Lateral Siuits 39 



and turns in at the transverse fissure to form the velum interpositum 

 and the choroid plexuses. It adheres closely to the cerebral cortex, 

 whilst the arachnoid passes from convolution to convolution without 

 dipping into the sulci. 



Falx and tentorium. Sickle-shaped processes of the dura dip 

 between the hemispheres, down to the corpus callosum, and also be- 

 tween the lobes of the cerebellum ; and a horizontal layer, the tentorium, 

 forms a roofing to the cerebellum, and a support for the posterior lobes 

 of the cerebrum. The attachment of the tentorium may be marked by 

 a line from the external occipital protuberance to the external auditory 

 meatus. 



Venous sinuses are formed by a splitting of the dura ; being part 

 of the vascular system, they are of course completely lined with a flat- 

 tened endothelium. They receive emissary veins from the skull, as 

 well as from the cerebrum and cerebellum. 



The superior longitudinal sinus begins at the crista galli by a vein 

 which it receives from the nasal fossae through the foramen csecum. 

 The sinus grooves the middle of the frontal bone and the adjacent 

 edges of the parietals, and, descending on the occipital, communicates 

 with the torcular Herophili and turns, for the most part, into one of the 

 lateral sinuses. Trephining in the neighbourhood of the sinus may 

 give rise to serious bleeding, and should generally be avoided. The 

 vein which ascends to begin the sinus is in communication with the 

 vessels of the nose ; thus headache which is due to over-fulness of the 

 cerebral vessels may be relieved by epistaxis or by leeching the nose. 

 In its course the sinus receives the superior cerebral veins and a peri- 

 cranial communication through the parietal foramen. 



The course of the sinus may be marked by a line beginning at the 

 root of the nose, passing up the middle of the forehead, backwards 

 along the interparietal suture, and to the external occipital protuberance. 



The lateral sinuses carry the blood from the region of the in- 

 ternal occipital protuberance to the posterior lacerated foramen, and 

 so into the beginning of the internal jugular vein. After leaving 

 the occipital bone the sinus grooves the posterior inferior angle of the 

 parietal, and then the mastoid part of the temporal. As a rule, the 

 right sinus carries away the contents of the superior longitudinal, 

 whilst the left empties the straight sinus. In the case of injury, the 

 surgeon will be loth to trephine near the mastoid process ; but in the 

 case of disease he may have no choice. In its course the lateral 

 sinus receives the superior petrosal sinus, and at its termination in the 

 jugular vein the inferior petrosal. It communicates with the veins of 

 the pericranium by the mastoid vein^ and by small vessels which enter 

 through the posterior condylar foramen. The short mastoid vein 

 runs from the posterior auricular vein through the mastoid bone. 

 When, in the case of meningitis, leeches are applied behind the ear, 

 it is by this vein that the intracranial circulation is relieved. The 



