GRAY MATTER OF THE CEREBRAL CORTEX. 211 



many fine processes which interlace with one another to 

 form a network. Some glia-cells, the "spider-cells," have 

 long, fine unbranched processes ; others, the " mossy cells," 

 have shorter, thicker, and branching processes (Figs. 80, 81). 

 A homogeneous intercellular cement-substance is perhaps also 

 present, contributing to the sustentacular framework. 



The cerebro-spinal meninges are the dura mater, arachnoid, 

 and pia mater. 



The dura mater is a dense fibrous membrane forming the 

 outer covering of the brain and spinal cord ; its inner surface 

 is lined with endothelium. Within the cranium it is closely 

 applied to the cranial bones, forming their inner periosteum. 



The arachnoid is a delicate connective-tissue lamina and 

 reticulum. Externally it presents a continuous free surface 

 lined with endothelium ; this surface is opposed to the dura 

 and separated from it by a large lymph-space, the subdural 

 space. The deeper portion of the arachnoid is a network of 

 delicate fibrous lamina? and cords, lined on their free surfaces 

 with endothelium ; this network is connected and continuous 

 with the pia mater, and the cavities enclosed in it form a 

 system of communicating lymph-spaces, called the subarach- 

 noid space. 



The pia mater intimately invests and follows the surface of 

 the brain and cord ; it consists of delicate vascular connective 

 tissue, often containing irregular pigment-cells. At intervals 

 it sends supporting fibrous trabecula? inward into the sub- 

 stance of the brain and cord. 



The Cerebrum. 



The cerebrum consists of aggregations of white and gray 

 matter. 



The ivhite matter is composed of radiating masses of medul- 

 lated nerve-fibres, and occupies the interior and central por- 

 tions of the organ. 



The gray matter, containing the nerve-cells, covers the sur- 

 face of the cerebrum and also occurs in the central and basal 

 ganglion ic tracts. 



The gray matter of the cerebral cortex in general (though 

 with variations in some situations) exhibits the following 



