128 ' THE BRAIN AND ITS APPENDAGES. 



The third membrane has been compared to a spider's web, 

 in allusion to which the name of 'membrana arachnoides has 

 been given to it. It is placed between the pia and dura mater. 



The arteries which supply the brain with blood are the 

 two vertebrals, besides two other branches called the internal 

 carotids. Its blood is returned from the sinuses of the dura 

 mater by the vertebral and jugular veins. 



It is on the supply of the vertebral arteries, however, that 

 the brain mainly depends for its supply, for if ligatures are 

 placed on these arteries, the animal dies ; whereas, both the 

 carotids may be tied without occasioning any apparent ill 

 effects. If our memory serves us, Dr. J. C. Warren, of this 

 city, has performed the latter operation on the human subject 

 with success. 



In raising the bony covering of the brain, we meet with 

 two processes, called the falx, or longitudinal process, and the 

 tentorium, or transverse process. The former resembles the 

 blade of a scythe, hence its name. These processes are 

 formed from duplicatures of the dura neater ; the first descends 

 for a short distance between the lobes of the cerebrum. It takes 

 its rise from the crista galli, and terminates on the os occipitis. 



The tentorium is extended from the inner plate of the os 

 occipitis along the sides of the cranium to its base, whence 

 it may be traced to the sphenoid bone, and is lost in the 

 common covering of the dura mater. It is composed of two 

 laminae : one is continuous with the falx : the other forms 

 that portion of the membraue which covers the cerebellum. 

 The tentorium is equally divided by the falx into two lat- 

 eral portions. 



The sinuses. — The superior, or longitudinal sinus, runs 

 within the duplication of the falx, along its superior border. 



The two lateral sinuses are formed within the duplicative 

 of that part of the tentorium which is attached to the tem- 

 poral and occipital bones ; one extending to the right, and 

 the other to the left. They receive veins both from the cere- 

 brum and cerebellum. 



The cavernous sinuses, so named from the cavernous ap- 



