THE NERVOUS SYSTEM. 113 



nearly fill up the canal. White and gray matter make up 

 the cord, as in the brain, but the white matter is placed 

 outside and the gray within. It commences at the large 

 opening at the base of the skull, the part above this being 

 the medulla oblongata. 



The spinal cord is composed of two lateral halves, formed 

 by a fissure in front and behind. From each half nerves 

 branch off, and dividing and sub-dividing, are distributed 

 to the trunk and limbs of the body. 



Surrounding the cord are membranes similar to those of 

 the brain, only the outer membrane is not attached to the 

 bone, as in the brain. Such attachment would prevent the 

 several motions of the spine. This outer membrane of the 

 spinal cord is a loose sheath, attached at the top and at 

 intervals to the bony walls. Within this sheath is the 

 cerebro-spinal fluid, so that the cord is virtually suspended 

 in a flexible tube of fluid, and is thus protected from injury 

 or shock. 



11. The Nerves. Running throughout the body every- 

 where can be found slender, white, glistening cords. These 

 are the nerves. They look somewhat like the tendons of the 

 muscles, only smaller. They are not so tough and strong. 

 A nerve trunk is made up of a number of fibres running side 

 by side, like the threads in a skein of silk. These fibres are 

 individual nerves, which, bound for the same locality, are 

 held together by connective tissue in a single trunk. 



Besides the nerves which come off from the brain and 

 spinal cord, there are others which have their centre in knots 

 of nerve tissue scattered through the body, called ganglia. 

 These are all connected together and form the sympathetic 

 system. There are therefore three classes of nerves, spinal, 

 cranial and sympathetic. All three classes convey impres- 

 sions of a twofold kind. There are those fibres along which 

 impressions travel to the brain, and by which it is made 

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