156 A MANUAL OF ANATOMY. 



The Function of the Cerebrospinal Fluid. 



It forms a "water-bed " upon which the brain rests, and 

 is thus protected from injury by coming in contact with the 

 projections of the bones forming the base of the skull. It 

 equalizes the intracerebral pressure, by flowing away from 

 the brain when the blood supply is too great, and toward 

 the brain when it is insufficient. 



The nerves to the arachnoid are derived from the fifth, 

 seventh, and eleventh cranial nerves (Bochdaleck). The 

 lymphatics from the subarachnoid spaces (which are really 

 lymph channels) communicate with those of the internal ear, 

 mucous membrane of the nose, and the superior longitudi- 

 nal sinus ; but are not in direct communication with the 

 general lymphatic system of the head. 



The subarachnoid spaces receive the lymphatics from the 

 brain (and the spinal portion from) the cord. 



DISSECTION. 



Carefully remove the arachnoid covering in the anterior and posterior sub- 

 arachnoid spaces. It will be useless to attempt to remove it from any other 

 part of the brain. 



The dissection of the pia consists in tracing out its vessels. This should 

 be done with great patience ; haste means destruction of important parts. 



The Pia. Figs. 20 and 2 1 . 



This is the most internal membrane enveloping the brain. 

 It is really a layer of blood-vessels held together by suf- 

 ficient connective tissue to preserve their relations, which is 

 closely applied to all parts of the outer surface of the brain, 

 reaching into all the fissures and sulci, and even passing into 

 the interior of the brain as the velum interpositum and 

 choroid plexuses of the lateral, third, and fourth ventricles. 

 It is to be noted that these extensions from the general 

 pia, though apparently within the brain, are still really ex- 



