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 swellings, one extending from the fifth vertebra of the neck to the fourth vertebra of the back, and the other situated in the region of the loins. These enlargements of the nervous mass are rendered necessary in order to supply the great nerves distributed to the fore and hind limbs respectively. The spinal cord gives origin in the horse to forty-two or forty-three pairs of nerves, each of which arises by two roots, a superior and an inferior.
DORSAL OR INTERCOSTAL BRANCHES
Fig. 172. - The Spinal Cord and the Nerves to which it gives rise.
The position and arrangement of the successive pairs are shown in the accompanying diagram.
If the cord be divided transversely the appearance presented in fig. 173 will be seen.
The shaded area is named the gray substance, the light area the white substance, of the cord. The white substance is composed almost exclusively of nerve-fibres; the gray substance, whilst containing many fibres, presents also a large number of nerve-cells. The cord is seen to be divided into symmetrical lateral halves by two fissures, the superior fissure being narrow and deep, and the inferior wide and more shallow. The gray substance of the cord somewhat resembles the letter H, or a pair of inverted commas, placed back to back and united by a cross bar. The extremities of the comma - like bodies are named the cornu, and there are consequently an upper and a lower cornu on each side. The upper one is more pointed, and reaches nearer to the surface than the lower cornu. The isthmus or central portion, which joins the two lateral masses of gray substance, is perforated by a small hole, which represents the section of a tube, named the central canal, which runs the whole length of the cord. If the nerve roots be traced into the substance of the cord, it will be found that the fibres of which they are composed chiefly end in branches surrounding the nerve-cells of the sujjerior and inferior cornu of their own side, whilst some ascend towards the brain on that side, and others cross over to the opposite side. Each half of the white substance of the cord is obviously divided into three regions: 1, an upper, between the superior cornu and the superior median fissure; 2, a lateral, between the upper and lower cornu; and 3, a lower region, between the inferior cornu and the inferior median fissure.
Fig. 173. - Sections of Spinal Cord.
1 Superior Root. 2Inferior Root. 3 Ganglion. 4 Superior Nerve. 5 Inferior Nerve. 6 Choroid Plexus. 7 Nerve Substance. 8, 10 Inferior Cornua. 9 Superior Cornu. 11 Central Canal. 12 Superior Longitudinal Fissure. 13 Inferior Longitudinal Fissure.
In some instances the fibres constituting these divisions conduct impressions forwards, in others backwards. Some are chiefly made up of sensory nerves, whilst others are essentially motor. Others again are mixed, and convey impulses both upwards to the medulla oblongata, cerebrum, and cerebellum, and downwards from these centres to the muscles.