344 , NERVOUS SYSTEM. 



triangular or prismatic, and runs from the anterior margin of 

 the tentorium to the posterior, where it terminates in the extre- 

 mity of the longitudinal sinus. The general union which is 

 there formed between the longitudinal, the fourth, and the lateral 

 sinuses, constitutes the Torcular Hierophili. 



The anterior extremity of the fourth sinus, besides receiving 

 the inferior longitudinal, is joined by the Vena Galeni, a single 

 trunk, formed by the junction of the two veins of the middle of 

 the velum interpositum, and extending from the posterior margin 

 of the fornix to the beginning of the sinus quartus. The Sinus 

 Quartus, in its course, also receives the superior veins of the 

 cerebellum, with the exception of the most anterior ones, which 

 terminate in the Vena .Galeni. 



5. The Sinus Petrosi are small cylindrical cavities, and are 

 so called from being situated on the petrous bone. There are 

 two on each side; one above and the other below. The former 

 is the Superior, and runs from the cavernous sinus along the 

 superior margin of the petrous bone to join the lateral sinus, 

 where the latter quits the tentorium to descend towards the base 

 of the cranium. The other Petrous Sinus is the Inferior. It is 

 larger than the superior, and arises, also, fix>m the cavernous 

 sinus by its posterior margin. It then runs along the fissure be- 

 tween the occipital and the petrous bone, leaving its mark on 

 the margin of these bones, but principally on the former, and 

 then terminates in the lateral sinus just above the posterior fora- 

 men lacerum. 



6. The Sinus Cavernosi, one on each side, are also formed by 

 a separation of the two laminae of the dura mater, though their 

 shape is so different from that of the others. They are situated 

 at the sides of the sella turcica, and on the body of the sphenoid 

 bone. Their cavity is very irregular, and is furnished with a 

 number of filaments, which cross in every direction, and give 

 it a cellular arrangement. The internal carotid artery and the 

 sixth nerve traverse it, but are protected by its lining membrane 

 being reflected over them. 



The cavernous sinus anastomoses in front with the circular 

 sinus, and behind with the two petrous sinuses and the anterior 



