THE CEREBAL HEMISPHERES 



953 



I 



MOLECULAR LAYER 



ECTAL POLYMOR- 

 PHOUS CELL LAYER 



14- 



LAYER OF SMALL 

 PYRAMIDAL CELLS 



course. The cells, of various sixes and shapes, together with their clendrites and axones, are 

 embedded in a matrix of neuroglia. 



The nerve cells in a typical section of the cortex are arranged in five tangential layers, as fol- 

 lows: (1) the molecular layer; (2) the ectal polymorphous cell layer; (3) the layer of small 

 pyramidal cells; (4) the layer of large pyramidal cells; (5) the ental polymorphous cell layer. 



The molecular layer (neuroglia layer) lies imme- 

 diately subjacent to the pia, and is chiefly made up 

 of glia cells and fibres, among which the dendrites of 

 the subjacent layer of cells intrude. 



The ectal polymorphous layer cells are polygonal, 

 triangular, and fusiform in shape, and tend to gather 

 in groups in certain cortical regions. The fusiform 

 cells are placed with their long axes parallel (i. e., 

 tangential) to the gyral surface and are presumably 

 associative in function. 



The Layer of Small and the Layer of Large 

 Pyramidal Cells. The cells in the second and third 

 layers may be studied together, since, with the excep- 

 tion 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 pig- 

 ment, 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 axone, and this passes into the central white 

 substance, giving off collaterals in its course to be 

 distributed as a projection, commissural, or asso- 

 ciation fibre. Both the apical and basal parts of 

 the cell give off dendrites. The apical dendrite is 

 directed toward the surface, and ends in the molec- 

 ular layer by dividing into numerous branches, all 

 of which may be seen to be studded with projecting 

 bristle-like processes when prepared by the silver or 

 methylene-blue method. The larger pyramidal cells, 

 especially in the precentral gyre, may exceed 50 ft 

 in length and 40 /* in breadth, and are termed giant 

 cells. The chief function of the small pyramidal 

 cells is commissural and associative. The chief 

 function of the large pyramidal cells in the pre- 

 central and paracentral cortex is motor, but they 

 have also coinmissural and associative functions. 



Layer of Ental Polymorphous Cells. The cells 

 in this layer, as their name implies, vary greatly in 

 contour, the commonest varieties being of a spindle, 

 star, oval, or triangular shape. Their dendrites are 

 directed outward, toward, but do not reach, the 

 molecular layer; their axones pass into the subjacent 

 white substance. From this layer come commissural 

 fibres, long association fibres, and some projection 

 fibres. 



There are two other kinds of cells in the cerebral 

 cortex, but their axones pass in a direction opposite 

 to that of the pyramidal and polymorphous cells, 

 among which they lie. They are: (a) the cells of 

 Golgi, the axones of which do not become myelin- 

 ated, but divide immediately after their origin into 



a large number of branches, which are directed toward the surface of the cortex; (6) the cells 

 of Martinotti, which are chiefly found in the polymorphous layer. Their dendrites are short, 

 and may have an ascending or descending course, while their axones pass out into the molec- 

 ular layer and form an extensive horizontal arborization. 



Nerve Fibres in the Cortex. These fill up a large part of the intervals between the cells. 

 Some of these fibres form fasciculi; some are isolated, and others are arranged in plexuses. 

 They may be myelinic or amyelinic, the latter comprising the axones of the smallest pyramidal 

 oells and the cells of Golgi. In their direction the fibres may be either transverse, the transverse 



til 



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m 



LAYER OF LARGE 

 PYRAMIDAL CELLS 



ENTAL POLYMOR- 

 PHOUS CELL LAYER 



FIG. 



707. Typical arrangement of the cell 

 layers in the cerebral cortex. 



