THE SPINAL CORD.
The spinal cord is in direct continuation with the brain by means of the medulla oblongata, and passes down the back, lodged in the canal formed by the rings of the vertebrae. It is from .15 to 18 inches long, and terminates at the level of the first lumbar vertebra, tapering off to a fine thread. It is about the thickness of the little finger, but is thicker in the region of the neck, where the nerves for the upper limbs pass off, and also at the lower or lumbar end, where the nerves for the lower limbs emerge. Like the brain it is closely invested by a very delicate membrane, the pia mater, by which blood - vessels are conveyed to the substance of the cord, having also an outer tough, fibrous coating—the dura mater. Be tween these two are the delicate serous layers of the arachnoid membrane inolosing a space between the dura and pia mater. This space contains a certain amount of fluid—the cerebro spinal fluid, similar to the fluid in the ven tricles of the brain. Finally, between the cord inclosed by its three membranes and the bony walls of the spinal canal there is a consider able amount of fatty tissue, acting as a packing material, embedded in which are some large blood-vessels. Thus the cord is supported in the canal by its membranes ; and by means of them and of the fluid and packing of fatty tissue it is protected from shocks and jars. Nerves, the spinal nerves, pass out from the cord at regular intervals along each side. There are thirty-one of these on each side ; and they receive sheaths from the delicate membranes of the cord, but pierce the dura mater. They issue from the bony canal by apertures, the intervertebral foramina, left at the sides between the opposing surfaces of the vertebra, and, having escaped from the backbone, they pass backwards and forwards ramifying in the soft parts of the body. The first pair of nerves comes off between the skull and atlas, the next pair between atlas and axis, a.ld so on down the canal. Thus eight pairs come off in the region of the neck—the cervical nerves, twelve pairs are dorsal, five are lumbar, and five sacral, while the last pair cones off behind the coccyx. The upper pairs come off at intervals and pass directly outwards through the openings in the canal for them. The lower pairs, however, to which belong the large nerves for the lower limbs, come of very near one another at the lower end of the cord and then pass down the canal in a bundle called cauda equine, from their resemblance to the tail of a horse, one pair passing outwards, one after the other, as they reach their respective openings. (Refer to Fig. 87.) The structure of the spinal cord resembles the brain, consisting of gray and white matter, but the arrangement is different. In the brain the white matter is within and the gray matter is spread on the surface of the convolutions. In the spinal cord the gray matter, which is characterized by large cells with many processes represented in Fig. 80 (a and e), is gathered in the centre into two half -moon -shaped masses, the backs of the masses being connected at the central part of the cord. The white matter,
consisting mainly of fibres, is outside of and sur rounds these gray crescents. Iu the centre of the cord, in the midst of the bridge of gray substance that unites the gray crescents, is a microscopically small canal, the central canal of the cord, which is continuous with the fourth ventricle of the medulla oblongata. The cord itself is almost divided into two lateral halves by a fissure or cleft which passes backwards from the middle line in front to within a short distance from the central canal. A division is also made behind between the right and left half of the cord by a partition of the pia mater that dips inwards from the middle line behind, also to a very short distance from the central canal. Thus there is formed a division, as it were, between the right and left side, each side having its own gray crescent, the horns of which point one forwards and the other backwards. The horn pointing forward is called the anterior horn or cornu, the other is the posterior horn or cornu. Now from these horns there pass off strands of nervous substance which form the roots of the spinal nerves. Thus, from the an terior horn of one side there passes off a strand which issues from the cord towards the front, and is the anterior root of a spinal nerve; from the posterior horn of the same side a strand passes off behind to form the posterior root of a spinal nerve. These two strands, having issued from the cord, curve round, meet and join one another at the side, and by their union a spinal nerve of one side is formed, which then passes out of the canal by its intervertebral opening. Similarly from the other side a spinal nerve is formed by the union of an anterior and pos terior root. On the posterior root, before it joins the anterior, is a ganglion, g of Fig. 88.
Fig. 88 represents the spinal cord cut across, as it would appear on looking down on the cut surface of the section. I is the fissure in front, II is the division between right and left behind, caused by the dipping in of the pia mater. The shaded portion in the centre represents the gray crescents with the connecting bridge, in the middle of which is the central canal, cc; ar is the anterior root, pr the posterior root. The two roots unite to form the spinal nerve 8p, and the nerve afterwards divides into two divisions, one going to the back of the body, and the other going to the front of the body.
The roots of the spinal nerves map off the white matter of the cord into columns. Be tween the anterior root of each side and the anterior fissure is the anterior column, AC, between the two roots at each side is the lateral column, Lc, and between the posterior root and the posterior fissure is the posterior column, Pc. In the white matter there are no groups of nerve-cells, but in the gray crescents are numerous groups of nerve-cells, ac, specially in the anterior corm's,. These are roughly in dicated in the diagram.