ANATOMY, PHYSIOLOGY, OF NERVOUS SYSTEM. 411 



these in turn to still others, and so there would be produced 

 networks of fibers. These networks are called plexuses. 

 It is by means of these plexuses that an individual nerve 

 fibril of one trunk may find its way into other trunks and so 

 reach removed parts of the body. This makes it possible 

 for a nerve trunk to contain near its end fibers which it did 

 not have at its beginning, but which reached it along its 

 course. While such plexuses are very generally distributed 

 all over the body there are peculiarly large ones in the 

 shoulder and lumbar regions, forming respectively the spinal 

 nerve plexuses that go to the arms and limbs. 



2. Nerve Centers or Ganglia. When the course of a 

 nerve is followed inward it is soon found to end either in the 

 brain, spinal cord, sympathetic system, or in isolated gan- 

 glia over the body. All these structures named are nerve 

 centers; that is, they are nervous centers from which nerve 

 fibers arise. These nerve centers may be large, as many of 

 those in the brain, or they may be small aggregations of 

 nervous tissue like the individual centers of the sympa- 

 thetic system. Centers lying more or less separate and 

 having a distinct outline are called ganglia. 



A ganglion is in essence nothing more than a group of 

 nerve cells with which the nerve fibers entering the gan- 

 glion are physiologically connected. These groups of nerve 

 cells are of course usually surrounded with a more or less 

 complete coat of connective tissue intended for protection. 

 While anatomically this description suits all ganglia, phy- 

 siologically there are distinct kinds. 



From this it will be seen that the entire nervous system 

 is composed of nerve cells more or less grouped into dis- 

 tinct ganglia and of nerve fibers which run out from these 

 ganglia connecting them with each other, or connecting the 

 ganglia with distant glands, muscles, skin, etc. A detailed 

 histological description of these nerve fibers and their 

 coats, and of the nerve ganglia and their aggregation in 

 turn into larger centers is reserved for the chapter on the 



