10 THE MENINGES OF THE BRAIN. 



fibro- elastic areolar tissue (Fig. 9). The endothelium covering its 

 outer surface is continuous with that ensheathing the subarachnoid 

 trabeculae. The pia closely follows the brain surface (Fig. 6). 

 Internally, it sends supporting trabeculae into the brain, which 

 transmit blood-vessels; and externally it forms an investing 

 sheath for each cerebral nerve. 



Folds. Two important processes are formed by the pia mater : 

 (i) The chorioid tela of the third ventricle (tela chorioidea 

 ventriculi tertii) is pushed forward into the anterior part of the 

 transverse fissure of the cerebrum between the fornix and the 

 interbrain (Fig. 6). Hence the old name, velum interpositum. It 

 is triangular in shape, with apex directed forward (Fig. 7). Each 

 lateral border is tucked into the chorioidal fissure of the cerebral 

 hemisphere and enters into the floor of the lateral ventricle, while 

 the median part of the fold is in the roof of the third ventricle. 

 Between the two layers of this chorioid tela is some areolar tissue 

 through which run backward the two internal cerebral veins and 

 unite near the base of the tela to form the great cerebral vein. 

 The chorioid plexuses of the lateral and the third ventricles oc- 

 cupy, respectively, the lateral borders and the median area of this 

 chorioid tela. (2) A second fold of pia mater is tucked .into 

 the transverse fissure of the cerebellum, dorsal to the medulla 

 oblongata and ventral to the posterior median part of the cere- 

 bellum (Fig. 6). It is called the chorioid tela of the fourth 

 ventricle (tela chorioidea ventriculi quarti) because its inferior 

 layer enters into the roof and contains the chorioid plexus of that 

 ventricle. This lower layer invests the posterior surface of the 

 medulla and the roof -epithelium of the fourth ventricle (Fig. 8). 

 It is pierced by three foramina which are situated as follows: 

 One over each lateral angle of the fourth ventricle, the lateral 

 apertures (Key and Retzii), and one over its inferior angle. The 

 latter is the largest and is called the median aperture (Magendi). 

 Those three foramina establish communication between the pos- 

 terior subarachnoid space and the fourth ventricle. 



The arteries of the pia mater supply the brain (Figs. 9,- 10, 



11 and 12). They are the anterior, middle and posterior cerebrals; 

 the anterior and posterior chorioidals; and the anterior and pos- 



