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Membranes of the Spinal Cord

pia, dura, mater, arachnoid, surface, anterior, posterior and subarachnoid

MEMBRANES OF THE SPINAL CORD Dura Mater.—Through the foramen magnum the membranes of the brain are continuous with those of the cord with which they are very similar in structure. The dura mater spinalis is attached to the margin of the great foramen and to the bodies of the first two or three cervical vertebre; elsewhere, though joined to the vertebrx by fibrous bands, its surface is free from immediate bony attachment and it does not possess the periosteal layer. Thus suspended, it hangs as an open sac, or sheath (Fig. 138) and reaches down to the third sacral vertebra where it is constricted to a fibrous cord which blends with the periosteum on the posterior surface of the coccyx. The arachnoid and pia, and the spinal cord and cauda equina are contained in the dural sac (Figs. 135, 136 and 138). Ex ternally, the surface of the dura is separated from the wall of the spinal canal by the internal vertebral plexus of veins, areolar tissue and fat. The outer surface is composed of flat polygonal cells, like the inner surface. Its internal, serous surface is bathed with a small amount of cerebrospinal fluid which separates it from the arachnoid. For every segment of the spinal cord, the dura presents, on either side, a pair of foramina, through which run the anterior and posterior roots of the spinal nerves (Fig. 135). Those nerve roots are invested by a sheath of dura prolonged from the margins of the foramina. The dura mater of the cord does not separate into two lay ers and forms neither sinuses nor processes. It performs no periosteal function and possesses no arachnoid granula tions (pacchionian bodies). Its two surfaces are formed by endothelium.

Arachnoid.—The arachnoid of the spinal cord (arachnoidea spinalis) forms a sac of the same length as the dural sheath, with whichitis externally in contact (Figs. 135, 136 and 138). It presents two serous surfaces. Internally, bands of fibro-elastic tissue attach it to the pia mater along the posterior median line of the cord and form the subarachnoid septum (Fig. 135). The external spinal veins and a considerable space separate the arach noid from the pia mater. That subarachnoid space is filled with fluid. By the ligamenta denticulata it is divided into the anterior and posterior subarachnoid spaces, which, through the foramen magnum, are continuous with the same spaces in the cranial cavity (Figs. 6 and 135).

Lumbar Puncture.—For diagnosis, for the relief of pressure, and to make room for subarachnoid medication, a certain amount of subarachnoid fluid may be drawn off through a lumbar puncture. The puncture is made

either between the third and fourth, or the fourth and fifth lumbar arches; in children always at the latter location. The wide separation of the lumbar arches affords easiest access in this region, and puncture below the fourth lumbar vertebra cannot injure the cord. The normal amount of cerebro spinal fluid is zoo—r30 cc., of which 20-30 cc. are usually withdrawn before introducing such medicinal agents as neo-salvarsan or salvarsanized serum in the treatment of syphilis.

Pia Mater.—The pia of the cord (pia mater spinalis) is much stronger than that of the brain (Figs. 135 and 136). It has two distinct layers, the inner of which is continuous with the brain pia and forms an epineurium for the cord and roots of the spinal nerves. The outer is the more vascular layer. Both layers dip into the anterior median fissure; they form the anterior septum which contains the anterior spinal artery. The inner layer is attached to the septum in the posterior median fissure. The pia mater forms the Linea splendens along the front of the cord and the ligamentum denticulatum on either side. The denticu late ligament is a longitudinal band whose straight medial border is continuous with the pia along the middle of the lateral surface of the cord; its lateral border is notched and its twenty teeth, invested with arachnoid, are attached to the dura opposite the first twenty vertebrae. The two ligaments subdivide the space between the pia and arachnoid into anterior and posterior subarachnoid spaces. A filamentous extension of the pia, 15 cm. long, helps to form the filum terminale internum. It descends in the arachno-dural sheath with the roots of the lumbar and sacral nerves and all together constitute the cauda equina (Fig. 138). For some distance, about 7.5 cm., the filum terminale internum contains gray matter and rudimentary fibers continuous with the spinal cord. The filum unites with the arachnoid and dura at the third sacral vertebra in forming the filum terminale externum which forms a sort of ligament for the spinal cord. The ligament is inserted into the coccyx. The pia mater of the cord contains the trunks and large branches of the anterior and the two posterior spinal arteries, and the tributaries of the external spinal veins.

Nerve Supply.—The membranes of the spinal cord are sup plied by recurrent branches of the spinal nerves and by the sympathetic. The recurrent branches are sensory in function.