THE MENINGES AND BLOOD SUPPLY 559 



(granulations arachnoidales) which protrude into the venous sinuses 

 to such an extent as often to produce corresponding depressions 

 in the inner surface of the bones of the cranial vault, into which 

 they push, carrying before them a much attenuated portion of 

 the dura mater. These villi are similar in structure to the mem- 

 branous portion of the arachnoid and are abundantly supplied with 

 small blood vessels and lymphatics. 



Fluid injected into these lymphatics or into the neighboring 

 portions of the subarachnoid space passes readily into the lym- 

 phatic 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 natural channels, it seems quite 

 possible that the cerebro-spinal fluid may during life find its way 

 along such channels into the venous sinuses to the relief of exces- 

 sive 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 invaginations form a double fold of pial tissue ; in the smaller, 

 the invaginated portions fuse to form a thin septum-like prolonga- 

 tion 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 fissures. It differs also from the dura mater 

 which, with the exception of the falces and tentorium, is not pro- 

 longed 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 fibres of the thin 

 inner layer are circularly disposed. 



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 trabecuiae 



