284 THE HUMAN BODY. 



Nerve-centres give origin to nerve-trunks ; these radiate 

 all over the body, branching and becoming smaller and 

 smaller as they proceed from the centre ; finally they end 

 in or among the cells and fibres of the various organs. 

 The general arrangement of the nerve-centres and of the 

 larger nerve-trunks of the body is shown in Fig. 79. 



The Main Nerve-Centres, The great majority of the 

 nerve-trunks take their origin from the brain and spinal 

 cord, which together form the great cerebro-spinal centre. 

 Some nerves, however, commence in rounded or oval 

 masses, which vary in size from that of the kernel of an 

 almond down to microscopic dimensions, and which are 

 widely distributed in the body. Each of these smaller 

 centres is called a ganglion. A considerable number of the 

 largest ganglia are united directly to one another by nerve- 

 trunks, and give off nerves especially to blood-vessels and 

 to the organs in the thoracic and abdominal cavities. These 

 ganglia and their branches form the sympathetic nervous 

 system (Figs. 1 and 2), as distinguished from the cerebro- 

 spinal nervous system, consisting of the brain and spinal 

 cord and the nerves proceeding from and to them. 



The Cerebro-Spinal Centre and its Membranes, Lying 

 in the skull is the brain, and in the neural canal of the 

 vertebral column the spinal cord or spinal marroiv, 

 the two being continuous through the foramen magnum 



To what do nerve-centres give origin? What becomes of nerve- 

 trunks? 



From what organs do most nerves arise? What is meant by the 

 cerebro-spinal centre? From what do those nerves arise which are 

 not directly connected with brain and spinal cord? What are the 

 smaller nerve-centres named? What is the sympathetic nervous sys, 

 tern? Where is the brain placed? What organ lies in the neural canal 

 of the backbone? Through what opening do brain and spinal core} 



