SPINAL PI A MATER. 541 



cord loosely, so as to leave a considerable interval between the two and leaves 

 the sulw racJt f'uid apace. The loose sheath is largest at its lower JeneaSi. 

 part, where it envelops the mass of nerves forming the cauda 

 equina. Around the roots of each nerve the arachnoid forms a 

 short tube, which is lost as they perforate the dura mater. 



Dissection. The subarachnoid space may be made evident by To show 

 placing the handle of the scalpel beneath the membrane, or by noidspace. 

 putting a piece of the cord in water and blowing air between the 

 arachnoid and pia mater. 



The subarachnoid space separates the arachnoid membrane from Subarach- 

 the spinal cord invested by the pia mater. It is larger below than uc 

 above, and is occupied by the cerebro-spinal fluid. Superiorly it is contains a 

 continuous with the cranial subarachnoid space ; and it com- U1 ' an 

 municates with the cavity in the interior of the brain by means opens into 

 of an aperture in the lower part of the roof of the fourth 

 ventricle (the foramen of Majendie\ Along the back of the cord the 

 space is imperfectly divided by a median partition (septum posticum) an imperfect 

 composed of bundles of fibrous tissue, which is most developed in behind* 

 the neck. Similar fine trabeculae pass between the posterior nerve - 

 tnd the arachnoid. The subarachnoid space also contains the 

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

 some vessels. 



Dissection. In order to see the next covering of the cord, with Dissection 

 the ligamentum denticulatum, the arachnoid membrane is to be covering. 

 taken away ; and two or three of the anterior roots of the upper 

 dorsal nerves may be cut through and reflected, as in fig. 195. 



The PIA MATER (fig 195,/i) is much less vascular on the spinal Pia mater 

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

 membrane closely surrounds the cord with a sheath, and sends a supports 

 thin fold into the anterior median fissure ; it furnishes coverings to 

 the roots of the spinal nerves. gives offsets. 



The outer surface of the pia mater is rough. Along the front is Fibrous 

 a median fibrous band (tinea splendens ; fig. 195, g) ; and on each 

 side another fibrous band, the ligamentum denticulatum, is attached 

 to it. In the cervical region the membrane has usually a rather 

 dark colour, due to the presence of pigment cells in it. 



Where the spinal cord ceases, viz., about the lower edge of the it ends 

 body of the first lumbar vertebra, the sheath of the pia mater con- j^JfiJrous 

 tracts, and gives rise to a slender thread-like prolongation, the cord, the 

 filum terminate or central ligament of the cord (fig. 197 d p. 546). 



This contains a little nervous substance in its upper part ; and 

 below, it blends with the central impervious process of the dura 

 mater. A vein and artery accompany the filum terminate, and 

 distinguish it from the surrounding nerves. 



The ligamentum <l>niiculatum (fig. 195, c) is a white, fibrous band The dentate 

 on each side of the spinal cord, and has received its name from its U 8ament 

 serrated appearance. It serves to support the cord, which is fixed 

 by it to the sheath of the dura mater. 



Situate between the anterior and posterior roots of the nerves, the is fixed on 

 band reaches upwards to the beginning of the medulla oblongata, J? cold? 



