370 ARACHNOID MEMBRANE. 



irregular channels which receive the venous blood. These are the sinuse* 

 of the dura mater, which have been described at page 338. 



The student cannot see the tentorium and falx cerebelli until the brain 

 is removed ; but he should consider the attachments of the former on the 

 dried skull, for he will have to incise it in the removal of the brain. He 

 should now proceed to that operation, for which purpose the dura mater 

 is to be incised all round, on a level with the section through the skull, 

 and the scissors are to be carried deeply between the hemispheres of the 

 brain in front, to cut through the anterior part of the falx ; then draw the 

 dura mater backwards, and leave it hanging by its attachment to the ten- 

 torium. Raise the anterior lobes of the brain carefully with the hand, and 

 lift the olfactory bulbs from the cribriform fossae with the handle of the 

 scalpel. Then cut across the two optic nerves and internal carotid arte- 

 ries. Next divide the infundibulum and third nerves, and carry the knife 

 along the margin of the petrous bone at each side, so as to divide the ten- 

 torium near its attachment. Cut across the fourth, fifth, sixth, seventh, 

 and eighth nerves in succession with a sharp knife, and pass the scalpel as 

 far down as possible into the vertebral canal, to sever the spinal cord, cut- 

 ting first to one side and then to the other, in order to divide the vertebral 

 arteries and first cervical nerves. Then let him press the cerebellum 

 gently upwards with the fingers of the right hand, the hemispheres being 

 supported with the left, and the brain will roll into his hand. 



The Jirteries of the dura mater are the anterior meningeal from the 

 ethmoidal, ophthalmic, and internal carotid. The middle meningeal and 

 meningea parva from the internal maxillary. The inferior meningeal from 

 the ascending pharyngeal and occipital arteries ; and the posterior menin- 

 geal from the vertebral. 



Its Nerves are derived from the nervi molles and vertebral plexus of 

 the sympathetic, from the Casserian ganglion, the ophthalmic nerve, and 

 sometimes from the fourth. The branches from the two latter are given 

 off while those nerves are situated by the side of the sella turcica ; they 

 are recurrent, and pass backwards between the layers of the tentorium, to 

 the lining membrane of the lateral sinus. Purkinje describes a sympa- 

 thetic plexus of considerable size, as being situated around the vena 

 Galeni at its entrance into the fourth sinus. The filaments froni this 

 plexus are distributed to the tentorium. 



Arachnoid Membrane. 



The Arachnoid (fya.'xyv ^?> like a spider's web), so named from its 

 extreme tenuity, is the serous membrane of the cerebro-spinal centre ; 

 and, like other serous membranes, a shut sac. It envelopes the brain and 

 spinal cord (visceral layer) and is reflected upon the inner surface of the 

 dura mater (parietal layer), giving to that membrane its serous investment. 



On the upper surface of the hemispheres the arachnoid is transparent, 

 but may be demonstrated as it passes across the sulci from one convolu- 

 tion to another by injecting, with a blow-pipe, a stream of air beneath it. 

 At the base of the brain the membrane is opalescent and thicker than in 

 other situations, and more easily demonstrable from the circumstance of 

 stretching across the interval between the middle lobes of the hemispheres. 

 The space which is included between this layer of membrane and those 

 parts of the base of the brain which are bounded by the optic commissure 



