LIGAMENTUM DENTICULATUM. 



189 



blends with both the epineurium and perineurium of the nerves. Accordingly it is 

 found that injection driven into either the subdural or the subarachnoid space passes 

 readily along the nerves even as far as the limbs. There thus exists a continuity 

 between the ventricles of the brain, the subarachnoid space, and the lymphatic 

 spaces within the nerve-sheaths. 



Structure. When examined under the microscope, the arachnoid membrane is 

 found to consist of distinct riband-like bundles of fine fibrous tissue interlaced with 

 one another. The intervals between these bundles are filled up by delicate 

 membranes, composed of expanded cells, the nuclei of which persist and are scattered 

 over the structure. Several layers of this tissue, arranged in a complex way, 



Fig. 132. SECTION OF THE SPINAL CORD 



WITHIN ITS MEMBRANES (UPPER 



DORSAL REGION). (Key and Retzius.) 

 Magnified. 



a, dura mater ; b, arachnoid ; c, sep- 

 tum posticum ; d, c, f, subarachnoid 

 trabeculae, those at /, /, supporting bun- 

 dles of a posterior nerve-root ; g, liga- 

 mentum denticulatum ; h, sections of 

 bundles of an anterior nerve-root ; k, I, 

 subarachuoid space. 



constitute the arachnoid mem- 

 brane proper. The subarachnoid 

 trabeculae consist of bundles of 

 similar fine fibrillar tissue, each 

 of which is surrounded by a deli- 

 cate nucleated sheath, also com- 

 posed of cells, and continuous 

 with the intertrabecular cell- 

 membranes of the arachnoid itself. The finer trabeculae when swollen by acetic 

 acid very frequently show the well-known ring-like constrictions. The subarachnoid 

 membranous expansions have a similar structure. In the spinal arachnoid the 

 fibril-bundles have for the most part a longitudinal direction. 



Volkmann described a rich plexus of nerves in the arachnoid membrane of 

 certain ruminants. Kolliker failed to detect their presence ; but they have been 

 again described by Bochdalek, who traces them to the portio minor of the fifth, the 

 facial, and accessory nerves ; and they have likewise been followed by Luschka. 



Ligament-am denticulatum. This is a narrow fibrous band which runs along 

 each side of the spinal cord in the subarachnoid space, between the anterior and 

 posterior roots of the nerves, commencing above at the foramen magnum, and 

 reaching down to the lower pointed end of the cord (fig. 133, 9, and fig. 132, g\ 

 By its inner edge this band is connected with the pia mater of the cord, while its 

 outer margin is widely denticulated ; its denticulations are attached by their points 

 to the inner surface of the dura mater, and thus serve to support the cord along 

 the sides, and to maintain it in the middle of the cavity. The first or highest 

 denticulation is fixed opposite the margin of the foramen magnum, between the 

 vertebral artery and the hypoglossal nerve (shown in fig. 182 of Vol. II.) ; the others 

 follow in order, alternating with the successive pairs of spinal nerves. In all, there 

 are about twenty-one of these points of insertion, but the lower six or seven arc less 

 regular. The points of the lower denticulations are prolonged into threads, and 

 ascend slightly to their attachments. At the lower end, the ligamentum denticu- 

 latum may be regarded as continued into the terminal filament of the spinal cord, 

 which thus connects it to the dura mater at the extremity of the sheath. The free 

 edge, in the intervals between the denticulations, is slightly thickened, and in many 



