[ 266 ] [Book IX. 



CHAP. XXVII. 



OF THE BRAIN, &c. 



fbe Dura Mater. "The Falx.Sinufes of the Brain. The Pie 

 Mater. The Cerebrum and Cerebellum. Source of the Optic 

 Nerves. The Pineal Gland. The fuppofed Seat of the Sou/. 

 The Medulla Oblongata. Source of the Nerves. The Spinal 

 Marronu. 



TH E cavity of the cranium is every way fur- 

 rounded with ftrong bones, which have been 

 already defcribed. Within thefe, before we arrive at 

 the fubftance of the brain, we meet with two mem- 

 branes, called by the antients the dura and pia mater, 

 from an opinion that they were the fource of the other 

 membranes of the body. The fame names are ftill 

 applied to them by the moderns, though, as in many 

 other cafes, the opinion which gave rife to them is 

 exploded. 



The dura mater is a thick, firm, infenfible mem- 

 brane, extremely full of blood veiTels. Its external 

 furface performs the part of a periofteum to the in- 

 ternal part of the fkull, to which it adheres by nume- 

 rous blood veffels, particularly at the futures, where 

 they pafs through the cranium to communicate with 

 thofe of the external periofteum. Its internal fur- 

 face is moiftened by the exhalation of a thin fluid, 

 which prevents its adhefion to the membrane within. 



The dura mater forms feveral projections, which 



ferve very important purpofes. One of thefe, from 



its refemblance to the blade of a fcythe, is called th 



falx. Its narroweft end is attached to the crifta galli 



8 of 



