The _outer_ table is the most elastic layer of the calvarium, and it varies greatly in thickness in different skulls and in different parts of the same skull. It is nourished chiefly from the pericranium which is firmly bound down along the lines of the sutures. The _inner_ or vibreous table is thin and fragile, and its smooth internal surface is grooved by the middle meningeal and other arteries of the dura mater, and by the large venous sinuses. The intermediate layer--the _diploë_--is highly vascular, branches of the meningeal vessels anastomosing freely in its open porous substance with branches derived from the pericranial vessels. Some of its veins open into the external veins, and others into the intra-cranial sinuses, and they communicate with the emissary veins as these pass through the bone, which explains the spread of infective processes from the structures outside the skull to those within. The possibility of withdrawing blood from the interior of the skull by leeching, bleeding, or cupping depends on the existence of the emissary veins. _The Membranes of the Brain._--The _dura mater_ is a fibro-serous membrane, the outer, fibrous layer constituting the endosteum of the skull, the inner, serous layer forming one of the coverings of the brain. Between the fibrous layer and the bone the meningeal vessels ramify; and along certain lines the two layers split to form channels in which run the cranial venous sinuses. Inside the dura, and separated from it by a narrow space--the _sub-dural space_--lies the _arachno-pial membrane_, consisting of an outer (_arachnoid_) layer which envelops the brain but does not pass into the sulci, and a highly vascular inner layer--the _pia mater_--which closely invests the brain and lines its entire surface. The space between these layers--the _sub-arachnoid space_--is traversed by a network of fine fibrous strands, in the meshes of which the cerebro-spinal fluid circulates. Each nerve-trunk as it leaves the skull or spinal canal carries with it a prolongation of each of these membranes and their intervening spaces. The membranes gradually become lost in the fibrous sheaths of the nerves, and the sub-dural and sub-arachnoid spaces become continuous with the lymph spaces of the nerves. The _cerebro-spinal fluid_ is secreted by the choroid plexuses and fills the cerebral ventricles, the central canal of the cord, the sub-dural and sub-arachnoid spaces, and the sheaths of the intra-cerebral blood vessels. At the base of the brain, particularly in the posterior fossa, the sub-arachnoid space is wider than elsewhere, forming "cisterns" filled with cerebro-spinal fluid which supports the cerebral structures. Through the foramen of Magendie in the roof of the fourth ventricle the sub-arachnoid fluid of the cranial cavity communicates with that of the vertebral canal.