DISSECTION OP THE BRAIN, OR ENCEPHALON. 241 



Where the two foregoing sinuses meet they form the whirlpool of 

 Herophilus {torcular Herophili), from which the blood is drained away 

 by the transverse sinuses. 



The Transverse Sinuses pass right and left at the periphery of the 

 tentorium cerebelli, and enter the parieto-temporal conduit. In that 

 canal each is continued as the parieto-temporal confluent, from ^yhioh 

 the blood is drained away by the roots of the temporal veins. 



The Cavernous Sinuses. Each of these is placed in the dura mater 

 at the side of the sella turcica of the sphenoid bone. Anteriorly each 

 receives the ophthalmic vein, and posteriorly the right and left sinuses 

 become continuous behind the pituitary gland. The venous arch which 

 th6y thus form discharges its blood through the foramen lacerum basis 

 cranii into the sub-sphenoidal coniiuent. The internal carotid artery 

 traverses the cavernous sinus, and forms while in it a sigmoid curve. 



The Petrosal Sinuses are small, and pass in the tentorium cerebelli 

 on each side, between the transverse and cavernous sinuses. 



The Occipital Sinuses. These are placed in or external to the dura 

 mater lining the cerebellar division of the cranial cavity. They are 

 continuous through the foramen magnum with the spinal sinuses, and 

 their contained blood is drained away by a large vein that passes through 

 the condyloid foramen to join the occipital vein. 



The Meningeal Arteries. These are derived from the meningeal 

 branch of the ophthalmic artery, which enters the forepart of the cavity 

 at the internal orbital foramen ; and from the great niemngeal or sphe?io- 

 spinous branch of the internal maxillary. The spheno-spinoiis artery 

 enters by the foramen lacerum basis cranii, and,- after detaching 

 meningeal branches, enters the parieto-temporal conduit to anastomose 

 with the mastoid artery. Some meningeal twigs are also furnished by 

 the prevertebral branch of the occipital artery (page 196). 



The Meningeal Nerves. Filaments from^the 4th, .5th, 9th, and 10th 

 cranial nerves, and from the sympathetic, are said to have been traced 

 to the dura mater. 



The Arachnoid. This, like the same membrane of the spinal cord, 

 is a delicate transparent membrane. In structure and disposition it is 

 comparable to a serous membrane. Its parietal layer is represented by 

 the endothelial lining of the dura mater ; its visceral layer invests the 

 brain and pia mater ; and the parietal and visceral portions together 

 enclose a space, which is the arachnoid cavity, or subdural space. The 

 free surface of the membrane bounding this space is smooth and moist 

 like a serous membrane. Between the visceral arachnoid and the pia 

 mater another space is left, which is termed the subarachnoid space. 

 This space is most evident over the intervals between the cerebral 

 convolutions, and over surface depressions at the base of the brain, for 

 at these points the arachnoid does not dip down to line the hollows, but 



