Each half of the spinal cord is divided lengthwise into three nearly equal parts, which are termed the anterior, lateral, and posterior columns, by the lines which join together two parallel series of bundles of nervous filaments, which compose the roots of the spinal nerves. The roots of those nerves, which are found along that line nearest the posterior surface of the cord, are termed the posterior roots; those which spring from the other line are known as the anterior roots. Several of these anterior and posterior roots, situated at about the same height on opposite sides of the spinal cord, converge and combine into what are called the _anterior_ and _posterior bundles_; then two bundles, anterior and posterior, unite and form the trunk of a spinal nerve. The nerve trunks make their way out of the spinal canal through apertures between the vertebra, called the _inter-vertebral foramina_ and then divide into numerous branches, their ramifications extending principally to the muscles and the skin. There are thirty-one pairs of spinal nerves, eight of which are termed cervical, twelve dorsal, five lumbar, and six sacral, with reference to that part of the cord from which they originate. When the cord is divided into transverse sections, it is found that each half is composed of two kinds of matter, a white substance on the outside, and a grayish substance in the interior. The _gray matter_, as it is termed, lies in the form of an irregular crescent, with one end considerably larger than the other, and having the concave side turned outwards. The ends of the crescent are termed the _horns_, or _cornua_, the one pointing forward being called the _anterior cornu_, the other one the _posterior cornu_. The convex sides of these cornua approach each other and are united by the bridge, which contains the central canal. There is a marked difference in the structure of the gray and the white matter. The white matter is composed entirely of nerve fibers, held together by a framework of connective tissue. The gray matter contains a great number of ganglionic corpuscles, or nerve-cells, in addition to the nerve-fibers. When the nerve-trunks are irritated in any manner, whether by pinching, burning, or the application of electricity, all the muscles which are supplied with branches from this nerve-trunk immediately contract, and pain is experienced, the severity of which depends upon the degree of the irritation; and the pain is attributed to that portion of the body to which the filaments of the nerve-trunk are distributed. Thus, persons who have lost limbs often complain in cold weather of an uneasiness or pain, which they locate in the fingers or toes of the limb which has been amputated, and which is caused by the cold producing an irritation of the nerve-trunk, the filaments, or fibers of which, supplied the fingers or toes of the lost member.