974 



THE NERVE SYSTEM 



These bodies are not found in infancy, and very rarely until the third year. 

 They are usually found after the tenth year; and from this period they increase 

 in number as age advances. Occasionally they are wanting. 



The Pia of the Brain (Pia Mater Encephali) (Figs. 722, 723). 



The pia of the brain is a vascular membrane, and derives its blood from the 

 internal carotid and vertebral arteries. It consists of a minute plexus of blood- 

 vessels, held together by an extremely fine areolar tissue. It invests the entire 

 surface' of the brain, dipping down between the convolutions and laminae, and 

 is prolonged into the interior, forming the velum interpositum and the choroid 

 plexuses of the lateral and fourth ventricles. 



SUPERCALLOSAL 

 FISSURE 



OCCIPITAL 

 FISSURE 



ANTERIOR CERE- 

 BRAL ARTERY 



OPTIC 

 NERVE 



CALCARINE 

 FISSURE 



ANTERIOR COMMU- 

 NICATING ARTERY 



INTERNAL POSTERIOR POSTERIOR 

 CAROTID COMMUNICATING CEREBRAL 

 ARTERY ARTERY ARTERY 



FIG. 725. The arteries of the medial surface of the right cerebral hemisphere. (Spalteholz.) 



The velum interpositum or the tela chorioidea superior (tela chorioidea ven- 

 triculi tertii) (Fig. 723) is the prolongation of the pia into the interior of the brain 

 through the medium of the transverse fissure. It is a double triangular vascular 

 fold, that lies between the body of the fornix above and the thalami and the 

 epithelial roof of the third ventricle below, and passes forward to the foramen of 

 Monro. At each edge of the velum interpositum is the choroid plexus (plexus 

 chorioideus ve?itriculi lateralis; paraplexus') of the corresponding lateral ventricle. 

 In front the two plexuses join behind the foramen of Monro, and at the point of 

 junction two lesser choroid plexuses pass back along the under surface of the velum 

 interpositum to the third ventricle, the diaplexus or median plexus (plexus chori- 

 oideus ventriculi tertii). The velar veins or veins of Galen (p. 722) are two veins 

 which lie on either side of the middle of the velum interpositum and pass back. 

 Each velar vein is formed by the union of the vein from the corpus striatum and 

 the choroid vein from the choroid plexuses. The two velar veins unite and form 

 the vena magna (Galeni), which empties into the straight sinus. 



