THE SKULL. 13 



the tip of the process and again curve forward to end in the jugular foramen and be 

 continued as the internal jugular vein. The S-shaped curve which they make in this 

 part of their course has given rise to the name sigmoid sinus. In its course along 

 the superior curved line the sinus rises above the level of a line drawn from the inion 

 to the centre of the external auditory meatus. 



In operating for cerebellar abscess, care should be taken to place the trephine 

 opening sufficiently low down to avoid wounding this sinus. It is in great danger of 

 being wounded in operating for septic conditions involving the mastoid antrum and 

 cells. Its distance from the surface of the skull varies in different individuals, and it 

 gets farther from it as it descends to the level of the tip of the mastoid process. It 



st olfactory 



sncl optic 



3rd oculomotor 



4th trochlear (pathetic) 



Superior petrosal sinus 

 Inferior petrosal sinus 



7 h facial 



h auditory 



h glossopharyngeal 

 oth vagus (pneumogastric) 

 ith spinal accessory 



2th hypoglossal 



. Occipital sinus 

 Tansverse or lateral sinus 



Superior longitudinal sinus 



FIG. 19. Exit of cranial nerves and venous sinuses at the base of the skull. 



receives the blood from the posterior lower portion of the cerebrum and upper 

 portion of the cerebellum, and communicates with the veins outside the skull through 

 the mastoid and posterior condyloid foramina. 



Running along the upper posterior edge of the petrous portion of the temporal 

 bone, in the attachment of the tentorium, is the superior petrosal sinus. It connects 

 the lateral or transverse sinus about its middle with the cavernous sinus. More 

 deeply situated, and running from the cavernous sinus to the lateral sinus, just as it 

 enters the jugular foramen, is the inferior petrosal sinus. 



The petrosal and lateral sinuses are frequently torn in fractures of the skull. A 

 fracture passing through the petrous portion of the temporal bone may tear the 

 petrosal sinuses, and hemorrhage from the ear might come from this source. A 

 fracture through the posterior cerebral fossa may tear the lateral sinus. Leeches 

 are sometimes applied behind the ear in inflammation of the brain, with the idea of 

 drawing blood from the lateral sinus through the mastoid vein. 



The occipital sinus is usually small and brings the blood up from the region of 

 the foramen magnum to the torcular Herophili. 



The straight sinus runs along the line of juncture of the tentorium and falx 

 cerebri. It receives the blood from the ventricles of the brain which are drained by 



