THE NERVOUS SYSTEM 1405 



continuous with the pia mater, which has entered through the 

 transverse fissure, situated between the splenium of the corpus 

 callosum and the corpora quadrigemina. Chi either side the velum 

 interpositum projects beyond the lateral border of the fornix, and 

 appears as a vascular fringe in the lateral ventricle, where it is 

 covered by the ventricular epend^Tua. This fringe is known as the 

 choroid plexus of the lateral ventricle. Posteriorly it is prolonged 

 into the descending comu. Anteriorly it approaches its fellow 

 of the opposite side, and the two unite in the median line behind the 

 foramina of Monro. From this junction two other choroid plexuses 

 extend backwards on the inferior surface of the velum interpo- 

 situm, one on either side of the median line. They form the 

 choroid plexuses of the third ventricle, and lie superficial to the 

 ependjTnal roof of that cavity. 



The principal veins in connection with the velum interpositmn 

 are the two choroid veins and the two veins of Galen. The choroid 

 vein of each side is situated in the choroid plexus of the lateral 

 ventricle. It passes fonvards and inwards to a point behind the 

 corresponding foramen of Monro, where it joins the vein of the 

 corpus striatum, which lies bet^veen the optic thalamus and the 

 corpus striatum. In this manner the vein of Galen of one side is 

 formed. 



The veins of Galen are two in number, right and left. Each 

 vein is formed, as just stated, by the union between the choroid 

 vein and the vein of the corpus striatum behind the corresponding 

 foramen of Monro. The two veins of Galen pass backwards 

 within the velum interpositiun, one on either side of the median 

 line. At first they are near each other ; then they diverge ; but 

 subsequently they again come together and unite to form one 

 vessel, called the vena magna Galenl, which opens into the 

 anterior extremity of the straight sinus. Each vein of Galen 

 receives numerous tributaries from the corresponding choroid 

 plexus of the third ventricle, optic thalamus, corpus callosima, 

 corpora quadrigemina, and pineal body. Near its termination it 

 is joined by the large basilar vein, which is formed at the locus 

 perforatus anticus by the union of the anterior cerebral vein with 

 the deep Sylvian vein. The vena magna Galeni receives tributaries 

 from the upper surface of the cerebellum and from the occipital 

 lobes of the cerebral hemispheres. 



Lateral Ventricles. 



The lateral ventricles are the ventricles of the cerebral hemi- 

 spheres, and are two in number, right and left. They are of 

 irregular shape, and each occupies about two-thirds of the length of 

 the corresponding hemisphere. They are lined with ependyma 

 (epithelium), and contain a certain amount of cerebro-spinal 

 fluid. Each ventricle communicates with the third ventricle by 

 the foramen of Monro of its own side, which is situated between the 



