THE CENTRAL NERVOUS SYSTEM. 383 



channels ; these canals open into the sub- or the epi-dural spaces 

 on the one hand, and stand in close relation with the blood-vessels 

 on the other. The veins of the dura are of much greater size than 

 the corresponding arteries. 



The nerves of the dura are not numerous, but consist of both 

 medullated and pale fibres, chiefly distributed to the walls of the 

 blood-spaces. 



The arachnoid is a connective-tissue membrane of great delicacy, 

 the component fibres being loosely held together rather than arranged 

 as distinct bundles. The free surfaces of the membrane, including 

 the numerous trabeculae on its inner side, are covered with endo- 

 thelium. The arachnoid lies closely applied, but slightly attached, 

 to the inner surface of the dura, while between the arachnoid and the 

 pia the considerable subarachnoidean space exists. 



Scattered over the outer surface of the arachnoid small villous 

 elevations project into the subdural space ; a core of connective 

 tissue, covered by a reflection of the endothelium, constitutes 

 these structures. In various situations, but particularly in the 

 neighborhood of the superior longitudinal sinus, the arachnoidal 

 villi become hypertrophied and form the Pacchionian bodies : these 

 press against the opposed dural surface and push the latter before 

 them where least resistant ; such points occur where the lamina of 

 the dura separate. The arachnoidal projections encroach upon 

 the dura to such a degree that its tissue is largely absorbed, the 

 cauliflower excrescence being separated from the venous current by 

 an extremely thin layer. In localities where the projections press 

 against the bones, conspicuous depressions on the inner cranial sur- 

 face mark the positions of the Pacchionian bodies ; these latter not 

 infrequently contain small, hard, calcareous concretions, the 

 * ' brain-sand. ' ' The arachnoid contains neither blood-vessels nor 

 nerves. 



The pia is the vascular membrane, and consists of two lamellae, 

 an outer layer, rich in blood-vessels, and an inner stratum, less 

 vascular, but closely associated with the nervous tissue, to which it 

 contributes a partial supporting framework. The pial stroma is 

 composed of interlacing fibro-elastic bundles, between which lie 

 the numerous blood-vessels, surrounded by perivascular lymphatics ; 

 the vessels, invested by delicate prolongations of both connective 

 tissue and lymph-sheath, pass into the nervous tissue. The free sur- 

 face of the pia is covered by endothelium, as are also the trabeculae 

 subdividing the subarachnoidean space and connecting the arach- 

 noid and the pia. The pia of the cord is composed of coarser fibres 

 than that of the brain. 



The dura and the arachnoid do not follow the irregularities of 



