The dura mater of the cord, continuous with that which invests the brain, is a loose sneatn wnicn surrounds it, being separated from the bony walls of the spinal canal by a quantity of loose areolar adipose tissue, and a plexus of veins. It is attached, above, to the circumference of the foramen magnum, and extends, below, as far as the top of the sacrum; but, beyond this point, it is impervious, being continued, in the form of a slender cord, to the back of the coccyx, where it blends with the periosteum. This sheath is much larger than is necessary for its contents, and its size is greater in the cervical and lumbar regions, than in the dorsal. Its inner surface is smooth, being lined by the arachnoid membrane; and 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 of a tubular prolongation on them as they issue from these apertures, and becoming lost upon them. 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 laminae do not separate, to form venous sinuses, as in the brain.

Structure. The dura mater consists of white fibrous tissue, arranged in bands, which intersect one another. It is sparingly supplied with vessels, as compared with the dura mater of the brain; and no nerves have as yet been traced into it.   The Arachnoid is exposed by slitting up the dura mater, and reflecting this membrane on either side (fig. 233). It is a thin, delicate, serous membrane, which invests the outer surface of the cord, and is then reflected upon the inner surface ot the dura mater, to which it is intimately adherent. That portion which surrounds the cord, is called the visceral layer of the arachnoid; and that which lines the inner surface of the dura mater, the parietal layer ; the interval between the two, is called the cavity of the arachnoid. The visceral layer forms a loose sheath around the cord, so as to leave a considerable interval between the two, which is called the sub-arachnoidean space. This space is largest at the lower part of the spinal canal, and encloses the mass of nerves which form the cauda equina. It contains an abundant serous secretion, the cerebro-spinal fluid, and usually communicates with the general ventricular cavity of the brain, by means of an opening in the fibrous layer of the inferior boundary of the fourth ventricle. This secretion is sufficient in amount to expand the arachnoid membrane, so as to completely fill up the whole of the space included in the dura mater. The sub-arachnoidean space is crossed, at the back part of the cord, by numerous fibrous bands, which stretch from the arachnoid to the pia mater, especially in the cervical region, and is partially sub-divided by a longitudinal membranous partition, which serves to connect the arachnoid with the pia mater, opposite the posterior median fissure. This partition is incomplete, and cribriform in structure, consisting of bundles of white fibrous tissue, interlacing with each other. The visceral layer of the arachnoid surrounds the spinal nerves where they arise from the cord, and encloses them in a tubular sheath as far as their point of exit from the dura mater, where it becomes continuous with the parietal layer.

  The arachnoid is not very vascular. No nerves have as yet been traced into this membrane.

  The Pia Mater of the cord is exposed on the removal of the arachnoid (fig. 233). It is less vascular in structure than the pia mater of the brain, with which it is continuous, being thicker, more dense in structure, and composed of fibrous tissue, arranged in longitudinal bundles. It covers the entire surface of the cord, to which it is very intimately adherent, forming its neurilemma, and sends a process downwards into its anterior fissure, and another, extremely delicate, into the posterior 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, which descends through the centre of the mass of nerves forming the cauda equina, and is blended with the impervious sheath of dura mater (before mentioned), on a level with the top of the sacral canal. 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 nervous substance, which may be traced for some distance into its upper part, and is accompanied by a small artery and vein.

  Structure. The pia mater of the cord, though less vascular than that which invests the brain, contains a network of delicate vessels in its substance. It is also supplied with nerves, which, according to Purkinje, are derived from the sympathetic; but Remak states that they are chiefly supplied from the posterior roots of the spinal nerves. At the upper part of the cord, it presents a greyish, mottled tint, which is owing to yellowish or brown pigment cells being scattered within its tissue.

  The Ligamentum Denticulatum (fig. 233) 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, having 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, serving to unite together the two layers of the arachnoid membrane. These serrations are about twenty 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 corresponds to nearly the lower end of the cord. Its use is to support the cord in the fluid by which it is surrounded.