566 CEREBRO-SPINAL MEMBRANES. 



space in various situations both within the spinal canal and at the base of 

 the brain, stretching thus from the arachnoid membrane to the pia mater. 



Fig. 385. TRANSVERSE SECTION OF THE SPINAL 

 CORD AND ITS ENVELOPES (from Sappey after 

 Hirschfeld and Leveille). 



1, dura mater or tbeca ; 2, parietal layer of 

 the arachnoid membrane: 3, internal or loose 

 .arachnoid ; 4 and 7, snbarachnoid cavity or 

 space ; 5, hinder part of the antero-lateral 

 column; 6, space between the arachnoid and the 

 dura mater, or between the inner and outer folds 

 of the arachnoid membrane : 8, reflection of the 

 one fold into the other; 9, sheath furnished to the spinal nerve by the dura mater; 10, 

 posterior ganglionic root; 11, smaller anterior root; 12, section of the ligamentum 

 denticulatum. This figure does not show the septum which posteriorly divides the 

 subarachnoid space into right and left parts : this would be placed between the arachnoid 

 at 3, and the pia mater covering the posterior surface of the cord. 



As the cerebral and spinal nerves proceed to their foramina of exit from 

 within the dura mater, they are loosely surrounded by tubular sheaths of 

 the arachnoid membrane, which extend along each nerve from the visceral 

 to the parietal layer. 



Structure. When examined under the microscope, the visceral or true arach- 

 noid is found to consist of very distinctly separated riband-like bundles of fibrous 

 tissue interlaced with one another, and a simple layer of scaly epithelium on the 

 surface. Volkmann has described a rich plexus of nerves in the arachnoid mem- 

 brane of certain ruminants. Kolliker has 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 accessorius nerves; and they have likewise been followed by 

 Luschka. 



Cerebro-spinal fluid. This is a very limpid serous fluid, which occupies the sub- 

 arachnoid space. When collected immediately after death, its quantity was found 

 by Magendie in the human subject to vary from two drachms to two ounces. It is 

 slightly alkaline, and consists, according to an analysis by Lassaigne, of 98 '5 parts 

 of water, the remaining 1 '5 per cent, being solid matter, animal and saline. In 

 experiments made on the dog, it was found by Magendie to be reproduced in thirty- 

 six hours, after it had been drawn off by puncturing the membranes at the lower 

 part of the cord. When pressure is made upon the brain, the quantity of fluid in the 

 spinal subarachnoid space is increased, and conversely, it may be forced from the 

 spinal cavity upwards into the cranium. 



Ligamentum 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 mag- 

 num, and reaching down to the lower pointed end of the cord. By its 

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

 outer margin is widely denticulated ; and its denticulations, traversing the 

 arachnoid space, with the arachnoid membrane reflected over them, 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 ; and the others follow in order, alternating with the 

 successive pairs of spinal nerves. In all, there are about twenty-two of 

 these points of insertion. At the lower end, the ligamentum denticulatum 



