MEMBRANES 



375 



Pia Mater Encephali. The pia mater forms the immediate 

 investment of the brain. It is finer and more delicate than 

 the corresponding membrane of the spinal medulla, and it 

 follows closely all the inequalities on the surface of the brain. 

 Thus, in the case of the cerebrum, it forms a fold within 

 every sulcus and lines both sides of the cleft. On the cere- 

 bellum the relation is not so intimate ; it is only the larger 

 fissures of the cerebellum which contain folds of pia mater. 



It has been noted that the larger blood vessels of the 

 brain run in the subarachnoid space j the finer twigs enter the 



Lateral lacuna 



Arachnoideal 

 granulation 



Arachnoideal granulation 

 Lateral lacuna 



Dura mater 



mater 

 Subarachnoid space 



Arachnoid 



Falx cerebri 



FIG. 142. Diagram of a frontal section through the middle portion of the 

 cranial vault and subjacent brain to show the membranes of the brain 

 and the arachnoideal granulations. 



pia mater, where they ramify and anastomose before passing 

 into the substance of the brain. As they enter the brain they 

 carry with them sheaths derived from the pia mater. Con- 

 sequently, if the dissector raises a portion of that membrane 

 from the surface of the cerebrum, a number of fine processes 

 will be seen to be withdrawn from the cerebral substance. 

 They are the blood vessels, and they give the deep surface 

 'of the membrane a rough and flocculent appearance. 



The pia mater is not confined to the exterior of the brain. 

 A fold is carried into its interior. This will be exposed in 

 the dissection of the brain, and will be described under the 

 name of the tela chorioidea (O.T. velum interpositum) of the 

 third ventricle (Fig. 174). 

 in 24 c 



