The cord is covered by a continuation downward of the cerebral meninges. It has a dura mater, arachnoid, and pia mater.

Dura Mater

The outer or endosteal layer of the cerebral dura mater ends posteriorly at the edge of the foramen magnum but anteriorly at the third cervical vertebra. The inner or meningeal layer continues downward as a tough fibrous tube from the foramen magnum to the second or third sacral vertebra, and thence is pro-longed downward as a fibrous cord (coccygeal ligament) to be attached to the periosteum over the coccyx. The dura mater in the spine does not, as in the skull, act as a periosteum. The vertebrae have a separate periosteum in addition. Between the dura mater and the bodies of the vertebrae is a somewhat loose space filled with fat, fibrils of connective tissue, and a venous plexus. In injuries these vessels are ruptured and bleed and give rise to clots; the blood, however, does not get inside the membranes and the effusion rarely assumes a sufficient size to produce compression of the cord. These veins pierce the ligamentum subflavum and thus communicate with the dorsal spinal veins. The dura mater is almost never torn in injuries even though the cord may be crushed (Fig. 487).

Arachnoid

The arachnoid of the spinal cord is a stouter membrane than the cerebral arachnoid. Above it is continuous with the cerebral arachnoid at the foramen magnum. Below it blends with the dura at about the level of the third sacral vertebra. Thus it is seen that while the cord itself ends at the lower border of the first lumbar vertebra the subarachnoid cavity is prolonged nearly or quite to the third sacral. As in the brain, the cavity between the arachnoid and the dura above is slight, the two membranes being practically in contact, so that there is almost no subdural space. Between the arachnoid and pia, however, there is a considerable cavity which is continuous with the same space beneath the cerebral arachnoid. It communicates with the fourth ventricle just above the calamus scriptorius by the foramen of Ma-gendie in the median line, and at the sides by the foramina of Key and Retzius, and also by slits at the descending horns of the lateral ventricles. Hence it is that the ventricular fluid can be drained by a lumbar puncture.

Through this subarachnoid space pass the septum posticum behind and the ligamenta denticulata on each side from the pia to the dura mater. It is also traversed by the anterior and posterior roots of the spinal nerves, the former being in front and the latter behind the liga-mentum denticulatum. The arachnoid contains neither vessels nor nerves (Fig. 488).

Pia Mater

The spinal pia mater is thin and closely invests the cord. It carries the blood-vessels of the cord and sends prolongations posteriorly to the dura as the septum posticum, laterally as the two ligamenta denticulata, and also around the anterior and posterior roots of the spinal nerves.