SPINAL CORD, the posterior or inferior portion of the cerebrospinal axis, entirely located in the spinal canal. In those animals which stand upright it may be termed the lower portion. The spinal cord is directly continuous with the medulla oblongata (q.v.) and, varying with different animals, it extends practically the length of the spinal canal, giving off numerous paired spinal nerves (q.v.) and terminates by breaking up into a series of terminal nerves, the so-called cauda equina. With reference to its length and thickness, its divisions and ar rangements, both internal and external, con siderable variation exists in different animals. (See ANATOMY, COMPARATIVE). In man, how ever, the spinal cord comprises all that portion which lies between the cauda equina and the decussation of the pyramids in the medulla. II lies within the bony framework of the spine, covered on the outside by a thick membrane, the dura mater, which is continuous with the dura mater of the brain. Within this is a thin delicate layer of tissue, the pia mater, which covers the cord closely, being continuous with the pia mater of the brain. Between the dura and pia a certain amount of cerebrospinal fluid is found, which serves as a protection-sheath. Passing out and coming into the cord are the paired motor and sensory nerves that carry motor impulses to all the muscles of the body below the head and sensory impulses to the brain from all the sense organs of the skin, mucous membranes and viscera. In simple language it is a large cable which carries in coming and outgoing messages. The incoming messages come through various organs of sense, the stimuli come in through receptors, they then travel in nerve fibres, enter the cord, form connections, etc., with the brain or other nervous centrals and outgoing messages travel down to make effective action in the environ ment. The general shape of the spinal cord is cylindrical. It is slightly flattened front and back and tapers at its caudal end into a long thin strip, the filum terminale. In its course it shows two enlargements, the cervical and dorsal enlargements, corresponding to the loca tion of increased amounts of nervous tissue that go and come from the upper and lower extremities. The length of the cord proper, apart from its terminal filament, averages only about 18 inches; it, therefore, falls short several inches of filling the entire spinal canal. It is about one-half inch in width at the widest pail and weighs in an average adult about one pound. The spinal cord is nothing but a collec tion of nerve ganglia and nerve-fibres, but in the human cord these are not as simple in their arrangement as in the nervous cords of insects, for instance. The cord preserves its bilateral symmetry, but its metameric symmetry is much modified, particularly in its internal structure. Thus the spinal cord consists of nerve-cells and nerve-fibres, like all other parts of the nervous system. It is divided almost into two halves by deep fissures in front and behind; and there are several secondary fissures that further sub divide the cord into segments corresponding somewhat to its anatomical arrangement. In order to understand somewhat of this ana tomical arrangement it is important first to trace the different fibre-tracts that go up and down the spinal cord and which make up its white matter. The first nerve-fibres to develop are sensory. These collect into bundles and, through the posterior spinous processes, pass into the cord and constitute two bundles of fibres, the columns of Gall and the columns of Burdach. These columns are at first very small, but as they continue to receive more and more fibres passing upward they become larger. After their entrance into the cord the sensory fibres give off many collaterals, making ex tremely complicated anatomical connections with the different segments of the cord. One set of these collaterals, however, makes a dis tinct bundle of fibres, which has been called the column of Gowers, or ascending lateral tract. All three of these fibre-tracts carry sensory impressions from the surface of the body to the brain; they constitute the sensory neuron paths. The column of Gowers is known to carry pain-sensory stimuli. These sensory neuron-paths terminate in the sensory areas (q.v.) in the brain, having numerous interrup tions or stations, on the way up. The typical sensory neuron-path is made up of three or four of these stations. The first ganglionic centre is in the posterior ganglia; the second is in the medulla, constituting the nuclei of the columns of Gall and Burdach; the third is in the optic thalamus and the fourth is in the cortex of the brain. As to the part of the cortex, it varies according to the character of the incoming stimuli: gravity stimuli are mostly located in the cerebellum, sight stimuli in the occipital lobes, sound stimuli in the temporal lobes, where the sensations for liver, spleen, kidney, heart, etc., are, is still under discussion.
Passing down from the brain a large number of motor tracts make up the larger part of the re maining white matter of the cord. The most important of these bundles are the direct and indirect or crossed pyramidal tracts. The de scending anterolateral tract and direct cere bellar tract are other smaller fibre-masses. Still other minute fibre-bundles are found in the cord, but their description belongs to more extensive treatises. The main function of these descending tracts is to carry motor im pulses down from the motor area to the gray masses of nuclei located on the interior of the spinal cord. From these spinal centres the im pulses are transmitted to the motor nerves which pass out the entire length of the cord, from in front, where they later bend backward and join the sensory fibres and together form mixed nerve-trunks. The motor nerves finally terminate in the muscles by specially modified muscle-plates. As the nuclei of the columns of Gall and Burdach in the medulla were the first ganglionic terminals of the sensory neuron chains, so the gray masses in the anterior columns of the cord are the first ganglionic terminals of the motor neuron-chain. If the spinal cord be cut across, these ganglionic masses appear on the cut surface as a gray letter H, or as a butterfly-wing, according to the region where the cut is made. In these wings certain definite groups of ganglion-cells may be made out, running short distances in and down the spine. These constitute definite ganglionic centres for certain muscle-groups, and their destruction causes the well-known spinal paralysis of children. Throughout the entire length of the spinal cord there is a small cavity around which there are bridges of fibres connecting the two halves of the cord. Thus the spinal cord is seen to represent a mass of cells and fibres, of vastly complicated structure, and subserving all of the nervous functions of the muscles of the skeleton and of the viscera below the diaphragm, save the stomach. The spinal cord is in intimate connection through out its entire length with the sympathetic nervous system. Any interruption in the spinal cord interferes with the carrying capacity of its fibres and cells. There then results disease of the spinal cord the symptoms of which will differ according to the place where the inter ruption takes place. As the pathways of the different kinds of sensation messages and mo tion messages are extremely complex they can not all be spoken of here. Only some of the more striking types of loss will be spoken of. If the entire cord is cut across, as happens in broken back from diving into shallow water, from bullet wounds of war, or sometimes from a tumor of the spinal cord, or an inflammation, there results a paralysis of all motion below the place where the injury takes place. If in the upper neck region the paralysis is of the entire body below the neck. If the injury is lower down the lower extremities alone are involved with such portions of the trunk as lie below the site of the injury. All above the site of the lesion is a loss of sensation. This would naturally follow from what has been said that the pathways are carrying sensory messages up and motor messages down. An incomplete section of the spinal cord introduces a number of complicated results which only the nervous specialist can analyze. He is like a telephone expert. He is a trouble hunter on the line and from the symptoms which the patient shows he is able to accurately locate the place where the interruptions are occurring or have oc curred.
In some disease processes localized parts of the spinal cord are picked out by the process and a limited type of injury takes place. Thus in the disease known as infantile paralysis [poliomyelitis] the virus of the disease chiefly involves the connections of the motor pathways as they make a switchboard connection with the muscle groups in the anterior horns of the spinal cord. This results in a partial or com plete paralysis of one or more groups of muscles. Sometimes the sensory parts of the cord are cut off or diseased. This cuts off in formation from switchboard operators higher up and various types of loss of feeling, in ability to localize, inability to know what one is doing with one's muscles, etc., results. A great number of complicated troubles can result and bring about the clinical pictures called syn dromes, in neurology, such as locomotor ataxia, syringomyelia, multiple sclerosis, paraplegia, muscular atrophies, etc., which must be con sulted in a work on diseases of the spinal cord or diseases of the nervous system. Con sult Barker, 'Nervous System' (1901) ; De jerine, 'Anatomic du Systeme Nerveux' (1901-04) • ' Van Gehuchten,