376 DISSECTION OF THE SPINAL CORD. 



The outer or parietal part is inseparably joined to the inner surface of 

 the dura mater, and gives to that membrane a shining appearance. 1 



The inner or visceral layer surrounds the cord loosely, so as to leave a 

 considerable interval between the two (sub-arachnoid space). At the 

 lower part of the spinal canal the loose sheath is largest, and envelops the 

 mass of nerves forming the cauda equina. As the different spinal nerves 

 extend to the intervertebral foramina, they receive loose tubes from the 

 arachnoid, but lose the same when they perforate the dura mater. 



Dissection. The sub-arachnoid space may be made evident by placing 

 the handle of the scalpel beneath the visceral layer ; or by putting a piece 

 of the cord in water, with the posterior aspect uppermost, and blowing air 

 beneath the serous membrane. 



The sub-arachnoid space separates the loose or visceral part of the 

 arachnoid membrane, from the spinal cord invested by pia mater. Larger 

 at the lower than the upper part of the spinal canal it contains a special 

 fluid cerebro-spinal ; and it communicates with the cavity in the interior 

 of the brain by an aperture in the fourth ventricle. Crossing the space, 

 at the posterior part of the cord, are bundles of fibrous tissue, which are 

 most marked in the neck. In the space are contained the serrations of 

 the ligamentum denticulatum, and the roots of the spinal nerves, with 

 some vessels. 



Dissection. For the purpose of seeing the next covering of the cord 

 with the ligamentum denticulatum, the arachnoid membrane is to be taken 

 away ; and two or three of the anterior roots of the upper dorsal nerves 

 may be cut through and reflected, as in the wood-cut 122. 



The pia mater (fig. 122, h) is much less vascular on the spinal cord 

 than on the brain. Thicker and more fibrous in its nature, the membrane 

 closely surrounds the cord with a sheath, and sends a thin prolongation 

 into the anterior median fissure ; it furnishes coverings to the roots of the 

 spinal nerves. 



The outer surface of the pia mater is rough. Along the front is a cen- 

 tral, anterior fibrous band (linea splendens, Haller) ; and on each side 

 another fibrous band, the ligamentum denticulatum, is attached to it. 

 Scattered through the membrane are branched pigment cells, which give a 

 dark appearance in the cervical region. 



Where the medulla spinalis ceases, viz., about the lower part of the 

 body of the first lumbar vertebra, the tube of the pia mater is suddenly 

 reduced in size, and has the appearance of a round fibrous cord (fig. 124, 

 b). This cord-like part (filum terminale) is provided with nervous sub- 

 stance above ; and blends, below, with the central impervious prolongation 

 of the dura mater. It serves to fix the lower end of the medulla spinalis, 

 and has been named, from that circumstance, the central ligament of the 

 cord. A vein and artery accompany this fibrous piece, and distinguish it 

 from the surrounding nerves. 



The lit/amentum denticulatum (fig. 122, c) is the white, fibrous toothed 

 band, on each side of the spinal cord. It is named from its serrated ap- 

 pearance ; and it has the same structure as the dura mater, except that it 

 wants an epithelial covering. 



Situate between the anterior and posterior roots of the nerves, the band 

 reaches upwards to the medulla oblongata, and ends inferiorly on the 



1 According to the opinion of Kb'lliker, the arachnoid membrane is a simple 

 tube corresponding with the visceral layer in the text. 



