524 BIOLOGICAL PHYSICS 



foramina, provided in it for the passage of nerves and 

 blood-vessels which openings, however, are not to be 

 recognised as openings in the strict sense of the word, 

 but as peripheral or lateral continuations of the dural 

 membrane. Thus, it is obvious we have to deal here 

 with a meningeal texture, continuous from its inception, 

 as the lining and covering respectively of the cerebral 

 and spinal cavities and their contained structures, to its 

 termination in the neurilemmar sheaths and spaces, where 

 the nerve terminals or arborisations constitute its external 

 or peripheral boundary. It goes without saying, there- 

 fore, that the spaces enclosed by it must be equally 

 continuous, and that the fluid contents which circulate 

 therein or therethrough, must in turn conform in their 

 movements or circulation to their solid environments. 



The second meningeal texture, the arachnoid, the 

 individuality of which is explained away by some authori- 

 ties, follows the anatomical disposition and distribution 

 of the first or dura mater with undeviating regularity, 

 and is underlaid by a space of like regularity and con- 

 tinuity, the fluid contents of which are possessed of the 

 same or even greater facilities of circulation than are 

 those of the sub-dural space. These two meninges are 

 sparingly vascular, their functional role being mainly 

 mechanical, or supporting and protecting, hence fibrous 

 tissue constitutes the main portion of their substance, 

 their inter-spaces being lined or overlaid by endothelial 

 or epithelial cell investments. 



The third meningeal texture, the pia mater^ is entirely 

 different from the two just described, in histological 

 character being highly vascular and supported by abun- 

 dance of fibrous tissue, it intimately connects itself with 

 the surrounding arachnoid as the arachnoid connects 

 itself with the overlying dura mater and with the fibrous 

 meshes of the underlying neuroglial matrix of brain and 

 cord. It is anatomically continuous, therefore, as a 

 covering or envelope, with the peripheral layer or cortex 

 of the brain and cord, as well as with the endoneurium 

 of the nerves and textures carrying the vascular supplies, 

 by which the neuroglial matrix is maintained and the 

 pabulum of the nervous system proper is supplied. 



