THE CEREBRAL CORTEX. 417 



From without inward the following layers may be differentiated 

 in the cerebral cortex : (i) a molecular layer ; (2) a layer of small 

 pyramidal cells ; (3) a layer of large pyramidal cells ; (4) a layer 

 of polymorphous cells ; and (5) medullary substance or underlying 

 nerve-fibers. 



Aside from neurogliar tissue, we find in the molecular layer a 

 large number of nerve-fibers, which cross one another in all direc- 

 tions, but, as a whole, have a direction parallel with the surface of 

 the brain. Within this layer there are found : (i) the tuft-like telo- 

 dendria of the chief dendritic processes of the pyramidal cells ; (2) 

 the terminations of the ascending neuraxes, arising mostly from the 

 polymorphous cells ; and (3) autochthonous fibers /. e., those which 

 arise from the cells of the molecular layer and terminate in this 

 layer. The cells of the molecular layer may be classed in three 

 general types polygonal cells, spindle-shaped cells, and triangular 

 or stellate cells. The polygonal cells have from four to six den- 

 drites, which branch out into the molecular layer and may even 

 penetrate into the underlying layer of small pyramidal cells. Their 

 neuraxes originate either from the bodies of the cells or from one 

 of their dendrites, and take a horizontal or an oblique direction, 

 giving off in their course a large number of branching collaterals, 

 which terminate in knob-like thickenings. The spindle=shaped 

 cells give off from their long pointed ends dendrites which extend 

 for some distance parallel with the surface of the brain. These 

 branch, their offshoots leaving them at nearly right angles, the 

 majority passing upward, assuming as they go the characteristics 

 of neuraxes having collaterals. The arborization is entirely within 

 the molecular layer. The triangular or stellate cells are similar 

 to those just described, but possess not two, but three, dendrites. 

 The triangular and spindle-shaped cells, with their numerous den- 

 dritic processes resembling neuraxes, are characteristic of the cere- 

 bral cortex. 



The elements which are peculiar to the second and third layers 

 of the cerebral cortex are the small (about 10 // in diameter) and 

 large pyramidal cells (from 20 //to 30 , in diameter). They are 

 composed of a triangular body, the base of the triangle being down- 

 ward and parallel to the surface of the brain, of a chief, principal, or 

 primordial dendrite ascending toward the brain-surface, of several 

 basilar dendrites arising from the basal surface of the cell-body, and 

 of a neuraxis which passes toward the medullary substance and 

 which has its origin either from the base of the cell or from one of 

 the basilar dendrites. The ascending or chief dendrite gives off a 

 number of lateral offshoots which branch freely and end in terminal 

 filaments. The main stem of the dendrite extends upward to the 

 molecular layer, in which its final branches spread out in the form 

 of a tuft. The neuraxis, during its course to the white substance, 

 gives off in the gray substance from six to twelve collaterals, which 

 divide two or three times before terminating. 

 27 



