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CHAPTER VIII. 



DISEASES OF THE NERVOUS SYSTEM. 



JUST as the arteries given off by the great aorta divide and subdivide, and go on dividing and 

 subdividing, like the branches of a great oak-tree, until they end in the minutest capillaries, 

 so minute that we cannot pierce the skin with the finest needle without thrusting the point of 

 it through some one or more of the ultimate ramifications of the largest artery in the body, so 

 also do the branches of the nerves that are given off by the brain or spinal cord subdivide, and 

 re-and-re-subdivide, until they end in branches more hair-like and tiny than the capillaries 

 themselves. More hair-like and tiny, because every capillary artery, or capillary vein, is supplied 

 with its nervelet. Again, as every artery is supplied with nerves, so does every nerve receive 

 blood from some artery. 



If we dissect any portion of the body, say the fore-arm, and search for the principal nerve, 

 we shall find it generally running alongside the principal vein and artery a simple-looking, 

 whitish cord, composed of some soft, fatty kind of matter, and not unlike a violin string ; and yet 

 that which you behold is one of the telegraph wires of life and mind. 



Each nerve contains, bound up in one sheath, two kinds of fibres, just as a thread might 

 be composed of silk and cotton. One set of fibres is called the motor, the other the sensory. 

 When we will do a particular action, say to clench the fist, the impression, the will, the wish, 

 or the command, is sent from the brain along the motor fibres to the flexor muscles of the 

 hand ; contraction is the result, the muscular fibres obey the order, shorten themselves, and 

 the fist is clenched. The sensory fibres, on the other hand, convey impressions to the 

 brain. If a dog bites one, he quickly draws away his hand ; but, quick as was the 

 motion, the impression of pain had first to be flashed upwards to the brain by the sensory 

 fibres, and another impression, the will, to be sent back again along the motor fibres, before 

 the person had the power to draw the hand away. Every nerve, then, is a kind of double 

 lectric wire. 



Now this set of nerves that we have just mentioned are called the nerves of voluntary 

 motion and sensation, because the will has power over them. But there is one other set of 

 nerves in the body, over which the mind has no direct power. These are called the involuntary 

 nerves, or the nerves of organic life, and they are even more important than the voluntary 

 nerves, because they regulate and keep going the inner wheels of life itself. They spring from a 

 row of ganglia, or nerve-knots, that lie alongside the spine. These ganglia seem, in fact, a whole 

 row of smaller nerve-centres, and they give their nerves to all the internal organs of the body, 

 keep up the peristaltic motion of the bowels, and the beating of the heart itself. They are of 

 all importance in the animal economy, and it is to a great extent this system of nerves that 

 suffer, when an animal is ill for want of properly oxygenated blood, or from bad feeding, or any 

 other cause which causes the health to deteriorate. 



When we have said that the nerves of motion and sensation and the nerves of organic life 

 are intimately connected, and that the one system cannot suffer without the other being affected, 



