PIA MATER. . 409 



layer of membrane and those parts of the base of the brain which are 

 bounded by the optic commissure and fissures of Sylvius in front, and 

 the pons Varolii behind, is termed the anterior subarachnoidean space. 

 Another space formed in a similar manner, between the under part of 

 the cerebellum and the medulla oblongata, is the posterior sub-arachnoi- 

 dean space; and a third space, situated upon the corpora quadrigemina, 

 may be termed the superior sub-arachnoidean space. These spaces 

 communicate freely with each other, the anterior and posterior across 

 the crura cerebelli, the anterior and the middle around the crura cere- 

 bri, and the latter and the posterior across the cerebellum in the course 

 of the vermiform processes. They communicate also with a still 

 larger space formed by the loose disposition of the arachnoid around 

 the spinal cord, the spinal sub-arachnoidean space. The whole of these 

 spaces, with the slighter spaces between the convolutions of the hemi- 

 spheres, constitute one large and continuous cavity which is filled with 

 a limpid, serous secretion, the sub-arachnoidean fluid* a fluid which is 

 necessary to the maintenance and protection of the cerebro-spinal mass. 

 The quantity of the sub-arachnoidean fluid is determined by the re- 

 lative size of the cerebro-spinal axis and that of the containing cavity, 

 and is consequently very variable. It is smaller in youth than in old 

 age, and in the adult has been estimated at about two ounces. The 

 visceral layer of the arachnoid is connected to the pia mater by a de- 

 licate areolar tissue, which in the sub-arachnoidean spaces is loose and 

 filamentous. The serous secretion of the true cavity of the arachnoid 

 is very small in quantity as compared with the sub-arachnoidean 

 fluid. 



The arachnoid does not enter into the ventricles of the brain, as 

 imagined by Bichat, but is reflected inwards upon the venae Galeni for 

 a short distance only, and returns upon those vessels to the dura mater 

 of the tentorium. It surrounds the nerves as they originate from the 

 brain, and forms a sheath around them to their point of exit from the 

 skull. It is then reflected back upon the inner surface of the dura 

 mater. 



There are no vessels apparent in the arachnoid, and no nerves have 

 been traced into it. 



Pia mater. 



The Pia mater is a vascular membrane composed of innumerable 

 vessels held together by a thin layer of fibro-cellular tissue. It invests 

 the whole surface of the brain, dipping into the sulci between the con- 

 volutions, and forming a fold in its interior called velum interpositum. 

 It also forms folds in other situations, as in the third and fourth ven- 

 tricles, and in the longitudinal grooves of the spinal cord. 



* The presence of a serous fluid beneath the arachnoid has given rise to the 

 conjecture that a sub-arachnoid serous membrane may exist in that situation. 

 Such a supposition is quite unnecessary to explain the production of the secre- 

 tion, since the pia mater is fully adequate to that function. 



