156 THE HUMAN BODY. 



Plexuses. Very frequently several neighboring nerve- 

 trunks send off communicating branches to one another, 

 each branch carrying fibres from one trunk to the other. 

 Such networks are called plexuses (Fig. 65*), and through 

 the interchanges taking place in them it often happens 

 that the distal branches of w nerve-trunk contain fibres 

 which it does not possess as it leaves the centre to which it 

 is connected. 



Nerve-Centres. The great majority of the nerves take 

 their origin from the brain and spinal cord, which together 

 form the great cerebro- spinal centre. Some, however, com- 

 mence 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 scattered centres is called a ganglion, and the 

 whole of them are arranged in three sets. A considerable 

 number of the largest are united directly to one another by 

 nerve-trunks, and also give off nerves to various organs, 

 especially to the blood-vessels and the viscera in the thoracic 

 and abdominal cavities. These ganglia and their branches 

 form the sympathetic nervous system, as distinguished from 

 the eerebro-spinal nervous system consisting of the brain 

 and spinal cord and the nerves springing from them. Of 

 the remaining ganglia some are connected with various 

 eerebro-spinal trunks near their origin, while the rest, for 

 the most part very small and connected with the peripheral 

 branches of sympathetic or other nerves, are known as the 

 sporadic ganglia. 



The Cerebro-Spinal Centre and its Membranes. Ly- 

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

 vertebral column the spinal cord or spinal marrow, the 

 two being continuous through the foramen magnum of the 

 occipital bone and forming the great eerebro-spinal nerve- 

 centre. This centre is bilaterally symmetrical throughout 

 except for slight differences on the surfaces of parts of the 

 brain, which are often found in the higher races of mankind. 

 Both brain and spinal cord are very soft and easily crushed; 

 the connective tissue which pervades them being of the deli- 

 cate retiform variety; accordingly both are placed in nearly 



