263 Blood Veffcls of the Brain, [Book IX. 



into two others, called lateral finufes, which, paffing 

 through the bafe of the cranium, terminate in the ju- 

 gular veins. Near the concourfe of tl* fuperior and 

 lateral finufes, we obferve an opening, which is the ori^- 

 fice of a finus, fituated along the union of the falx and 

 tranfrerfe feptum. 



Thefe finufes are triangular veins, which, being con- 

 veyed through fo firm a membrane as the dura mater, 

 are much lefs liable to be ruptured or diftended ; thefe 

 accidents are flill further guarded againil by certain 

 filaments, which pafs from one fide of the finufes to 

 the other, and give ftill further fecurity againft the bad 

 effects which are found to arife from the preffure of the 

 brain. The veins, which pour their blood into the 

 finufes, enter them in fuch a manner as to produce the 

 effect of a valve, and to prevent the' blood from re^ 

 turning into the tender veffcls of the brain, and thus 

 over-diftending them. 



Befides the finufes above-mentioned there are others 

 of a fmaller fize, which anfwer the fame important pur- 

 poles. All thefe communicate with each other and 

 with the great lateral finufes, and therefore difcharge 

 their blood into the internal jugular veins. 



The cavernous or lateral finufes of the os fphenoides 

 are refervoirs of a particular kind, containing confider- 

 able vcflels and nerves j and likewife a cavernous and 

 fpongy ftructure, which for fome unknown purpofe is 

 conftantly filled with blood. 



The pia .mater is a much fofter and thinner mem- 

 brane than the former ; it is connected to the dura 

 mater only by the veins which open into the finufes. 

 The pia mater confifts of two laminae ; the external 

 of thefe, from its extreme thinnefs, has been compared 

 to the fpider's web, and is named tunica arachnoidea ; 

 ac the upper part of the brain it is connected both tq 



the 



