ARACHNOID MEMBRANE. 365 



wards with the fingers of the right hand, the hemispheres 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 internal carotid. The middle meningeal and meningea parva 

 from the internal maxillary. The inferior meningeal from me as- 

 cending pharyngeal and occipital arteries ; and the posterior menin- 

 geal from the vertebral. 



The 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 the 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. 



Arachnoid Membrane. 



The Arachnoid (d^a^vyj, sTSos, 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, and is reflected upon the inner surface of 

 the dura mater, giving to that membrane its serous investment. 



The arachnoid is thin and transparent on the upper surface of the 

 brain, and may be demonstrated by inserting a blowpipe, and inject- 

 ing beneath it a stream of air. In other situations, as at the base of 

 the brain and between the cerebellum and medulla oblongata, it is 

 semi-transparent and dense in structure, and is rendered very evi- 

 dent by passing across from one convexity to another, and leaving 

 a considerable space between it and the brain. The space which is 

 thus formed between the arachnoid membrane and the interval of 

 the base of the brain between the two middle lobes of the hemi- 

 spheres, has been called by Cruveilhier the anterior sub-arachnoidean 

 space ; and that intervening between the posterior and under part of 

 the cerebellum and the medulla oblongata, the posterior sub-arach- 

 noidean space. Both these spaces communicate with each other 

 across the crura cerebelli. In inflammation of the meninges, this 

 membrane is often thickened and opaque. 



The arachnoid is attached to the pia mater of the brain by a loose 

 cellular tissue, the sub-arachnoidean. This tissue is filamentous at 

 the base of the brain, and between the hemispheres. Around the 

 spinal cord the arachnoid is disposed very loosely so as to leave a 

 considerable space between it and the spinal cord. The spinal sub- 

 arachnoidean space is divided by a partial longitudinal septum which 

 serves to 'connect the arachnoid with the posterior surface of the 

 spinal cord. 



The Sub-arachnoidean cellular tissue and the sub-arachnoidean 

 spaces are the seat of an abundant serous secretion, the sub-arach- 

 noidean fluid, which fills all the vacuities existing between the 

 arachnoid and pia mater, and distends the arachnoid of the spinal 

 cord so completely, as to enable it to occupy the whole of the space 

 included in the sheath of dura mater. 



