236 THE NERVOUS SYSTEM 



and the medulla oblongata. These different parts are con- 

 nected 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 purpose of receiving impressions and generating 

 nerve force. 



Membranes. The encephalon and cord are covered by mem- 

 branes for protection and for the support of vessels belonging 

 thereto. These are (i) the dura matter, (2) the arachnoid 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 cere- 

 brum (falx cerebri), between the cerebrum and cerebellum (ten- 

 torium 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 covers 

 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 sub- 

 arachnoid fluid. This fluid serves a mechanical purpose, equal- 

 izing pressure in different parts of the cerebro-spinal axis and 

 protecting the nervous substance from injury by concussion, etc. 

 Besides being found in the subarachnoid space, it occupies the 

 ventricles of the brain and the central canal of the cord, com- 

 munication between these being furnished by a small opening at 

 the inferior angle of the floor of the fourth ventricle. 



The pia mater is a very delicate structure dipping between the 

 convolutions of nervous matter and lying in close contact with 

 the external surface of the encephalon and cord. It is exceed- 



