THE NERVOUS SYSTEM 163 



Nerve-substance is of two distinctly different kinds as may 

 be seen with the naked eye when a cross section of the spinal- 

 cord or brain is made. The white matter, which is composed 

 mostly of nerve-fibers closely packed in parallel bundles, is soft 

 and contains about 70 per cent, of water; the gray matter which 

 consists largely of nerve-cells, is brown in color and of even softer 

 consistency than the white substance, containing over 80 per 

 cent, of water. There is a decided difference in opinion among 

 histologists in regard to the finer features of the gray matter. 



THE ORGANS OF THE NERVOUS SYSTEM 



The organs comprising the nervous system may be arranged 

 under the following subsystems: (1) the central nervous system, 

 comprising the brain and spinal cord which are encased by bones; 



(2) the -peripheral nervous system, comprising the 12 pairs of 

 cranial and forty-two pairs of spinal nerves with their ganglia 

 which are distributed throughout the various organs of the body; 



(3) the sympathetic nervous system, comprising two chains of 

 ganglionated nerve cords extending on either side of the lower 

 surface of the thoracic vertebrae and their various branches to 

 all the soft organs of the large body cavities. These three 

 systems of nerves have intimate relations with each other by 

 means of connecting fibers. 



The brain is the chief organ of the central nervous system 

 (Fig. 53) . It is located in the cranial cavity, and is composed of 

 the cerebrum, the cerebellum, the pons, and the medulla ob- 

 longata. Completely enveloping these parts and continued 

 onto the spinal cord, with which the brain is connected, are three 

 membranes or meninges (Fig. 54). 



The dura mater is the most external of these membranes. It 

 is a thick, tough, fibrous covering, bluish white in color. It 

 lines the cranial cavity and extends backward to form a tube- 

 like sheath for the spinal cord. 



The arachnoid is the middle coat. It is a delicate, serous mem- 

 brane. Between its two layers is a space containing the cerebro- 

 spinal fluid. This fluid acts as a sort of water-jacket to take jar 

 and pressure off the sensitive nerve structures which it encloses. 



The pia mater closely covers and adheres to both the brain 

 and spinal cord. It contains numerous small blood-vessels for 

 these structures. 



Digitized by Microsoft® 



