TUNICA ARACHNOIDEA. PIA MATER. 373 



nerves as it passes out the foramina of the cranium and spine, 

 to assist in forming their neurileraa or sheath. — 



Tunica Arachnoidea. 



The Tunica Arachnoidea is an exceedingly thin, tender, 

 and transparent membrane, in which no vessels have been 

 hitherto observed. 



It is spread uniformly over the surface of the brain, enclosing 

 all its convolutions, without insinuating itself between any of them. 



At the upper part of the brain it adheres so closely to the 

 subjacent coat by fine cellular substance, that it can scarcely be 

 separated from it ; but in different parts of the base of the brain, 

 particularly about the tuber annulare and medulla oblongata, 

 it is merely in contact with the membrane under it, and may 

 readily be raised from it by the assistance of the blow-pipe. 

 — The Tunica Arachnoidea belongs to the class of serous 

 membranes and forms a double sac — one covering the surface 

 of the brain or rather of the pia mater, and the other giving an 

 internal polished facing to the dura mater. It does not however like 

 the pia mater pass in to line the ventricles. — A separate arachnoid 

 membrane is formed in these cavities. The arachnoid of the 

 outer surface of the brain passes down through the basilar fora- 

 men, to form one of the coverings of the spinal marrow. A 

 loose cellular tissue serves to connect it to the pia mater, both of 

 the brain and spinal marrow, which contains in the healthy state 

 a limpid secretion, amounting to one and a half or two ounces 

 called the sub-arachnoid fluid, which is supposed to serve as a 

 protection to the delicate cerebro-spinal mass. The greater 

 looseness of the arachnoid above noticed over the anterior part 

 of the tuber-annulare and the medulla ablongata, form what are 

 called, the anterior and posterior sub-arachnoid spaces. A third 

 space called the superior is found above the tubercula quadri- 

 gemina. The looseness of the arachnoid over the spinal marrow 

 forms a spinal sub-arachnoid space of considerable dimensions. 



Pia Mater. 

 The Pia Mater, named from its tenderness, is an extremely 

 vascular membrane, made up exclusively of vessels and a deli- 

 cate cellular tissue which unites them together. 

 VOL. II. 32 



