PIA MATER. 371 



and fissures of Sylvius in front, and the pons Varolii behind, is termed 

 the anterior sub-arachnoidean space. Another space formed in a similar 

 manner, between the under part of the cerebellum and the medulla oblon- 

 gata, is the posterior sub-arachnoidean space ; and a third space, situated 

 over the corpora quadrigemina, may be termed the superior sub-arach- 

 noidean space. These spaces communicate freely with each other, the 

 anterior and posterior across the crura cerebelli, the anterior and the supe- 

 rior around the crura cerebri, 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 arach- 

 noid around the spinal cord, the spinal sub-arachnoidean space. The 

 whole of these spaces, with the lesser spaces between the convolutions of 

 the hemispheres, 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 relative 

 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 delicate 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 quan- 

 tity 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 venas Galeni for a short dis- 

 tance only, and returns upon those vessels to the dura mater of the tento- 

 rium. 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 re- 

 flected back upon the inner surface of the dura mater. 



According to Mr. Rainey,f vessels of considerable size, but few in 

 number ; and branches of cranial nerves are found in the arachnoid. He 

 also describes, in this membrane, numberless plexuses and ganglia, which 

 he considers to be analogous to those of the sympathetic nerve. The fibres 

 proceeding from this source are distributed on the arteries and nerves of 

 the cerebro-spinal axis, but particularly on the former. 



Pia Mater. 



The Pia mater is a vascular membrane composed of innumerable ves- 

 sels held together by a thin layer of areolar tissue. It invests the whole 

 surface of the brain, dipping into the sulci between the convolutions, and 

 forming a fold in its interior called velum interpositum. It also forms folds 

 in other situations, as in the third and fourth ventricles, and in the longi- 

 tudinal fissures of the spinal cord. 



This membrane differs very strikingly in its structure in different parts of 

 the cerebro-spinal axis. Thus, on the surface of the cerebrum, in contact 

 with the soft grey matter of the brain, it is extremely vascular, forming re- 

 markable loops of anastomoses in the interspaces of the convolutions, and 



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

 ture that a sub-arachnoid serous membrane may exist in that situation. Such a suppo- 

 sition is quite unnecessary to explain the production of the secretion, since the pia 

 mater is fully adequate to that function. 



f Medico-Chirurgical Transactions, vol. 29. 



