THE PI A MATER 



879 



rane, absent probably at the foramen of Majendie and the two foramina of Luschka 

 and perforated in a peculiar manner by all the bloodvessels as they enter or leave 

 the nervous system. In the perivascular spaces, the pia apparently enters as a 

 mesothelial lining of the outer surface of the space; a variable distance from the 

 exterior these cells become unrecognizable and are apparently lacking, replaced by 

 neuroglia elements. The inner walls of these perivascular spaces seem likewise 

 covered for a certain distance by the mesothelial cells, reflected with the vessels 

 from the arachnoid covering of these vascular channels as they traverse the sub- 

 arachnoid spaces. 



The Cranial Pia Mater (pia mater encephali; pia of the brain) invests the entire 

 surface of the brain, dips between the cerebral gyri and cerebellar lamina?, and is 

 invaginated to form the tela chorioidea of the third ventricle, and the choroid 

 plexuses of the lateral and third ventricles (pages 840 and 841) ; as it passes over 

 the roof of the fourth ventricle, it forms the tela chorioidea and the choroid 



lexuses of this ventricle. On the cerebellum the membrane is more delicate; the 



essels from its deep surface are shorter, and its relations to the cortex are not 



o intimate. 



Subdural cavity 



Pia mater 

 Arachnoid 

 Dura mater 



Subdural cavity 



FIG. 770. Diagrammatic transverse section of the medulla spinalis and its membranes. 



The Spinal Pia Mater (pia mater spinalis; pia of the cord) (Figs. 767, 770) is 

 thicker, firmer, and less vascular than the cranial pia mater: this is due to the fact 

 that it consists of two layers, the outer or additional one being composed of bundles 

 of connective-tissue fibers, arranged for the most part longitudinally. Between 

 the layers are cleft-like spaces which communicate with the subarachnoid cavity, 

 and a number of bloodvessels which are enclosed in perivascular lymphatic sheaths. 

 The spinal pia mater covers the entire surface of the medulla spinalis, and is very 

 intimately adherent to it; in front it sends a process backward into the anterior 

 fissure. A longitudinal fibrous band, called the linea splendens, extends along the 

 middle line of the anterior surface; and a somewhat similar band, the ligamentum 

 denticulatum, is situated on either side. Below the conus medullaris, the pia mater 

 is continued as a long, slender filament (filum terminate), which descends through 

 the center of the mass of nerves forming the cauda equina. It blends with the 

 dura mater at the level of the lower border of the second sacral vertebra, and extends 

 downward as far as the base of the coccyx, where it fuses with the periosteum. It 

 assists in maintaining the medulla spinalis in its position during the movements 

 of the trunk, and is, from this circumstance, called the central ligament of the 

 medulla spinalis. 



The pia mater forms sheaths for the cranial and spinal nerves; these sheaths 

 re closely connected with the nerves, and blend with their common membranous 



vestments. 



