408 ARACHNOID MEMBRANE. 



mer upon 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 be- 

 tween the hemispheres of the brain in front, to cut through the ante- 

 rior part of the falx ; then draw the dura mater backwards, and leave 

 it hanging by its attachment to the tentorium. 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 arteries. Next divide 

 the infundibulum and third nerve, and carry the knife along the mar- 

 gin of the petrous bone at each side, so as to divide the tentorium 

 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, 

 cutting 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 forwards with the fingers of the right hand, the he- 

 mispheres being supported with the left, and the brain will roll into 

 his hand. 



The Arteries 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 

 meningeal 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 last are 

 given off while those nerves are situated by the side of the sella tur- 

 cica ; they are recurrent, and pass backwards between the layers of the 

 tentorium, to the lining membrane of the lateral sinus. 



Arachnoid membrane. 



The Arachnoid (&&%**} tfdos, like a spider's web), so named from 

 its extreme tenuity, is the serous membrane of the cerebro-spinal 

 centre, and, like dther 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 mem- 

 brane its serous investment. 



On the upper surface of the hemispheres the arachnoid is transpa- 

 rent, but may be demonstrated as it passes across the sulci from one 

 convolution 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 



