258 - NERVOUS SYSTEM. 



whicli act as nerve-centres, receiving impressions and gen- 

 erating the so-called nerve-force. The gray matter of the 

 spinal cord also serves, to a greater or less extent, as a con- 

 ductor. 



The cerebro-spinal axis is enveloped in membranes, for 

 its protection and for the support of its nutrient vessels. It 

 is surrounded, to a certain extent, with liquid, and presents 

 cavities, as the ventricles of the brain and the central canal 

 of the cord, which contain liquid. The gray matter is dis- 

 tinct from the white, even to the naked eye. In the spinal 

 cord, the white substance is external and the gray is internal. 

 The surface of the brain presents an external layer of gray 

 matter, the white substance being internal. In the white 

 substance of the brain, also, we find collections of gray 

 matter. As we should expect from the similarity in func- 

 tion between the white matter and the nerves, this por- 

 tion 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. In 

 the human subject and in many of the higher animals, its 

 surface is marked by numerous convolutions, by which the 

 extent of its gray substance is very much increased. The 

 cerebrum, the cerebellum, and all of the encephalic ganglia 

 are connected with the white substance, and are contin- 

 uous with the spinal cord. With the encephalon and the 

 cord, all of the cerebro-spinal nerves are connected. The 

 cerebro-spinal axis acts as a conductor, and its different col- 

 lections of gray matter, or ganglia receive impressions con- 

 veyed by the sensory conducting fibres, and generate nerve- 

 force, which is 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, fibrous 



