236 THE NERVOUS SYSTEM 



of connected ganglia. In the cord the white matter is situ- 

 ated externally; in the brain the gray. The encephalon is 

 situated in the cranial cavity and consists of the cerebrum, 

 the cerebellum, the pons Varolii, and the medulla oblongata. 

 These different parts are connected with each other and 

 with the cord by nerve fibers, and all the cranial and spinal 

 nerves are connected with gray matter either in the brain or 

 in the cord, or in both. This gray matter exists for the pur- 

 pose of receiving impressions and generating nerve force. 



Membranes. The encephalon and cord are covered by 

 membranes for protection and for the support of vessels be- 

 longing thereto. These are (i) the dura mater, (2) the ar- 

 achnoid, and (3) the pia mater. 



The dura mater is a dense fibrous structure surrounding 

 the encephalon and adherent to the inner surfaces of the 

 cranial bones. At certain points .the two layers of which it 

 is composed separate to form the venous sinuses. Processes 

 of the internal layers also are sent inward between the two 

 lobes of the cerebrum (falx cerebri), between the cerebrum 

 and cerebellum (tentorium cerebelli) and between the lateral 

 halves of the cerebellum (falx cerebelli). This membrane 

 passes through the foramen magnum to cover also the spinal 

 cord, and to follow as a sheath the spinal nerves at their 

 foramina of exit. 



The arachnoid resembles the serous membranes: It cov- 

 ers the brain and cord underneath the dura' mater without 

 dipping into the sulci of the brain. Between it and the pia 

 mater is what is known as the subarachnoid space containing 

 the subarachnoid fluid. This fluid serves a mechanical pur- 

 pose, equalizing pressure in different parts of the cerebro- 

 spinal axis and protecting the nervous substance from in- 

 jury by concussion, etc. Besides being found in the subarach- 

 noid space, it occupies the ventricles of the brain and the 

 central canal of the cord, communication between these being 

 furnished by a small opening at the inferior angle of the 

 floor of the fourth ventricle. 



