Nerve-fibres are processes or outrunners from the cells. At the point where a fibre springs from a cell it is exceedingly fragile and delicate, but as it travels away from the cell it gradually acquires a protective covering or sheath, which calls to mind that employed to insulate a telegraph wire. It loses it, however, again as it approximates its destination, becoming reduced to a very attenuated thread. If examined in the middle of its course, say, for example, in the sciatic nerve, each fibre will be found to consist of a central core or axis cylinder, believed to be the path along which all nervous impressions and impulses are propagated. Covering this is a layer of white substance of a fatty nature, named the medullary sheath, or white substance of Schwann, and outside this again is a delicate but firm and resistant membrane, which is the neurilemma; such nerve-fibres are named " medullated fibres". The central core or cylinder axis often runs for long distances without division, but at times gives off collateral branches, and may even divide into a set of branches like a bouquet. Medullated fibres vary in diameter from 1-1500th to 1-12,000th of an inch. The fibres are bound together into bundles by connective tissue, and these bundles are associated into groups, the whole having a strong investment of connective tissue and constituting a nerve.
The results of injury to the central parts of the nervous system are so disastrous that they are everywhere protected from mechanical violence with the greatest care. The spinal cord, cerebellum, and brain are contained in a strong case of bone particularly well adapted to preserve these soft parts from blows or pressure. The head bones are composed of an outer and an inner layer of compact bone, between which is a layer of loose cancellous or spongy bone, the whole requiring great force to cause fracture, whilst the solid bodies and projecting spines of the backbone, together with the successive layers of thick skin, fibrous tissue, and muscles that cover it, are equally competent to protect the spinal cord. The skull will not indeed resist the penetrating power of a bullet, nor will the spine resist the weight and shock of a heavy rider in leaping into a ravine, hut 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 opposed 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 fluid bed.
Fig. 171. - Section of Nerve.
B, Medullary Sheath.
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 reinforced 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.