GENEKAL, COMPOSITIOK OF THE NERVOUS SYSTEM 385 



commimicated to the others. Some central system of con- 

 trol must also exist, in order that the various activities of 

 the body may be directed by us. The system of organs 

 in our body by means of which this direction and control 

 are exercised is spoken of collectively as the nervous system. 

 The relation of this system to voluntary and involuntary 

 actions and the structure of the parts will be considered 

 in the following paragraphs. 



General composition of the nervous system. — The ner- 

 vous system may be said to consist of three parts: nerve 

 centers, nerves, and nerve-end organs. The nerve centers 

 in our bodies are the brain, the spinal cord, and little 

 patches of nervous matter scattered in different parts of 

 the body, called ganglia. The nerves arise from these 

 centers, and with their branches penetrate all organs and 

 tissues of the body. Finally, each nerve ends in some 

 form of organ called an end organ. Some of these organs 

 are very simple in structure; others, such as those at 

 the ends of the sight and hearing nerves, are exceedingly 

 complex. 



For convenience, the cord and brain, together with the 

 nerves which arise in them, are spoken of as the central 

 nervous system, while the sets of ganglia on each side of 

 the spinal column, with their nerve connections, are called 

 the sympathetic system. All nervous centers are, how- 

 ever, in communication, and these two systems are sepa- 

 rated in name for convenience of study, and not because 

 of any true separation in the systems. 



In general, then, the brain and cord may be likened to 

 the "central" of a telephone system, the nerves to the 

 wires, and the end organs to the individual telephones. 

 Such an arrangement makes it necessary that all communi- 



EDST. PHTS. — 25 



