DISSECTION OF THE BRAIN, OR ENCEPHALON. 235 



Where the two foregomg 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 parietotemporal conduit. In that 

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

 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 

 they thus form discharges its blood through the foramen lacerum basis 

 cranii into the sub-sphenoidal confluent. 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 simises, 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, Avhich enters the forepart of the cavity 

 at the internal orbital foramen ; and from the great meningeal or 

 sphe7io-spinous branch of the internal maxillary. The sjyheno-spinous 

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

 meningeal branches, enters the pai'ieto-temporal conduit to anastomose 

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

 the prevertebral branch of the occipital artery (page 191). 



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 2)arietal 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 sei-ous 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 convolu- 

 tions, 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 bridges 



