THE MEMBRANES OF THE CORD. 



709 



Dura mater 



Arachnoid 

 Post, root 



Ant. root 



Dura mater 



Plexus venosus 



FIG. 378. Transverse section of the spinal 

 cord and its membranes. (Gegenbaur.) 



lymph carried to it by the peri vascular lymphatics is conveyed back into the 

 circulation. 



Structure. 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 (Fig. 377). 

 It covers the entire surface of the cord, to 

 which it is very intimately adherent, form- 

 ing its neurilemma, and sending a process 

 downward into its anterior fissure. It also 

 forms a shea.th 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 

 denticalatum, 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 ter- 

 minale), which descends through the centre 

 of the mass of nerves forming the cauda equina. It perforates the dura about 

 the level of the second or third lumbar vertebrge, 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 

 fibres. 



Structure. 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 fibres, 

 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 sympa- 

 thetic. 



The Ligamentum Denticulatum (Fig. 377) 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. 



Surgical Anatomy. Evidence of great value in the diagnosis of meningitis may be ob- 

 tained by puncturing the theca of the cord and withdrawing some of the cerebrospmal fluid, 

 and the operation is regarded by some as curative, under the supposition that the draining away 

 of the cerebrospinal fluid relieves the patient by diminishing the intracramal pressure. Ine 



