THE PIA MATER OF THE BRAIX. 



munications with the inferior horn of the lateral ventricle; the 

 posterior space communicates with the fourth ventricle (Fig. 8) 

 through the median aperture (apertura mediana ventriculi quarti, 

 Magendi} and the lateral apertures (apertura laterales ventriculi 

 quarti, Key and Retzii). 



The vessels seen for a short distance in the arachnoid belong 

 to the pia mater. Its nerves are doubtful. Perhaps branches 

 of the mandibular, of the facial and of the accessory supply it. 



Pia mater Subarachnoid space 



Arachnoid 



Third ventricle 

 Infundibulum 



Cisterna interpeduncularis 

 Cisterna pontis 



Fourth 

 ventricle 

 Cisterna cere- 

 bello-medul- 



laris 



Median apert- 

 ure (Magendi) 



Fig. 6. Diagram of pia and arachnoid, showing subarachnoid spaces. 

 (After Morris's Anatomy.) 



In the arachnoid of the cord fewer trabeculae join it to the pia; 

 and these, in great part, are collected to form a fenestrated sep- 

 tum hi the posterior median line (Fig. 97, A). The external 

 spinal veins are covered by the spinal arachnoid, they lie between 

 it and the pia. 



THE PIA MATER OF THE BRAIN. 



(Pia Mater Encephali.) 



Structure and Relations. It is a vascular membrane com- 

 posed of a close network of veins and arteries held together by 



