THE CHIEF NERVE-CENTRES. 1 93 



there might be some sort of telegraph-system in the 

 body. If there were something like telegraph-wires 

 running from all the organs to a central office or ex- 

 change, then word of the state and needs of any organ 

 might be sent from it to the central office, and proper 

 messages be sent out from the central office to those 

 other organs whose help was wanted. This is in fact 

 something very like what does take place. 



If the dead body be dissected, a great many white 

 cords are found which run all through it, and go into 

 the skin, and the mucous membranes, and the heart, and 

 the lungs, and^each muscle, and so forth. These cords are 

 nerves. If one be followed back from where it enters any 

 of the above parts, it will be found at last to join a 

 much larger mass to which other nerves are also united. 

 This mass is a nerve-centre. The nerves and nerve- 

 centres together make the nervous system. The nerves 

 answer to the telegraph-wires, and the centres to the 

 main offices from which the wires spread over the 

 country. 



4. The Chief Nerve-Centres are the brain, the spinal 

 cord, and the sympathetic ganglia. You have already learned 

 that the brain lies inside the skull (p. 19), and the spinal 

 cord runs down inside the back-bone. At the under 

 part of the skull, where it fits on the back-bone, is a 

 large hole, through which the brain and spinal cord 

 unite. Strictly speaking, therefore, the brain and spinal 

 cord make only one centre; they are often spoken of 



might occur to us ? What really does take place ? What are nerves ? 

 What is found when a nerve is traced back from a muscle or the skin ? 

 Name of the mass? Of what does the nervous system consist? To 

 what are nerves and nerve-centres compared ? 



4. What are the chief nerve-centres? Where does each lie? HOW 

 do they join ? What is the cerebro-spinal centre ? 



