THE NERVOUS SYSTEM X455 



The following parts are contained within the circle of Willis, in 

 order from behind forwards : (i) the locus perforatus posticus ; 

 (2) the corpora albicantia or mammillaria ; (3) the tuber cinereum 

 and infundibulum ; and (4) the optic commissure or chiasma. 



Veins of the Encephalon. 



The cerebral veins are arranged in two groups — superficial and 

 deep. 



The superficial cerebral veins are divided into two sets — superior 



and inferior. 



The superior cerebral veins return the blood from the upper parts 

 of the outer surfaces of the cerebral hemispheres. They lie in the 

 pia mater, and pierce the arachnoid membrane and inner layer of 

 the dura mater, after which they open into the superior longitudinal 

 sinus, having previously received the veins from the mesial surface 

 of either hemisphere. Their direction, for the most part, is forwards 

 and inwards, whilst the direction of the blood-current in the superior 

 longitudinal sinus is backwards. 



The inferior cerebral veins return the blood from the lower parts 

 of the cerebral hemispheres, and they terminate in the cavernous, 

 superior petrosal, and lateral sinuses. One of these veins is known 

 as the middle cerebral, or superficial Sylvian, vein. It passes along 

 the fissure of Sylvius, and opens into the front part of the cavernous 

 sinus. This vein commimicates posteriorly with (i) the superior 

 longitudinal sinus, through means of the great or superior anasto- 

 motic vein ; and (2) the lateral sinus, through means of the posterior 

 anastomotic vein. 



The deep cerebral veins are as follows : (i) the choroidal veins ; 

 (2) the veins of the corpora striata ; (3) the veins of Galen ; (4) the 

 anterior cerebral veins ; (5) the deep Sylvian veins ; and (6) the 

 basilar veins. 



The choroidal vein of each side commences in the choroid plexus 

 of the descending comu of the lateral ventricle. It ascends at first, 

 and then passes forwards in the lateral margin of the velum inter- 

 positum to the foramen of Monro, where it unites with the vein of 

 the corpus striatimi to form the corresponding vein of Galen. 



The vein of the corpus striatum is formed by branches which 

 issue from the corpus striatimi and optic thalamus. It runs for- 

 wards in the groove between these two bodies, hing superficial to 

 the taenia semicircularis, and at the foramen of Monro it joins the 

 choroidal vein to form, as stated, the corresponding vein of Galen. 



The vein of Galen, of each side, is formed close to the foramen of 

 Monro by the union of the choroidal vein, and the vein of the 

 corpus striatum. The two veins of Galen, right and left, pass back- 

 wards between the two layers of the velum mterpositvun, and they 

 unite, beneath the splenium of the corpus callosum, to form one 

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

 of the straight sinus. Each vein of Galen receives tributaries from 



