dendrites, while 

 its axon may arise directly from the cell or from one of its dendrites. The axons 

 and dendrites of these cells ramify in the molecular layer. Each triangular cell 

 gives off two or three dendrites, from one of which the axon arises, the dendrites 

 and the axon ramifying in the mole- 

 cular layer. The fusiform cells are 

 placed with their long axes parallel to 

 the surface and are mostly bipolar, 

 each pole being prolonged into a 

 dendrite, which runs horizontally for 

 some distance and furnishes ascending 

 branches. Their axons, two or three 

 in number, arise from the dendrites, 

 and, like them, take a horizontal course, 

 giving off numerous ascending collat- 

 erals. The distribution of the axons 

 and dendrites of all three sets of cells 

 is limited to the molecular layer. 



The Layer of Small and the Layer 

 of Large Pyramidal Cells. The cells 

 in these two layers may be studied 

 together, since, with the exception of 

 the difference in size and the more 

 superficial position of the smaller cells, 

 they resemble each other. The body 

 of each cell is pyramidal in shape, its 

 base being directed to the deeper parts 

 and its apex toward the surface. It 

 contains granular pigment, and stains 

 deeply with ordinary reagents. The 

 nucleus is nucleolated, of large size, 

 and round or oval in shape. The base 

 of the cell gives off the axis-cylinder, 

 and this passes into the central white 

 substance, giving off collaterals in its 

 course, and is distributed as a projec- 

 tion, commissural, or association fibre. 

 Both the apical and basal parts of the 



cell give off dendrites. The apical dendrite is directed toward the surface, a 

 ends in the molecular layer by dividing into numerous branches, all of which mav 

 be seen, when prepared by the silver or methylene-blue method, to be studied with 

 projecting bristle-like processes. The larger pyramidal cells, especially in the 

 Rolandic area, may exceed 50 // in length and 40 p in breadth, and are termed 



giant cells. . r 



Layer of Polymorphous Cells. The cells in this layer, as their name implies, 

 are very irregular in contour, the commonest varieties being of a spindle, s 

 oval, or triangular shape. Their dendrites are directed outward, toward, but 

 not reach, the molecular layer; their axons pass into the subjacent whit< 



There are two other kinds of cells in the cerebral cortex, but their axons pas 

 in a direction opposite to that of the pyramidal and polymorphous cells, amor 

 which they lie. They are: (a) the cells of Golgi, the axons of which do i 

 become medullated, but divide immediately after their origin into a large numb* 



Polymorphous 

 layer. 



White medullary 

 substance. 



FIG 361 The four layers of cells in the cerebral cor- 

 tex. (After Cajal.) Modified from Testut. 



