970 



THE NERVE SYSTEM 



On either side of the superior longitudinal sinus, especially near its middle, and also near the 

 lateral and straight sinuses, are numerous spaces in the dura which communicate with the sinus, 

 either by a small opening or a distinct venous channel. These spaces are the parasinoidal sinuses 

 (lacunae laterales) (Fig. 720). Many of the meningeal veins do not open directly into the 

 sinuses, but indirectly through the parasinoidal sinuses. These venous lacunae are often invagi- 

 nated by arachnoid villi, and they communicate with the underlying cerebral veins, and also 

 with the diploic and emissary veins. 



The nerves of the dura are filaments from the trochlear, the ophthalmic division of the tri- 

 geminal, the semilunar or Gasserian ganglion, the vagus, the hypoglossal, and the sympa- 

 thetic. 



The Arachnoid (Arachnoidea Encephali) (Fig. 721). 



The cranial arachnoid is a delicate membrane which envelops the brain, lying 

 between the pia internally and the dura externally; from this latter membrane 

 it is separated by a very fine slit or space, the subdural space (cavum subdurale). 

 The subdural space contains a very minute quantity of fluid of the nature of lymph. 



OLFACTORY 

 NERVE 



VERTEBRAL 

 ARTERY 



SPINAL 

 ARACHNOID 



FIG. 721. The arachnoid upon the base of the brain. On the right the arachnoid has been partly removed 

 to show the cerebrum and cerebellum with their superficial veins. (Poirier and Charpy.) 



It is prolonged upon emerging nerves and joins the lymph spaces of the nerves. 

 The subdural space does not communicate with the subarachnoid space. 



_The arachnoid invests the brain loosely, being separated from direct contact 

 with the cerebral substance by the pia, and a quantity of loose areolar tissue, the 

 subarachnoidean areolar tissue. On the upper surface of the cerebrum the arachnoid 



