398 TUNICA ARACHNOIDEA. 



the veins of the septum lucidum, choroid plexus, corpora 

 striata, and other inner portions of the brain, and termin- 

 ates at the junction of the longitudinal and straight sinus. 



The petrosal sinuses are four in number, two on each side. 

 The superior are between the superior grooved ridge of the 

 petrous bone and tentorium, the inferior at the root of the 

 petrous bone, and both discharge into the lateral sinuses. 



The cavernous sinuses are situated upon the sides of the 

 sella turcica, receive the ophthalmic veins anteriorly, and 

 connect posteriorly with the petrosal sinuses. Tendinous 

 fibres are seen intersecting this sinus, and through it 

 pass the internal carotid artery, the third, fourth, first 

 branch of the fifth and sixth nerves. 



The circular sinus of Ridley is situated in front and be- 

 hind the pituitary body, almost surrounding it, and con- 

 nected with the cavernous. 



The torcular Heropliili is situated upon the internal 

 occipital protuberance, and forms the common point of 

 junction of the longitudinal, lateral, straight, occipital and 

 transverse sinuses 



The occipital sinuses are situated in the falx cerebelli, 

 receive veins from the spinal canal, cerebellum, and adja- 

 cent bone, and terminate in the torcular Herophili. 



The transverse sinus extends across the cuneiform pro- 

 cess of the occipital bone, and connects with the petrosal, 

 cavernous, and lateral sinus of opposite sides. 



The tunica araclmoidea forms the second covering of the 

 brain, and is continuous with that of the spinal marrow. 

 Like all serous membranes, it forms a shut sac, one portion 

 of it covering the brain, called visceral, the other, or 

 parietal, is reflected upon the dura mater. It presents the 

 usual smooth, shining, transparent, and ,delicate appear- 

 ance of serous membranes in general. Its extent is com- 

 mensurate with the whole external surface of the brain, 

 covering all its convolutions, but not dipping down between 

 them. It also lines the interior of the dura mater, and 

 can be traced into the ventricles lining their internal sur- 

 face. The point at which this membrane is seen to enter 



