The cranial dura mater is not a simple bag. From its intemal surface partition-like processes pass inwards, which serve to separate certain subdivisions of the encephalon. These are, theft/ix cerebri, the tentoriumcerebelli, and the fills cerebelli.
Thefidx cerebri is a process of fibrous mem brane corresponding to the mesial plane and lying in the great median fissure of the brain, where it separates the lateral hemispheres trom each other. Its shape is falciform ; it,s superior convex border corresponds to the frontal and sagittal sutures, and encloses the great longitu dinal sinus; its inferior border is concave and much shorter than the superior, and corres ponds to the superior surface of the corpus callosum, which connects the hemispheres of the brain. In front the falx is very narrow and almost pointed; it embraces the crista gala of the ethmoid bone, which appears to be enclosed between its layem. As the falx proceeds back wards it increases considerably in depth ; its superior edge may be traced back to the internal occipital protuberance; its inferior edge termi nates at a point corresponding to the middle line of the posterior margin of the corpus callosum. The falx cerebri contains within it, along its posterior border, a large vein, which is called the inferior longitudinal sinus.
The falx cerebri is continuous at its posterior border on each side with the tentorium cere belli. This process is nearly horizontal in its direction ; it forms a vaulted roof to a cavity (the floor of which corresponds to the occipital fossm) in which the cerebellum is lodged. Its upper surface is convex on each side of the attachment of the posterior extremity of the falx cerebri : it supports the posterior lobes of the brain. The inferior surface is adapted to the upper convex surfaces of' the cerebellar hemispheres. Its posterior and outer edge adheres to the occipital bone and to the pos terior border of the petrous portion of the temporal bone, reaching as far inwards as the posterior elinoid processes of the sella Turcica. The occipital portion of this edge contains a considerable part of the lateral sinus (fig 362, e) and that portion which adheres to the petrous bone contains the superior petrosal sinus. The
anterior or inner margin of the tentorium is concave and free in the greater part of its extent; it forms the posterior and lateral boun dary of a large opening (which the sella Tur , cica completes in front), through which the crura cerebri and other parts connected with them pass. This margin is attached by its anterior extremities to the anterior clinoid processes, to reach which it crosses the posterior border. The crossing of these two edges at a point external to the sella Turcica gives rise to the formation of a little triangular space, the base of which corresponds to the sella Turcica; its outer angle is perforated for the transmission of the third pair of nerves, and its anterior one for that of the fourth pair.
From the inferior surface of the tentorium cerebelli at its posterior edge, a short and thick I fold of very slight depth descends to the pos 1 terior edge of the foramen magnum. This is thefalx cerebelli ; it corresponds to the, -ned ian notch between the hemispheres of the cerebellum. Its anterior border is slightly concave. Two veins called occipital sinuses are contained in it.
The internal surface of the cranial dura mater presents the same smooth appearance as we have noticed in the spinal membrane of the same name. We observe, however, an excep tion to this on each side of the line along the great longitudinal sinus. Here it is very com mon to find the membrane presenting a peculiar cribriform appearance, which occupies a space of from half an inch to two inches in length and not more than a quarter of an indi in breadth, but exhibiting great difference in various sub jects as to the number and depth of the foramina or depressions upon which the sieve-like struc ture depends. These depressions are caused by the presence of little bodies which grow from the layer of arachnoid that covers the brain, glandalce Pacchioni, which will be described by-and-bye. The anatomist may expect to find in a large proportion of adult brains a greater or less degree of adhesion between these parts of the dura mater and the edges of the hemispheres of the brain.