874 HUMAN ANATOMY. 



— chiefly into the tributaries of the lateral sinus. The two sinuses through the 

 superior petrosal sinus and other venous channels, have free anastomotic connection 

 which effectually tends to equalize or distribute blood-pressure. 



The communication between the two cavernous sinuses through the basilar sinus 

 — or plexus — and the circular sinus, is an important portion of the mechanism by 

 which the pressure of venous blood within the skull is equalized. This same com- 

 munication may, however, in a case of anterio- venous aneurism {vide supra) bring 

 about involvement of the orbit on the other side, the blood from the aneurism entering 

 the opposite sinus by way of these intercommunicating sinuses, or infection may 

 follow the same channel. 



* 



7. The Spheno-Parietal Sinus. — The spheno-parietal sinus T sinus spheno- 

 parietalis), also known from its position as the sinus alee parvcB, arises at the outer 

 extremity of the lesser wing of the sphenoid from one of the meningeal veins and 

 passes horizontally inward, under cover of the posterior border of the lesser wing, to 

 reach the cavernous sinus near its anterior extremity. It receives dural, diploic, and 

 some of the anterior cerebral veins. 



8. The Superior Petrosal Sinus. — The superior petrosal sinus (sinus petrosus 

 superior) is the smaller of the two sinuses into which the cavernous divides at the 

 apex of the petrous portion of the temporal. It passes outward and backward along 

 the superior border of the petrous bone and opens into the lateral sinus just at the 

 point where it leaves the line of attachment of the tentorium cerebelli to become the 

 sigmoid sinus. The superior petrosal sinuses receive some small tympanic veins and 

 some branches from the cerebellum and cerebrum. 



9. The Inferior Petrosal Sinus. — The inferior petrosal sinus (sinus petrosus 

 inferior) is the larger terminal branch of the cavernous sinus, and extends from the 

 posterior extremity of that sinus, at the apex of the petrous portion of the temporal 

 bone, along the petro-occipital suture to the jugular foramen, where it opens into the 

 superior bulb of the jugular vein, or, frequently, into the vein below the bulb. 



In addition to small branches from the neighboring portions of the dura and 

 from the cerebellum, pons, and medulla oblongata, the inferior petrosal sinus 

 receives some internal auditory veins and an anterioi- condyloid vein which arises 

 from a plexus surrounding the hypoglossal nerve in its course through the anterior 

 condyloid foramen. In its anterior portion the sinus is also in communication with 

 the basilar sinus. 



10. The Basilar Sinus. — The basilar sinus (plexus basilaris), also termed the 

 transverse sinus, is usually a plexus of sinuses rather than a single, distinct sinus. 

 It occupies the dura mater which covers the basilar process of the occipital 

 bone and communicates with the inferior petrosal and posterior intercavernous 

 sinuses in front, and behind, at the anterior border of the foramen magnum, with 

 the anterior spinal plexus. It receives branches from the medulla oblongata and 

 from the diploe. 



Practical Considerations. — Fracture of the base of the skull through the 

 posterior (cerebellar) fossa may involve the basilar plexus of sinuses and be followed 

 by an intracranial hemorrhage which slowly oozes through the line of fracture and, 

 following the lines of vessels or nerves, ultimately causes swelling and ecchymosis of 

 the skin of the neck ; the latter is apt to show first anterior to the tip of the mastoid, 

 to which region the blood is conducted by the cellular tissue around the auricular 

 artery. It spreads thence upward and backward in a curved line. 



The Diploic Veins. 



The spaces, of the diploe are traversed by a rich plexus of veins, characterized 

 by the thinness of their walls and opening by numerous small communicating 

 branches either into the veins of the scalp, the middle meningeal veins, or the 

 cranial sinuses. Some larger, although rather inconstant, stems also arise from the 

 plexus and form what are termed the diploic veins. Of these, four are usually 

 recognized (Fig. 758). 



