The Spinal Cord is the cylindrical, elongated part of the cerebro-spinal axis, which is contained in the vertebral canal. Its length is usually about 42-45 centimeters, and its weight, when divested of its membranes and nerves, about one ounce and a half, its proportion to the encephalon being about 1 to 33. It does not nearly fill the canal in which it is contained, its investing membranes being separated from the surrounding walls by areolar tissue and a plexus of veins. It occupies, in the adult, the upper two-thirds of the vertebral canal, extending from the upper border of the atlas to the lower border of the body of the first lumbar vertebra, where it terminates in a slender filament of gray substance, which is continued for some distance into the filum terminale. In the faetus, before the third month, it extends to the bottom of the sacral canal, but after this period it gradually recedes from below, as the growth of the bones composing the canal is more rapid in proportion than that of the cord, so that in the child at birth the cord extends as far as the third lumbar vertebra. Its position varies also according to the degree of curvature of the spinal column being raised somewhat in flexion of the spine. On examining its surface it presents a difference in its diameter in different parts, being marked by two enlargements, an upper or cervical, and a lower or lumbar. The cervical enlargement extends from about the third cervical to the first or second thoracic vertebra: its greatest diameter is in the transverse direction (13 mm.), and it corresponds with the origin of the nerves, which supply the upper extremities. The lumbar enlargement is situated opposite the last two or three thoracic vertebra, and corresponds with the origin of the nerves which supply the lower extremities. Below the lumbar enlargement the cord gradually tapers to form a cone, the conus medullaris, the apex of which is continuous with the filum terminale. In form, the spinal cord is a cylinder, flattened before and behind.

The membranes, which envelop the spinal cord, are three in number. The most external is the dura mater, a strong fibrous membrane, which forms a loose sheath around the cord. The most internal is the pia mater, a cellulo-vascular membrane, which closely invests the entire surface of the cord. Between the two is the arachnoid membrane, a non-vascular membrane, which envelops the cord and is connected to the pia mater by slender filaments of connective tissue.

The Dura Mater of the cord represents only the meningeal or supporting layer of the cranial dura mater. The endocranial or endosteal layer ceases at the foramen magnum posteriorly, but reaches as low as the third cervical vertebra in front; below these levels its place is taken by the periosteum. It forms a loose sheath, which surrounds the cord, and is separated from the bony walls of the spinal canal by a quantity of loose areolar tissue and a plexus of veins. The situation of the veins between the dura mater of the cord and the periosteum of the vertebrae corresponds therefore to that of the cranial sinuses between the endocranial and supporting layers. It is attached to the circumference of the foramen magnum, and to the axis and third cervical vertebra: it is also fixed to the posterior common ligament, especially near the lower end of the spinal canal, by fibrous slips; it extends below as far as the second or third piece of the sacrum; here it becomes impervious, and, ensheathing the filum terminale, descends to the back of the coccyx, where it blends with the periosteum. The dura mater is much larger than is necessary for its contents, and its size is greater in the cervical and lumbar regions than in the thoracic. Its inner surface is smooth. On each side may be seen the double openings which transmit the two roots of the corresponding spinal nerve, the fibrous layer of the dura mater being continued in the form f a tubular prolongation on them as they pass through these apertures. These prolongations of the dura mater are short in the upper part of the spine, but become gradually longer below, forming a number of tubes of fibrous membrane, which enclose the sacral nerves, and are contained in the spinal canal.

The chief peculiarities of the dura mater of the cord, as compared with that investing the brain, are the following:

• The dura mater of the cord is not adherent to the bones of the spinal canal, which have an independent periosteum.
• It does not send partitions into the fissures of the cord, as in the brain.
• Its fibrous laminas do not separate to form venous sinuses, as in the brain.

The dura mater consists of white fibrous and elastic tissue arranged in bands or lamellae, which, for the most part, are parallel with one another and have a longitudinal arrangement. Its internal surface is covered by a layer of endothelial cells, which gives this surface its smooth appearance. It is sparingly supplied with vessels, and some few nerves have been traced into it.

The Arachnoid is exposed by slitting up the dura mater and reflecting that membrane to either side. It is a thin, delicate, tubular membrane, which invests the surface of the cord, and is connected to the pia mater by slender filaments of connective tissue. Above, it is continuous with the cerebral arachnoid, on each side it is continued on the various nerves, so as to form a sheath for them as they pass outward to the intervertebral foramina. The outer surface of the arachnoid is in contact with the inner surface of the dura mater, and the two are, here and there, joined together by isolated connective-tissue trabeculse, especially on the posterior surface of the cord. For the most part, however, the membranes are not connected together, and the interval between them is named the subdural space. The inner surface of the arachnoid is separated from the pia mater by a considerable interval, which is called the subarachnoidean space. The space is the largest at the lower part of the spinal canal, and encloses the mass of nerves, which form the cauda equina. Superiorly it is continuous with the cranial subarachnoid space, and communicates with the general ventricular cavity of the brain by means of an opening in the pia mater, in the roof of the fourth ventricle (foramen of Majendie and foramina of Key and Retzius). It contains an abundant serous secretion, the cerebro-spinal fluid. This secretion is sufficient in amount to expand the arachnoid membrane, so as to fill up completely the whole of the space included in the dura mater. The subarachnoidean space is occupied by trabeculae of delicate connective tissue, connecting the pia mater on the one hand with the arachnoid membrane on the other. This is named subarachnoid tissue. In addition to this it is partially subdivided by a longitudinal membranous partition, the septum posticum, which serves to connect the arachnoid with the pia mater, opposite the posterior median fissure of the spinal cord, a partition which is incomplete and cribriform in structure, consisting of bundles of white fibrous tissue interlacing with each other. This space is to be regarded as, in reality, a great lymph space, from which the lymph carried to it by the perivascular lymphatics is conveyed back into the circulation.

The arachnoid is a delicate membrane made up of closely arranged interlacing bundles of connective tissue in several layers.

The Pia Mater of the cord is exposed on the removal of the arachnoid. It covers the entire surface of the cord, to which it is very intimately adherent, forming its neurilemma, and sending a process downward into its anterior fissure. It also forms a sheath for each of the filaments of the spinal nerves, and invests the nerves themselves. A longitudinal fibrous band extends along the middle line on its anterior surface, called by Haller the linea splendens; and a somewhat similar band, the ligamentum denticulatum, is situated on each side. At the point where the cord terminates the pia mater becomes contracted, and is continued down as a long, slender filament (filum terminale), which descends through the center of the mass of nerves forming the cauda equina. It perforates the dura about the level of the second or third lumbar vertebrae, receiving a sheath from it, and extends downward as far as the base of the coccyx, where it blends with the periosteum. It assists in maintaining the cord in its position during the movements of the trunk, and is from this circumstance called the central ligament of the spinal cord. It contains a little gray nervous substance, which may be traced for some distance into its upper part, and is accompanied by a small artery and vein. At the upper part of the cord the pia mater presents a grayish, mottled tint, which is owing to yellow or brown pigment cells scattered among the elastic fibers.

The pia mater of the cord is less vascular in structure, but thicker and denser, than the pia mater of the brain, with which it is continuous. It consists of two layers: an outer composed of bundles of connective-tissue fibers, arranged for the most part longitudinally; and an inner, consisting of stiff bundles of the same tissue, which present peculiar angular bends, and is covered on both surfaces by a layer of endothelium. Between the two layers are a number of cleftlike lymphatic spaces which communicate with the subarachnoid cavity, and a number of blood-vessels which are enclosed in a perivascular sheath, derived from the inner layer of the pia mater, into which the lymphatic spaces open. It is also supplied with nerves, which are derived from the sympathetic.

The Ligamentum Denticulatum is a narrow fibrous band, situated on each side of the spinal cord, throughout its entire length, and separating the anterior from the posterior roots of the spinal nerves. It has received its name from the serrated appearance, which it presents. Its inner border is continuous with the pia mater at the side of the cord. Its outer border presents a series of triangular, dentated serrations, the points of which are fixed at intervals to the dura mater. These serrations are twenty-one in number on each side, the first being attached to the dura mater, opposite the margin of the foramen magnum between the vertebral artery and the hypoglossal nerve, and the last near the lower end of the cord. Its use is to support the cord in the fluid by which it is surrounded.

Each half of the spinal cord is thus divided into four columns: an anterior column, a lateral column, a posterior column, and a postero-median column. This division, however, is very imperfect, since the limit between the so-called anterior and lateral columns cannot be defined on account of the bundles of the anterior roots being spread over a considerable area. It is three-fore customary to divide each half of the spinal cord into two columns, separated by the postero-lateral groove:

• A small posterior column, which is bounded internally by the posterior median fissure, and externally by the postero-lateral fissure;
• A large antero-lateral column, which comprises the rest of the cord.

The posterior column is further divided, at all events at its upper part, by the posterior intermediate septum, into a postero-median column and a postero-lateral column.

If a transverse section of the spinal cord be made, it will be seen to consist of white and gray nervous substance. The white matter is situated externally, and constitutes the greater part. The gray substance occupies the center, and is so arranged as to present on the surface of the section two crescentic masses, placed one in each lateral half of the cord, united together by a transverse band of gray matter, the gray commissure. Each crescentic mass has an anterior (ventral) and a posterior (dorsal) horn. The posterior horn is long and narrow, and approaches the surface of the postero-lateral fissure, near which it presents a slight enlargement, the caput cornu: from this it tapers to form the apex cornu, which at the surface of the cord becomes continuous with some of the fibers of the posterior roots of the spinal nerves. The anterior horn is short and thick, and does not quite reach the surface, but extends toward the point of attachment of the anterior roots of the nerves. Its margin presents a dentate or stellate appearance. Owing to the projections toward the surface of the anterior and posterior horns of the gray matter, each half of the cord is divided, more or less completely, into three columns, anterior, middle, and posterior, the anterior and middle being joined to form the antero-lateral column, as the anterior horn does not quite reach the surface.

The commissure of the spinal cord is composed of white and gray matter, and is therefore divided into the white and gray commissures. The white commissure is situated at the bottom of the anterior median fissure, and is formed of medullated nerve fibers, which pass between the gray matter of the anterior horn and the anterior white column of the one side into similar parts on the other. The fibers are oblique in direction; many which enter at the posterior part of the commissure on the one side leave it at the anterior part of the commissure on the other, and vice versa, a decussation taking place in the middle line.

The gray commissure, which connects the two crescentic masses of gray matter, is separated from the bottom of the anterior median fissure by the anterior white commissure. It consists of transverse medullated nerve-fibers, with a considerable quantity of neuralgia between them. The fibers when they reach the lateral crescents diverge: some pass backward to the posterior roots; others spread out, at various angles, into the crescent.

Running through the gray commissure of the whole length of the cord is a minute canal, which is barely visible to the naked eye in the human cord, but is proportionately larger in some of the lower vertebrata. It is called the central canal; it opens above into the fourth ventricle, and terminates below in a somewhat dilated extremity. An area of neuralgia, which in the recent state, has a gelatinous appearance, and in which there are no nerve-fibbers, surrounds it. This is sometimes called the substantia gelatinosa centralis. When hardened in alcohol or chromic salts it has a finely reticulated appearance. The canal is lined in the fetus by columnar ciliated epithelium, but in the adult the cilia have disappeared, and the canal is filled with their remains.

The mode of arrangement of the gray matter, and its amount in proportion to the white, vary in different parts of the cord. Thus, the posterior horns are long and narrow in the cervical region; short and narrower in the thoracic; short, but wider, in the lumbar region. In the cervical region the crescentic portions are small, and the white matter more abundant than in any other region of the cord. In the dorsal region the gray matter is least developed, the white matter being also small in quantity. In the lumbar region the gray matter is more abundant than in any other region of the cord. Toward the lower end of the cord the white matter gradually ceases. The crescentic portions of the gray matter soon blend into a single mass, which forms the only constituent of the extreme point of the cord.

The cord consists of an outer part, composed of medullated nerve-fibers, which is the white substance; and of a central part, the gray matter, both supported in a peculiar kind of tissue, called neuroglia.

The neuroglia consists of a homogeneous transparent matrix, of a network of very delicate fibrillae, and of small stellate or branched cells, the neuroglia-cells.

In addition to forming a ground substance, in which the nerve-fibers, nerve-cells, and blood-vessels are imbedded, a considerable accumulation of neuroglia takes place in three situations:

1. On the surface of the cord, beneath the pia mater;
2. Around the central canal, the substantia gelatinosa centralis;
3. Over the extremity of the posterior horn, forming the substantia cinerea gelatinosa.

The white substance of the cord consists of medullated nerve-fibers, mostly disposed longitudinally, with blood-vessels and neuroglia. When stained with carmine it presents a very striking appearance on transverse section. It is seen to be studded all over with minute dots, surrounded by a white area (Fig. 387). This is due to the longitudinal medullated fibers seen on section. The dot is the axis-cylinder, the white area the substance of Schwann. Externally, the neuroglia forms a sheath closely investing the outer surface of the cord immediately beneath the pia mater; from it numerous septa pass inward and separate the respective bundles of fibers and extend between the individual nerve-fibres, acting as a supporting medium, in which they are imbedded.

There are, however, also oblique and transverse fibers in the white substance. These principally consist of:

1. The fibers of the white commissure;
2. Horizontal or oblique fibers passing from the roots of the nerves into the gray matter;
3. Fibers leaving the gray matter and pursuing a longer or shorter horizontal course.

It is impossible to trace the course of the nerve-fibers in their passage through the cord; but the investigation of pathological lesions has shown that the white matter of the cord consists of certain columns or tracts of fibers; for it has been found that certain lesions are strictly limited to certain well-determined parts of the cord without involving neighboring regions. That these parts or fasciculi correspond to so many distinct anatomical systems, each endowed with special functions, seems abundantly proved by the researches of Flechsig and others on the development of the spinal cord during the later periods of utero-gestation and in the newly born infant. By these researches several tracts can be traced along the greater part of the cord and into or from the encephalon. Thus in the antero-lateral column of the cord, on either side of the anterior median fissure, a portion of the column may be divided off as the direct pyramidal tract. This tract is only found in the upper part of the cord; it gradually diminishes as it is traced downward, and disappears about the middle of the dorsal region. It consists of centrifugal or descending fibers, which can be traced downward from the pyramid of the medulla of the same side, and are derived from the motor area of the cerebral cortex. The fibers of this tract decussate in their course down the cord, passing across the middle line through the anterior white commissure; this explains the gradual diminution and eventual disappearance of the tract.

In the hinder part of the antero-lateral column is a somewhat triangular area, larger than the preceding, which is named the crossed pyramidal tract. This also consists of descending fibers, which are derived from the pyramid of the medulla of the opposite side, and which have crossed in the decussation of the pyramids. The fibers are derived from the motor area of the cerebral cortex of the opposite side. Thus it will be seen that all the fibers from the motor area, which descend through the internal capsule, the crus cerebri, and the pons Varolii to the pyramidal body of the medulla, decussate; some at the upper part of the cord, and these descend through it as the crossed pyramidal tract; and others, which descend as the direct pyramidal tract and cross through the anterior commissure of the cord to reach the crossed pyramidal tract of the opposite side. Although this is the usual method of describing the crossing of the direct pyramidal tract in the cord, it seems probable that its fibers cross in the anterior commissure and pass directly to the anterior horn of gray matter, to end by forming synapses around its cells.

The antero-lateral ascending tract (Gower's tract) is an extensive crescent-shaped strand, which skirts the circumference of the anterior three-quarters of the antero-lateral column of the cord. Behind, where it is thickest, it lies in the angle formed by the direct cerebellar and crossed pyramidal tracts, becoming narrower as it passes forward toward the direct pyramidal tract. It consists of centripetal or ascending fibers, which arise from cells situated at the base of the posterior horn and which cross to the opposite side of the cord in the anterior gray commissure. They can be traced upward through the medulla and pons to the cerebellum, reaching the latter through its superior peduncles. If the spinal cord is divided in the cervical region, some scattered fibers in this column degenerate in a downward direction. This would seem to prove therefore that it contains some descending fibers, which are believed to be derived from the same side of the cerebellum.

The direct cerebellar tract is situated at the circumference of the cord behind the preceding and external to the crossed pyramidal tract, occupying a narrow area which extends backward as far as the postero-lateral fissure or nearly so. It commences at the level of the upper lumbar region, and increases in size as it ascends and passes through the restiform body of the medulla to the cerebellum. Its fibers are derived from the cells of the posterior vesicular column of Clarke in the gray matter of the cord.

Close to the point where the posterior roots enter the cord, in the antero-lateral column, is a small collection of fibers, which is known as the tract of Lissauer; it is formed by some of the fibres of the posterior roots which run upward in the tract for a short distance, and then enter the posterior horn of the gray matter.

The rest of the antero-lateral column of the spinal cord is occupied by the antero-lateral ground bundle. It surrounds the anterior cornu and separates the antero-lateral tract and the crossed pyramidal tract from the gray matter of the cord. It consists of longitudinal commissural fibers, which unite the groups of cells in the gray matter with one another; of fibers, which pass across the anterior commissure from the gray-matter of the opposite side; and horizontal fibers belonging to the anterior roots of the nerves, which pass through it before leaving the cord.

In the posterior column of the cord there are two tracts. The posterior intermediate furrow on the surface of the cord marks them off from each other. The part, which has been described, previously as the posterior median column pretty nearly corresponds to the one tract, the tract of Goll, and the remainder of the posterior column corresponds to the other, the tract of Burdach.

The tract of Goll increases as it ascends, and consists of long, but small, fibers derived from the posterior roots of the spinal nerves, which ascend to the medulla oblongata, where they end in the nucleus gracilis.

The tract of Burdach consists of shorter, but larger, fibers than the preceding; they are, however, derived from the same source, the posterior roots; some ascend only for a short distance in the tract and then enter the gray matter and come into close relationship with the cells of the posterior vesicular column of Clarke ; others incline toward the mesial plane, and, entering Goll's column, can be traced as far as the medulla. In the cervical and upper thoracic regions there is contained in the substance of Burdach's column a small strand of fibers, called the descending comma tract. It presents, on transverse section, the appearance of a comma, the blunt extremity of which is directed forward. The fibers forming it probably represent in part descending portions of the dorsal nerve-roots, together with descending commissural fibers within the cord itself. A small strand of similar descending fibers is seen, in the lower part of the cord, lying in the inner part of Goll's column.

The gray substance of the cord occupies its central part in the shape of two crescentic horns, joined together by the gray commissure. Each of these crescents has an anterior or ventral and a posterior or dorsal cornu.

The posterior horn consists of a slightly narrowed portion, at its base, where it is connected with the rest of the gray substance - this is the cervix cornu; from this it gradually expands into the main part of the horn, the caput cornu; around its extremity is a lamina or layer of gelatinous material, which covers the head like a cap, and from this it tapers almost to a point, which approaches the surface of the cord at the postero-lateral groove.

The gelatinous substance is a peculiar accumulation of neuroglia (Klein) similar to that found around the central canal, and has been named by Rolando the substantia cinerea gelatinosa. It probably takes its origin from the columnar cells, which line the posterior part of the embryonic spinal canal.

The anterior horn of the gray substance in the cervical and lumbar swellings, where it gives origin to the motor nerves of the extremities, is much larger than in any other region, and contains several distinct groups of large and variously shaped cells.

In addition to this, a lateral horn is found projecting outward, from the lateral region of the gray matter on a level with the gray commissure in the upper part of the dorsal region of the cord; in the cervical and lumbar regions this lateral horn blends with the anterior horn, which thus becomes broad and expanded. From the concavity of the crescent, between the anterior and posterior horns, processes of gray matter extend into the white substance, where they divide and anastomose to form a network, termed the formatio reticularis.

The gray commissure contains the central canal, and is situated behind the white commissure, which separates it from the bottom of the anterior median fissure.

The gray substance of the cord consists of:

1. Nerve-fibers of variable but smaller average diameter than those of the white columns;
2. Nerve-cells of various shapes and sizes, with from two to eight processes;
3. Blood vessels and connective tissue.

The nerve-fibers of the gray matter of the posterior horn are for the most part composed of a dense interlacement of minute fibrils, intermingled with nerves of a larger size. This interlacement is formed partly by the axons and dendrites of the cells of the gray matter, and partly by fibers which enter the gray matter and which come from various sources.

The nerve-cells of the gray matter are collected into groups as seen on transverse section, but they really form columns of cells placed longitudinally; or else they are found scattered throughout the whole of the gray matter.

In the anterior horn the cells consist of two chief groups: one mesial, the more constant, near the anterior column; the other lateral, near the lateral column. A second lateral group is present in the cervical and lumbar enlargements. At the base of the posterior horn on its inner side, adjoining the gray commissure is a group of nerve-cells, called Clarke's posterior vesicular column, which extends from the eighth cervical to the second lumbar nerve.

At the junction of the anterior and posterior cornu, in the outer portion of the gray matter, is a third group of cells, the lateral cell column; this is best seen in the thoracic region. In certain regions of the cord these cells extend in among the fibers of the white matter of the lateral column, and give rise to the lateral horn. In addition to these groups a few large scattered cells are found in the posterior horn and in the substantia gelatinosa of Rolando.

The roots of the spinal nerves are attached to the surface of the cord, opposite the horns of gray matter.

The posterior nerve-root enters the cord in two bundles, mesial and lateral. The mesial strand consists of coarse fibers, which enter the outer part of the column of Burdach. The lateral strand is sometimes divided into middle and an external bundle. The former contains large fibers, and passes through the gelatinous substance of Rolando into the posterior horn. The external bundle consists of fine fibers, which assume a longitudinal direction in Lissauer's tract. All the posterior root-fibers divide into ascending and descending branches on entering the cord, and these in their turn give off collaterals. The fibers and their collaterals terminate by forming arborescences, some around the cells in the posterior horn, and others around the cells of Clarke's column, while the long ascending branches pass up in the columns of Goll and Burdach, and end by arborizing around the cells in the gracile and cuneate nuclei. Some of the fibers, however, pass to the gray matter of the opposite horn, and others to the anterior horn of the same side of the cord.

The majority of the fibers of the anterior nerve-roots are the continuations outward of the axons of the large or small multipolar cells in the anterior horn of gray matter. Some, however, appear to pass across in the anterior white commissure to the cells in the anterior horn of the opposite side, while others extend backward to the posterior horn and outward to the lateral column of the same side.