SPINAL CORD. 383 



SPINAL CORD. 



The dissection of the spinal cord requires that the spinal column 

 should be opened throughout its entire length by sawing through the 

 laminae of the vertebras, close to the roots of the transverse processes, 

 and raising the arches with a chisel, after the muscles of the back 

 have been removed. 



The Spinal column contains the spinal cord, or medulla spinalis ; 

 the roots of the spinal nerves ; and the membranes of the cord, viz., 

 dura mater, arachnoid, pia mater, and membrana dentata. 



The Dura mater (tkeca vertebralis) is continuous with the dura 

 mater of the skull : it is closely attached around the border of the 

 occipital foramen, particularly in front, where it is connected with 

 the posterior common ligament. In the vertebral canal it is connected 

 only by loose cellular tissue, containing an oily fluid, somewhat 

 analogous to the marrow of long bones. On either side and below, 

 it forms a sheath, for each of the spinal nerves, to which it is closely 

 adherent. Upon its inner surface it is smooth, being lined by the 

 arachnoid ; and on the sides may be seen the double openings for 

 the two roots of each of the spinal nerves. 



The Arachnoid is a continuation of the serous membrane of the 

 brain. It encloses the cord very loosely, being connected to it only 

 by long slender cellular filaments, and by a longitudinal lamella 

 which is attached to the posterior aspect of the cord. It passes off 

 on either side with the spinal nerves, to which it forms a sheath ; 

 and is then reflected upon the dura mater, to constitute its serous 

 surface. A connexion exists in several situations between the arach- 

 noid of the cord and that of the dura mater. 



The space between the arachnoid and the spinal cord is identical 

 with that already described as existing between the same parts in 

 the brain, the sub-arachnoidean space. It is occupied in both by a 

 serous fluid, sufficient in quantity to expand the arachnoid, and fill 

 completely the cavity of the theca vertebralis. The sub-arachnoi- 

 dean Jluid keeps up a constant and gentle pressure upon the entire 

 surface of the brain and spinal cord, and yields with the greatest 

 facility to the various movements of the cord, giving to those deli- 

 cate structures the advantage of the principles so usefully applied 

 by Dr. Arnott in the hydrostatic bed. According to Majendie this 

 fluid communicates with the secretion contained in the lateral ven- 

 tricles, by means of an opening which exists in the fibrous layer of 

 the inferior boundary of the fourth ventricle. 



The Pia mater is the immediate investment of the cord ; and, like 

 the other membranes, is continuous with that of the brain. It is 

 not, however, like the pia mater cerebri, a vascular membrane ; but 

 is dense and* fibrous in its structure, and contains very few vessels. 

 It invests the cord closely, and sends a duplicate into the sulcus lon- 

 gitudinalis anterior, and another, extremely delicate, into the sulcus 

 longitudinalis posterior. It forms a sheath for each of the filaments 



