THE PI A MATER 



921 



both the brain and spinal cord ramify in it as they give off the central branches 

 into the nervous substance. The structure and arrangement of the membrane 

 varj' somewhat in the cranial and spinal regions. 



The spinal pia mater consists of two layers, an inner and an outer. It is 

 thicker and more compact than that of the encephalon, due to the extra develop- 

 ment of its outer layer, which is in the form of a strong, fibrous layer with the 

 fibres arranged for the most part longitudinally. 



The spinal pia mater also appears less vascular than the cranial from the fact that the 

 blood-vessels composing the plexus lying in it are obviously much smaller than those of the 

 encephalon. Its inner layer is a thin feltwork of fibres which is closely adherent to the surface 

 of the spinal cord throughout, sending numerous connective-tissue processes into it which 

 contributes to the support of the nervous tissues. The larger of these processes carry with them 

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

 connected with each other, and are distinguished by the difference in the arrangement of their 

 fibres. 



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

 extra thickening along it. This thickening appears as a band along the mid-line of the ventral 

 surface of the cord, the linea splendens (fig. 717). It carries, or ensheathes, the anterior spinal 

 artery, the largest of the arterial trunks of the superficial plexus (fig. 725). 



Fig. 726. — Diagram showing Relations of Meninges to Spinal Nerve-roots. 

 Denticulate ligament 



Body of vertebra 

 Periosteum 



Dura mater 

 Subdural cavity 

 Arachnoid 



Subarachnoid cavity 

 Pia mater 



Intervertebral foramen 



The pia mater contributes the innermost and most compact poi-tion of the epineurium of 

 each of the nerve-roots, and thus, upon the roots, it is prolonged laterally into the intervertebral 

 foramina, where the dura mater blends with it in producing the increased thickness of the 

 epineurium. 



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

 ticulate ligament, 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 about twenty-one pointed processes, which 

 extend through the arachnoid and are attached to the inner surface of the dura 

 mater. The dentations are usually inserted between the levels of exit of the roots 

 of the spinal nerves, the uppermost one a little cephalad to the first cervical nerve 

 and the region where the vertebral artery perforates the dura mater; the most 

 caudal one between the last thoracic and first lumbar nerves, or, between the last 

 two thoracic nerves. The Hgaments, aided slightly by the septum posticum, 

 serve to hold the spinal cord more or less suspended in the subarachnoid 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 caudalward into the sac formed by the dura 

 mater about the cauda equina, and at the end fuses with the dm-a mater in line 

 with the filum of the spinal dura mater (coccygeal ligament) of the outside (figs. 

 613, 715). 



