THE PI A MATER 



895 



for the most part longitudinally. The spinal pia materalsoappearslessvasciilarthan 

 (lie cranial from the fact lhal the Mood-vessels composing the plc.Mis lying in it are 

 oliviously much smaller than those of the encephalon. Its inner layer is a thin 

 felt work of lihres which is closely adherent to the surface of the spinal cord through- 

 out, sending numerous connective-tissue processes into it which contribute to the 

 >up|)ort of the nervous tissues. The larger of these process carry \\ith them the 

 numerous intrinsic blood-vessels from the superficial plexus. The two layers are 

 closely connected \vith each other, and are distinguished by the difference in the 

 arrangement of I heir fibres. 



The membrane dips into the anterior median fissure and bridges it over by 

 forming an extra thickening alon.r it. This thickening appears as a band along the 

 mid-line of the ventral surface of the cord, the linea splendens (fig. 057). Tt carries 

 the anterior spinal artery, the largest of the arterial trunks of the superficial plexus 



(fig. G( ;:.). 



The pia mater contributes the innermost and most compact portion of the epineu- 

 rivim of each of the nerve-roots, and thus, upon the roms. it is prolonged laterally into 

 the intervertebral foramina, where the dura mater blends with it in producing the 

 increased thickness of the epineurium. 



FIG. r>< iii. l)i U;KAM SHOWING RELATIONS OF MEXIXUKS TO SPIXAL XKRVE-HOOTS. 



Ligamentum dentlculatum 



" BODY OF VERTEBRA 



Peri oat eum 

 Dura mater 



81 I:KI-I:AL CAVITY 



Arachnoid 



A7 /:.t/:.\r//\OTD 



INTERVERTBBRAL 



i: i.i//..-.v 



Pia mater 



From each side of the cord the pia mater gives off a leaf-like fold, the ligamentum 

 denticulatum, which spreads laterally toward the dura mater midway between the 

 lines of attachment of the dorsal and ventral nerve-roots. The outer border of this 

 fold is dentate or scalloped into twenty-one pointed processes, which extend through 

 the arachnoid and are attached to the inner surface of the dura mater. The denta- 

 tions are usually inserted bet ween the levels of exit of the roots of the spinal nerves, 

 the uppermost one a little above the first cervical nerve and the region where the 

 vertebral artery perforates the dura mater; the lowermost one between the last 

 thoracic and first lumbar nerves, or, between the last two thoracic nerves. The liga- 

 menta denticulata. aided slightly by the septum posticum, serve to hold the spinal 

 cord more or less suspended in the subaractmoid cavity. 



Below, at the sudden, conical termination of the spinal cord in the lumbar portion 

 of the spinal canal, the pia mater is spun out into a thin, tubular filament, the 

 filum terminale, which continues downwards into the sac formed by the dura mater 

 about the cauda oquina, and at the end fuses with the dura mater in line with the 

 filum of the spinal dura mater (corcyireal ligament) of the outside (figs. 566 and 655). 



The cranial pia mater is closely applied to the external surface of the brain, 

 dipping into all the fissures, furrows, and sulci. It is connected with the arachnoid 

 by numerous filaments of the spongy subarachnoid tissue and by the blood-vessels 



