THE PIA OF THE CORD 349 



are not connected together, and the interval between them is named the subdural 

 space (car urn- subdurale). The subdural r ,pace contains a very small amount 

 of lymph-like fluid. There is no communication between the subdural and the sub- 

 arachnoid spaces. The subdural space is prolonged laterad for a short distance 

 on each emerging nerve and communicates with the lymph tract of the nerve. 

 The inner surface of the arachnoid is separated from the pia by a considerable 

 interval, which is called the subarachnoid space (cavum suharachnoideale). The 

 space is largest at the lower part of the spinal canal, and encloses the mass of 

 nerves which forms the cauda equina. Cephalad it is continuous with the cranial 

 subarachnoid space, and communicates with the general ventricular cavity of the 

 brain by means of openings in the pia, in the roof of the fourth ventricle, the 

 foramen of Majendie or metapore and foramina of Key and Retzius. It contains 

 an abundant serous secretion, the cerebrospinal fluid (liquor cerebrospinalis). 

 This secretion is sufficient in amount to expand the arachnoid, and thus to distend 

 completely the whole of the space included in the dura. The subarachnoid 

 space is occupied by trabeculae of delicate endothelial-covered connective tissue, 

 connecting the pia on the one hand with the arachnoid on the other. This is 

 named subarachnoid tissue. 



In addition to this the space is partially subdivided by a longitudinal mem- 

 branous partition, the septum posticum or the dorsal fenestrated septum (septum 

 subarachnoideale), which serves to connect the arachnoid with the pia, opposite 

 the dorsomedian fissure of the spinal cord. It is a partition, but an incomplete 

 and cribriform partition, which consists of bundles of white fibrous tissue inter- 

 lacing with each other, and is coated with endothelium. The dentate ligament 

 (ligamentum denticulatum), which runs from the pia to the dura on either side of 

 the cord, divides the subarachnoid space into an anterior or ventral and a posterior 

 or dorsal space (cavum subarachnoideale anterius et posterius), which joins like 

 spaces in the cavity of the cranium. The external spinal veins (venae spinales 

 externae) lie in the subarachnoid space. 



Structure. The arachnoid is a delicate membrane made up of closely arranged interlacing 

 bundles of connective tissue in several layers. It contains many elastic fibres, and is covered 

 on each side by endothelial cells. The arachnoid contains neither vessels nor nerves. 



The Pia of the Cord (Pia Spinalis). 



The pia of the cord is exposed on the removal of the arachnoid (Figs. 615 

 and 616). It covers the entire surface of the cord, to which it is very intimately 

 adherent, forming its neurilemma, and sending a fold into its ventral fissure. 

 It also forms a sheath for each of the filaments of the spinal nerves, and invests 

 the nerves themselves. A longitudinal fibrous band extends along the middle 

 line on its ventral surface, the linea splendens; and a somewhat similar band, which 

 forms the ligamentum denticulatum, is situated on each side. At the point where 

 the cord terminates the pia becomes contracted, and is continued caudad as a 

 long, slender filament, the filum terminale (Fig. 617), which descends within the 

 sheath of the dura and the arachnoid and through the centre of the mass of nerves 

 forming the cauda equina. It unites with the dura and arachnoid about the level 

 of the third sacral vertebra, and as the central ligament of the spinal cord, the coccy- 

 geal ligament, or the filum durae spinalis the fused membranes extend caudad as 

 far as the base of the coccyx, where they blend with the periosteum. It assists 

 in maintaining the cord in its position during the movements of the trunk. It 

 contains a little gray nerve substance, which may be traced for some distance 

 into its upper part, and is accompanied by a small artery and vein. At the upper 

 part of the cord the pia presents a grayish, mottled tint, which is due to yellow 

 or brown pigment cells scattered among the elastic fibres. 



54 



