1126 THE MEMBRANES. [BOOK m. 



vertebral canal the arachnoid lies close to the dura mater, so that 

 usually, when the dura mater is slit open and turned back, the 

 arachnoid is carried with it and the cavity exposed is that of the 

 subarachnoid space. The arachnoid, like the dura mater and the 

 pia mater, is continued for some distance over the nerves as they 

 leave the cerebro-spinal cavity ; so that each nerve at its exit is 

 surrounded by a tubular prolongation of the subdural space, and 

 within this a similar tubular prolongation of the subarachnoid 

 space. 



The subdural space is broken up to a slight extent only 

 by bridles carrying nerves and blood vessels, especially venous 

 sinuses, between the pia mater and dura mater, and, over the 

 surface of the brain, by villus-like projections of the arachnoid, 

 called Pacchionian glands, some of which pierce the venous 

 sinuses of the dura mater. It is lined throughout, both on its 

 dural and on its arachnoid wall, by an epithelium of flat epi- 

 thelioid cells, and may be compared to a serous cavity such as 

 that of the peritoneum. Like the serous cavities it contains 

 normally a small quantity only of fluid, and its size is potential 

 rather than actual. 



The subarachnoid space on the other hand is, especially in 

 certain regions, such as the dorsal portions of the vertebral canal 

 and the base of the brain, much broken up by bridles of con- 

 nective tissue passing from it to the pia mater, as well as by 

 a network or sponge-like arrangement of bundles of connective 

 tissue lying immediately beneath itself, and giving it when viewed 

 from below a honeycomb or fenestrated appearance. The under 

 surface of the membrane itself as well as all the trabeculae of the 

 sponge -work and the bridles are covered with an epithelium of 

 flat epithelioid cells, which is continued also over the pia mater 

 and the ligamentum denticulatum, and lines the tubular sheath- 

 like prolongations of the space along the issuing nerve roots. 

 The subarachnoid space therefore, like the subdural space, may be 

 regarded as a serous or large lymphatic space, but it is an actual 

 not a mere potential space ; it always contains an appreciable 

 quantity of fluid, which however is not ordinary lymph, but is 

 furnished in a particular way, and deserves special study. To 

 understand the nature and origin of this cerebro-spinal fluid, as it 

 is called, we must turn to some special arrangements of the pia 

 mater. 



694. The pia mater proper, consisting of interwoven bundles 

 of connective tissue, with some elastic fibres and a considerable 

 number of connective tissue corpuscles, serves as we have said as 

 the bearer of blood vessels to the nervous structures which it 

 invests. The small arteries as they pass into the nervous substance 

 by the way of the septa are surrounded by perivascular lymphatic 

 canals with which spaces in the neuroglial groundwork both of 

 the brain and spinal cord, especially spaces surrounding the larger 



