160 THE HUMAN BODY. 



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 num- 

 ber of the largest are united directly to one another by 

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

 'cially 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 

 cerebro-spinal nervous system consisting of the brain and 

 spinal cord and the nerves springing from them. Of the re- 

 maining ganglia some are connected with various cerebro- 

 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. Lying 

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

 vertebral column the xpinal cord or spinal marrow, the cwo 

 being continuous through the foramen magnum of the oc- 

 cipital oone and forming the great cerebro-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 con- 

 nective tissue and a peculiar supporting tissue (neuroglia) 

 which pervade them being delicate; accordingly both organs 

 are placed in nearly completely closed bony cavities and are 

 also enveloped by membranes which give them support. These 

 membranes are three in number. Externally is the dura 

 mater, very tough and strong and composed of white fibrous 

 and elastic connective tissues. In the cranium the dura 

 mater adheres by its outer surface to the inside of the skull 

 chamber, serving as the periosteum of its bones; this is 

 not the case in the vertebral column, where the dura mater 

 forms a loose sheath around the spinal cord and is only at- 

 tached here and there to the surrounding bones, which have 

 a separate periosteum of their own. The innermost membrane 

 of the cerebro-spinal centre, lying in immediate contact with 

 the proper nervous parts, is the pia mater, also made up of 

 white fibrous tissue interwoven with elastic fibres, but less 



