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NORMAL HISTOLOGY. 



CHAPTER XVI. 



THE CENTRAL NERVOUS SYSTEM. 

 THE MEMBRANES. 



THE spinal cord and the brain are surrounded by their enveloping 

 membranes, the dura, the arachnoid, and the pia ; these afford 

 additional protection and support the blood-vessels in their course to 

 the nervous tissue. 



The dura consists of interlacing bundles of dense fibro-elastic 

 tissue, in the interspaces between which lie numerous plate-like 

 connective-tissue cells ; many irregular granular elements resem- 

 bling plasma-cells occupy the more superficial layers. The narrow 

 clefts between the fibrous bundles represent lymph-spaces. 



The smooth unattached surfaces of the dura are clothed with 



a single layer of endothelial 



FIG. 317. plates, while the attached sur- 



faces, on the contrary, are rough 

 and without endothelium, but 

 covered with fibrous processes 

 for attachment. The inner sur- 

 face of the visceral layer forms 

 the outer wall of the subdural 

 space, the inner boundary of 

 which is contributed by the op- 

 posed surface of the arachnoid ; 

 the two surfaces, while usually in 

 apposition, are united by very few 

 intervening bands of connective 

 tissue. In some places the outer 

 dural layer is less intimately united 

 with the bone than usual, which 



arrangement produces the epidural spaces ; a more or less perfect 

 endothelial lining exists at such points. 



The dural layers vary in vascularity in different regions ; in addi- 

 tion to the intradural venous sinuses, on either side of the supe- 

 rior longitudinal sinus smaller venous clefts, the parasinoidal spaces, 

 occur ; into these the cerebral veins directly open, the entrance 

 of the veins into the sinus being thus indirect. The arteries of the 

 dura in many places are surrounded by perivascular lymphatic 



Section of membranes from brain of child : D, 

 A , f, respectively the dura, the arachnoid, and 

 the pia ; a, subdural space ; b, meshes of sub- 

 arachnoidean space ; c, blood-vessel within the 

 pia sending branch into cerebral cortex, d. 



