1442 



A MANUAL OF ANATOMY 



layer of small pyramidal cells; (3) the layer of large pyramidal 

 cells ; and (4) the layer of polymorphous cells. 



The molecular or plexiform layer, which is the most superficial, 

 is thin and consists of cells and fibres. Many of the cells are 

 neuroglia-cells, the others being nerve-cells. These nerve-cells are 

 for the most part fusiform, and are disposed horizontally. They 

 are known as the horizontal cells of Cajal. Their dendrons and axons 

 are long, the latter forming meduUated fibres which are disposed 

 horizontally or parallel to the surface. These furnish minute 

 branches which pass vertically towards the surface. The horizontal 

 cells, according to Cajal, receive impulses from the corticipetal 

 fibres which extend from the optic thalamus to the cerebral cortex. 



In addition to these 

 Terminal Ramifications fibres there are many 



others which enter the 

 molecular layer from 

 deeper sources, and 

 form a dense interlace- 

 ment by their ramifi- 

 cations. The sources 

 from which these ex- 

 traneous fibres are de- 

 rived are as follows: 

 (i) the terminal rami- 

 fications of the apical 

 dendrons of the pyra- 

 midal cells (small and 

 large) ; (2) the axons 

 of the cells of Martin- 

 otti; and (3) corticipe- 

 tal fibres derived from 

 the medullary centre 

 of the gyrus. 



The pyramidal layers 

 represent the second 

 and third layers, and 

 are composed of 

 characteristic pyramidal cells which are peculiar to the cerebral 

 cortex, those of the second layer being small, whilst those of the 

 third layer are large. The layer of small pyramidal cells is narrow, 

 but the layer of large pyramidal cells is of considerable thickness. 

 There is no well-marked line of demarcation between these two 

 layers, the one passing imperceptibly into the other. They con- 

 stitute the chief part of the cerebral cortex. 



The apex of each pyramidal cell is directed towards the surface 

 of the gyrus, and is prolonged into a long tapering dendrite, which 

 passes into the molecular layer, giving off delicate collaterals in its 

 course. Near the surface of the molecular layer it divides into 

 terminal filaments, which are disposed horizontally and mingle with 



Apical Dendrite, giving 



off Collaterals 



^-^— Body of Cell 



L/L Lateral Dendrites 



..Axon, giving off Collaterals 



Fig. 605. — A Pyramidal Cell of the Human 

 Cerebral Cortex (Ramon y Cajal). 



