546 NERVOUS SYSTEM 



Membranes of the Encephalon and Spinal Cord. The membranes of 

 the brain and spinal cord are the dura mater, the arachnoid and the pia 

 mater. 



The dura mater of the encephalon is a dense membrane in a single 

 layer and is composed chiefly of ordinary fibrous tissue. That portion 

 which lines the cranial cavity is adherent to the bones. In certain situ- 

 ations it is separated into two layers and bounds what are known as 

 the venous sinuses. The dura mater also sends off certain folds or 

 processes. One of these passes into the longitudinal fissure and is 

 called the falx cerebri ; another lies between the cerebrum and the cere- 

 bellum and is called the tentorium ; another is situated between the 

 lateral halves of the cerebellum and is called the falx cerebelli. The 

 dura mater is closely attached to the bone at the border of the foramen 

 magnum. From this point it passes into the spinal canal and forms a 

 loose covering for the cord. In the spinal canal, the membrane is not 

 adherent to the bones, which have, like most other bones in the body, a 

 special periosteum. At the foramina of exit of the cranial and the 

 spinal nerves, the dura mater sends out processes which envelop the 

 nerves, with the fibrous sheaths of which they soon become continuous. 

 The subdural space contains a small quantity of liquid. 



The arachnoid is a delicate membrane, resembling the serous mem- 

 branes, except that it presents but one layer. Its inner surface is cov- 

 ered with a layer of endothelium. There is a considerable quantity of 

 liquid between the arachnoid and the pia mater surrounding the cerebro- 

 spinal axis, in what is called the subarachnoid space. This is called the 

 cerebro-spinal, or cephalo-rachidian liquid. The arachnoid does not 

 follow the convolutions and fissures of the encephalon or the fissures of 

 the cord, but it simply covers their surfaces. There is a longitudinal, 

 incomplete, cribriform, fibrous septum in the cord, passing from the 

 inner layer of the arachnoid to the pia mater. A similar arrangement 

 is found in certain situations at the base of the skull. 



The pia mater of the encephalon is a delicate fibrous structure, very 

 vascular, seeming to present, indeed, only a skeleton network of fibres 

 for the support of vessels going finally to the nervous substance. This 

 membrane covers the surface of the encephalon immediately, follows the 

 sulci and fissures, and is prolonged into the ventricles, where it forms 

 the choroid plexus and the velum interpositum. From its internal sur- 

 face small vessels are given off which pass into the nervous substance. 



The pia mater of the encephalon is continuous with the correspond- 

 ing membrane of the cord ; but in the spinal canal the membrane is 

 thicker, stronger, more closely adherent to the subjacent parts, and its 

 bloodvessels are not so abundant. In this situation many of the fibres 



