THE MEMBRANES OF THE SPINAL CORD.
As is the brain, so also the spinal cord is surrounded by three envelopes—the dura mater, the arachnoid and the pia mater.
Dura mater spinalis. This membrane forms a strong fibrous investment consist ing of two layers, the outer, which fuses with the periosteum of the and an inner, which is the spinal dura proper. The space between these two layers is filled with loose connective tissue, contains the large venous plexus and is traversed by lymph spaces ; it is the cavum inlerdurale or cavum epidurale. The dura extends as a long wide sac over the conus medullaris, narrows at the level of the second or third sacral vertebra:, thence, as the filum durae matris spinalis, clothes the filum terminate and finally passes into the periosteum of the coccyx.
Arachnoidea spinalis. This is a delicate vascularless membrane, separated from the dura mater by the cavum subdurale and from the pia mater by the cavum subarachnoi deale. It is connected with the subarachnoidal fibres, which are particulary robust and numerous toward the sulcus medianus posterior ; in the lower cervical and in the thoracic region, they form a special partition, the septum subarachnoideale or septum cevicale intermedum. Within the subarachnoidal space the liquor cerebro-spinalis circulates.
Pia mater spinalis. This membrane encloses the spinal cord as a delicate vas cular envelope and forms, by penetrating within the anterior median fissure, the septum anterius. The pia is connected with the dura mater by means of the ligamentum dentic ulatum. The latter consists of from 19-23 small triangular processes, with their bases attached to the pia, which extend outward from the lateral surface of the cord between the anterior and posterior roots of the spinal nerves, to be attached by their points to the dura mater. The ligamentum denticulatum serves as a suspensory band that holds the spinal cord in position.