622 THE NERVOUS SYSTEM 



attenuated portion of the dura mater. These villi are similar in struc- 

 ture to the membranous portion of the arachnoid. They are said to be 

 absent at birth, small and inconspicuous in childhood, and to increase in 

 size and number as age advances. 



Fluid injected into the arachnoid or into the neighboring portions 

 of the subarachuoid space passes readily into the lymph spaces of the 

 dura mater, and may even be forced into the venous cavity of the cranial 

 sinuses. While fluid thus injected may follow artificial rather than nat- 

 ural channels, it seems quite possible that the cerebrospinal fluid may 

 during life find its way along such channels into the venous sinuses to 

 the relief of excessive intracranial pressure. 



The pia mater is intimately adherent to the surface of the brain 

 and spinal cord. It follows all the irregularities of their surfaces and 

 sends prolongations into all their sulci. In the larger fissures these in- 

 vaginations form a double fold of pial tissue; in the smaller, the in- 

 vaginated portions fuse to form a thin septum-like prolongation of the 

 pia. In this particular the pia mater differs from the arachnoid, which 

 bridges over all the sulci without dipping into any but the largest fis- 

 sures. It differs also from the dura mater which, with the exception of 

 the falces and tentorium, is not prolonged into any of the fissures or sulci 

 of either the brain or the spinal cord. 



The pia mater is a connective tissue membrane and is divisible into 

 an inner and an outer layer. The outer layer is composed of coarse 

 fibrous bundles the most of which in the pia mater of the spinal cord run 

 longitudinally, while the finer fibers of the thin inner layer are cir- 

 cularly arranged. 



Between the two layers are many blood-vessels and lymphatics, the 

 pia mater being typically a vascular membrane. The larger blood- 

 vessels are loosely embedded in the outer surface of the pia, some of 

 them projecting into or even lying entirely within the subarachnoid 

 space. The outer surface of the pia mater, as also the sheaths of the 

 vessels which are loosely attached to its surface, is covered with a layer 

 of very thin endothelial cells derived from the lining membrane of the 

 subarachnoid space. 



The inner surface of the pia is everywhere firmly adherent to the 

 surface of the brain and spinal cord. The slender trabeculas and septa- 

 like processes which extend into the superficial portions of these organs, 

 consist of connective tissues whose fibrous bands are continuous with 

 those of the membranous pia mater. In the spinal cord many of these 

 fibrous bundles extend inward as far as the gray matter, meanwhile be- 



