470 NEUROLOGY. 



In some places its layers separate to form irregular channels, the 

 sinuses of the dura mater, which are lined by a continuation of 

 the inner coat of the veins, for the reception of the venous 

 blood. On its external surface, the branches of the meningeal 

 arteries can be traced ; and from the inner surface, which is 

 smooth and lined by the arachnoid membrane, processes are 

 directed inwards for the support and protection of the different 

 parts of the brain. These membranous processes, regarded as 

 duplicatures of the inner layer of the' dura mater, are three in 

 number — viz., the falx cerebri, tentorium cerebelli, and falx 

 cerehelli. 



The falx cerebri, so named in the human subject from its 

 being sickle-shaped, is an arched process, which descends in the 

 longitudinal fissure between the hemispheres of the cerebrum. 

 Anteriorly it is attached to the crista galli process, and posteriorly 

 to the upper part of the ossific tentorium. Its convex upper 

 border, along which runs the superior longitudinal sinus, is 

 attached along the longitudinal auture joining the pairs of parietal 

 and frontal bones ; its under border is free. 



The tentorium cerebelli is a transverse partition, formed by 

 lateral inflections of the dura mater, between the cerebellum and 

 the hemispheres of the cerebrum. Each lamina presents an outer 

 convex adherent border, attached to the lateral ridges of the ossific 

 tentorium, and enclosing a lateral sinus ; an inferior concave, free 

 border, turned outwards and a little forward ; a superior extremity 

 attached to the ossific tentorium ; and an inferior extremity, 

 which reaches the upper part of the Gasserian ganglion, and 

 contains the petrosal sinus. 



The falx cerebelli is a short double process, proceeding from 

 the ossific tentorium towards the foramen magnum, between the 

 lobes of the cerebellum. The occipital sinus is found witliii* 

 its folds. 



The Aeachnoid membrane, like that of the cord, is a closed 

 sac, the parietal layer of which is firmly adherent to the dura 

 mater, and is reflected over the several processes. The visceral 

 layer is thin and transparent on the upper surface of the brain, 

 to which it is loosely attached by the subarachnoidean cellular 

 tissue ; at the base, where the subarachnoidean spaces are found, 

 it is thicker and more opaque. These spaces, three in number 

 (anterior, middle, and posterior), contain the cerebro-spinal fluid ;. 

 they communicate freely with each other, and with the spinal 



