14. THE SPINAL NERVES.--The spinal nerves, thirty-one pairs in number, spring from each side of the cord by two roots, an anterior and a posterior root, which have the same functions as the columns bearing similar names. The posterior root is distinguished by possessing a ganglion of gray matter, and by a somewhat larger size. The successive points of departure, or the off-shooting of these nerves, occur at short and nearly regular intervals along the course of the spinal cord. Soon after leaving these points, {157} the anterior and posterior roots unite to form the trunk of a nerve, which is distributed, by means of branches, to the various organs of that part of the body which this nerve is designed to serve. The spinal nerves supply chiefly the muscles of the trunk and limbs and the external surface of the body. [Sidenote: 15. The nerve tissue? Its character? Course of each nerve fibre?] 15. The tissue composing the nerves is entirely of the white variety, or, in other words, the nerve-fibres; the same as we have observed forming a part of the brain. But the nerves, instead of being soft and pulpy, as in the case of the brain, are dense in structure, being hardened and strengthened by means of a fibrous tissue which surrounds each of these delicate fibres, and binds them together in glistening, silvery bundles. Delicate and minutely fine as are these nerve-fibres, it is probable that each of them pursues an unbroken, isolated course, from its origin, in the brain or elsewhere, to that particular point which it is intended to serve. For, although their extremities are often only a hair's breadth distant from each other, the impression which any one of them communicates is perfectly distinct, and is referred to the exact point whence it came. [Sidenote: 16. How may we illustrate the fact? The fibre connecting the brain with a point in the foot?] 16. This may be illustrated in a simple manner, thus: if two fingers be pressed closely together, and the point of a pin be carried lightly across from one to the other, the eyes may be closed, and yet we can easily note the precise instant when the pin passes from one finger to the other. If the nerve-fibres were less independent, and if it were necessary that they should blend with and support each other, all accuracy of perception would be lost, and all information thus afforded would be pointless and confused. These silvery threads must, therefore, be spun out with an infinite degree of nicety. Imagine, for instance, the fibre which {158} connects the brain with some point on the foot,--its length cannot be less than one hundred thousand times greater than its diameter; and yet it performs its work with as much precision as fibres that are comparatively much stronger and less exposed. [Sidenote: 17. The sympathetic system of nerves? Of what does it consist?]