256 THE ESSENTIALS OF HISTOLOGY. 



adherent to the under surface of the bony cavity than is the case in 

 the vertebral canal. The arachnoid is in many places close to the dura 

 mater, and separated by a wide subarachnoid space, which is bridged 

 across by finely reticulating bands of areolar tissue (subarachnoid 

 trabeculae) from the pia mater. In the vicinity of the longitudinal 

 sinus, small rounded elevations (arachnoidal villi, Pacchionian glands) 

 project into the dura mater, and even become embedded in the skull 

 itself. The pia mater is closely adherent to the surface of the brain, 

 and dips into all the sulci, but without forming actual folds (Tuke). 

 Jn it the blood-vessels ramify before passing into the substance of the 

 brain, and they are accompanied, as they thus enter the cerebral sub- 

 stance, by prolongations of the pia mater, which do not, however, 

 closely invest them, but leave a clear space around each vessel, presum- 

 ably for the passage of lymph (perivascular space). The capillary net- 

 work is much closer in the grey than in the white matter. 



