. Cunningham's Text-book of anatomy. Anatomy. THE PIA MATEE. 673 sinus wall. On each side of the superior sagittal sinus there is a number of irregular spaces in the dura mater which communicate with the sinus either by a small aperture or a narrow channel. These spaces are called the lacunas laterales, and certain of the meningeal veins and some of the diploic veins open into them. Granulations push themselves into the lateral lacunae from below in such a manner that they receive a complete covering from the layer of dura mater which forms the sinus floor. Nor does the bone escape. As the gra
. Cunningham's Text-book of anatomy. Anatomy. THE PIA MATEE. 673 sinus wall. On each side of the superior sagittal sinus there is a number of irregular spaces in the dura mater which communicate with the sinus either by a small aperture or a narrow channel. These spaces are called the lacunas laterales, and certain of the meningeal veins and some of the diploic veins open into them. Granulations push themselves into the lateral lacunae from below in such a manner that they receive a complete covering from the layer of dura mater which forms the sinus floor. Nor does the bone escape. As the granulations enlarge they cause absorption of the cranial wall, and small pits are hollowed out on its internal surface for their reception. It must be clearly understood, however, that in such cases the granulation is separated from the bone by the following :—(1) A con- tinuation round the granulation of the subdural space ; (2) the thinned floor of the lateral lacuna; (3) the lumen of the sinus; and (4) the greatly thinned upper wall of the sinus. The granulations have a special function to perform. Through them fluid can pass from the subarachnoid space into the venous sinuses with which they stand in connexion. Whenever the pressure of blood in the sinuses is lower than that of the fluid in the subarachnoid space and the ventricles of the brain, the cerebro- Granulationes arachnoideales Mouth of a vein. Bone | Superior F sagittal sinus Fig. 598.—Median Section through the Cranial Vault in the Frontal Eegion. Displays a portion of the superior sagittal sinus and the arachnoideal granulations protruding into it (enlarged). spinal fluid filtrates through the granulations into the blood-sinuses. This is not the only way that subarachnoid fluid may obtain exit. The subarachnoid space is carried outwards for a short distance on the nerves in connexion with their arachnoideal sheaths, and communicates with the lymph channels of the nerves. This connexion is more complete in the
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