SPINAL CORD. 587 



internal. In the white substance of the brain, also, are collections of gray 

 matter. The white matter of the cerebro-spinal axis is composed largely of 

 fibres. The gray substance is composed chiefly of cells. 



The encephalon is contained in the cranial cavity and consists of the 

 cerebrum, 'cerebellum, pons Varolii and medulla oblongata. In the human 

 subject and in many of the higher animals, its surface is marked by convo- 

 lutions, by which the extent of its gray substance is much increased. The 

 cerebrum, the cerebellum and most of the encephalic ganglia are connected 

 with the white substance of the encephalon and with the spinal cord. All 

 of the cerebro-spinal nerves are connected with the encephalon and the cord. 

 The cerebro-spinal axis acts as a conductor, and its different collections of 

 gray matter, or ganglia, receive impressions conveyed by the sensory conduct- 

 ing fibres, and generate motor impulses which are transmitted to the proper 

 organs by the motor fibres. 



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 two layers, 

 composed chiefly of ordinary fibrous tissue, which lines the cranial cavity 

 and is adherent to the bones. In certain situations its two layers are sepa- 

 rated and form what are known as the venous sinuses. The dura mater 

 also sends off folds or processes of its internal layer. One of these passes in- 

 to the longitudinal fissure and is called the f alx cerebri ; another lies between 

 the cerebrum and the cerebellum and is called the tentorium ; another is sit- 

 uated between the lateral halves of the cerebellum and is called the falx cere- 

 belli. 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, this membrane is not ad- 

 herent 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 arachnoid is a delicate membrane, resembling the serous membranes, 

 with the exception that it presents but one layer. Its inner surface is cov- 

 ered with a layer of tesselated 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 cer- 

 ebro-spinal, or cephalo-rachidian fluid. The arachnoid does not follow the 

 convolutions and fissures of the encephalon or the fissures of the cord, but it 

 simply covers their surfaces. Magendie described a longitudinal, incom- 

 plete, 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 vas- 

 cular, seeming to present, indeed, only a skeleton net- work of fibres for 

 the support of the vessels going to the nervous substance. This membrane 

 covers the surface of the encephalon immediately, follows the sulci and fis- 



