380 HEALTH AND DISEASE 



tliat cover it, are equally competent to protect the spinal cord. The 

 .skull will not indeed resist the penetrating power of a bullet, nor will 

 tiie spine resist the weight and shock of a heavy rider in leaping into 

 a ravine, but they will preserve the nerve centres intact through all the 

 ordinary casualties of life. The bones are not the only means of 

 guarding these parts from injury, for the osseous case is lined by a thick 

 and extremely tough membrane, the dura mater, thin sheets of which 

 dip down between the two hemispheres of the cerebrum, and between 

 the cerebrum and cerebellum, and prevent the former from unduly 

 pressing upon the latter. Within the dura mater is a thin double 

 membrane, named the arachnoid, one part covering the inside of the 

 dura mater and the other the outer part of the brain. These two op- 

 posed surfaces are lubricated with a serous fluid, which permits a slight 

 gliding movement of the brain, such as accompanies the act of breathing 

 and the beating of the arteries, with the least possible friction. And 

 lastly, the whole surface of the brain is immediately invested with the 

 pia mater, which is a membrane of blood-vessels, the branches being 

 so numerous and so closely arranged in it that it may almost be said 

 the great nerve centres rest in every position of the body on a Huid 

 bed. 



The nerve centres and the nerves receive an abundant supply of blood. 

 The branches of the internal carotid artery and those of the vertebral 

 arteries are distributed to the brain and cerebellum, whilst the spinal cord 

 receives blood-vessels from the vertebral arteries, which run down the 

 upper and lower surfaces of the cord as the spinal arteries, and are rein- 

 forced as they descend by many branches from the intercostals and posterior 

 aorta. The blood thus distributed is returned from the head and spine 

 by the jugular and spinal veins. The nerves of the trunk and extremities 

 receive their blood from the nearest artery, and return it to the nearest 

 vein. 



The Spinal Cord. — The spinal cord or spinal marrow is a long, nearly 

 cylindrical mass of nerve substance which extends from the head to the 

 sacral region of the spine, and weighs about 10 ozs. It is contained 

 in a canal formed by the successive vertebrae, which is wider than itself, 

 so that there is no danger of any pressure being exerted upon it in the 

 various movements the body is capable of performing. It has the same 

 coverings as the brain, which are named dura mater, arachnoid, and 

 pia mater. In front the cord enlarges both in breadth and thickness, 

 and is continuous with the brain through the medulla oblongata. Behind 

 it terminates near the anterior third of the sacral region. It does not 

 preserve the same diameter from one end to the other, but presents two 



