MENINGEAL COVERINGS 525 



Besides, however, supplying nourishment for the nerve 

 structures, it is instrumental in excreting, or exuding, 

 into the cavities surrounding the central nervous system 

 in all their wide extent, as well as by its attached choroid 

 plexuses into the cavities inter-penetrating the brain and 

 cord, a fluid which has received the name of the cerebro- 

 spinal fluid, and a fluid the functional role of which is 

 of the greatest value in safeguarding the delicate textures 

 composing the central as well as the peripheral nervous 

 system, and of aiding and permitting the uninterrupted 

 performance of the complex functions, mechanical as well 

 as physiological. 



We would remark that the functional role of these 

 meningeal coverings or membranes is also of the highest 

 order, that they perform the important offices within the 

 head and spinal canal of the protection, support, and the 

 affording of facilities for the conveyance of material for 

 nutrition, as well as facilities for the removal of effete 

 matter for excretion from the vitally important organs 

 within their cavities, while they accompany in unbroken 

 continuity every nerve which leaves them. They more- 

 over, between their folds and within their inter-spaces, 

 afford room for the collection of pools, columns, or 

 layers of cerebro-spinal fluid, which become the buffers 

 and liquid supports of the most delicate and impression- 

 able organisms contained within the human body. 



The outer or dura mater is most adherent on its 

 attached, or outer, surface, hard and strong throughout its 

 substance, and covered with a silkily smooth and glistening 

 surface on its inner, like a wall of cartilage. The arach- 

 noid membrane belies not its name; a peculiarity attaches 

 to it, however, which to us seems to betoken that it may 

 take part in the work of excretion from within the head 

 and which is this along the immediate neighbourhood 

 of the longitudinal sinus on either side on the roof of 

 the skull, as well as on other parts, a series of small 

 organisms called " Pacchionian bodies " are observed, 

 consisting of duplications or projections of the arachnoid 

 membrane, which penetrate the overlying dura mater and 

 bone, at least its vitreous table reaching the diplo'e, or 

 even beyond. These bodies seem almost glandular, and 



