MEMBRANES OF THE BRAIN. 345 



occipital. It receives, in front, the ophthalmic veins; from above, 

 the anterior and inferior cerebral veins; and on the sides, some 

 veins from the dura mater. 



V 



7. The Sinus Circularis is placed in the sella turcica, and 



surrounds the pituitary gland. It is a small cavity which re- 

 ceives the veins of this gland, and, as just mentioned, commu- 

 nicates with the cavernous sinus. 



8. In the posterior part of the base of the cranium, there are 

 also some other sinuses, called, from their position, Occipital. 

 One of these, the Anterior, is upon the basilar process of the os 

 occipitis, and extends itself directly across the bone, from the 

 hind part of one cavernous sinus to the corresponding point of 

 the other; and is, therefore, a means of communication between 

 these two cavities. Another of these sinuses, the posterior, ex- 

 tends from the region of the torcular Hierophili, or the upper extre- 

 mity of the lateral sinuses, along the base of the falx cerebelli, 

 to the posterior margin of the occipital foramen, where it bifur- 

 cates, and then goes along the margin of this foramen to dis- 

 charge itself into the lateral sinus at the posterior foramen la- 

 cerum. 



The smaller sinuses about the base of the cranium, besides 

 the outlets mentioned, have collateral ones, which pass at dif- 

 ferent places through the base of the cranium, and run into the 

 branches of the internal jugular vein. These communications, 

 as mentioned in the account of the veins, were known to San- 

 torini, and are called his Emissaries. 



Of the Tunica Jlrachnoidea. 



This membrane is the second of the envelopes of the brain, 

 and is spread over the surface of the pia mater, adhering to it 

 closely in the greater part of its extent. It is so diaphanous 

 and thin, as its name implies, that it is distinguished with some 

 difficulty, wherever it adheres to the pia mater; which it does 

 all over, with the exception of some few places on the basis of 

 the brain, as, for example, just in front of the tuber annulare, 

 and behind the medulla oblongata. There this membrane may 



