132 THE HUMAN BODY. 



cerebellum, which isolates it from the cerebral lobes. It also 

 forms the falx cerebelli, which springs from the base of the 

 skull between the two hemispheres of the lesser brain ; and, 

 lastly, it extends beyond the cranium, and furnishes the 

 covering of the optic nerve, and the periosteum of the cavity 

 of the orbit. 



These folds and partitions formed by the dura mater 

 around and between the organs of the encephalon, serve to 

 maintain the different parts in place, to prevent their collision 

 'in shocks received on the body, and to prevent the parts from 

 falling upon each other in certain positions; as, for instance, 

 in lying on one side the falx cerebri prevents the weight 

 of one hemisphere from resting on or compressing the other. 

 The disposition of the sinuses of the dura mater is not less 

 remarkable; they are, as already stated, venous canals with 

 inextensible walls, through which the circulation is easy, and 

 in no danger from obstruction or suspension ; and when there 

 is an afflux of blood it cannot compress the brain, as would 

 be the case if the sinuses were replaced by veins with exten- 

 sible walls. 



Arachnoid. This membrane has been compared to a 

 spider's web from its extreme tenuity, and from this it 

 derives its name. It is a serous membrane, and lines the 

 dura mater throughout its whole extent. Like the other 

 serous membranes, it is a sac without an opening, the walls 

 of which, placed back to back, secrete a fluid. It adheres 

 very strongly, by its external wall, to the dura mater, to which 

 it moulds itself, and which it accompanies throughout its 

 whole extent. Its internal wall is united to the pia mater, 

 which separates it from the nervous substance at many points. 

 This union is so intimate that many have thought the arach- 

 noid has no existence except where it is detached from the 

 pia mater, as at the level of the fissure of Sylvius, and in the 

 cerebral sinuses, &c. In fact, the arachnoid does not enter 

 those intervals where the dura mater does not penetrate, but 

 is strictly confined to it. At these intervals there is a cavity 

 between the serous membrane and the nervous centre which 

 it surrounds, but does not touch, as is well illustrated by the 

 spinal cord. 



