THE BRAIN 137 



boundary of the foramen of Monro, then through the grey 

 matter on the outer side of the 3rd ventricle ; and on reaching 

 the base of the brain they make a half figure-of-eight turn, 

 forming the white envelope of the corpora albicantia ; they 

 then ascend and pass into the anterior nucleus of the optic 

 thalamus. The pillars receive near the foramen of Monro 

 the fibres of the peduncles of the pineal gland, and those of 

 the taenia semicircularis. 



The posterior pillars : the two strata of the fornix diverge 

 behind, being connected at first with the splenium of the 

 corpus callosum ; thence curving round the pulvinar they 

 pass into the descending cornu, the outer fibres being lost on 

 the hippocampus major, the inner ones forming the taenia 

 hippocampi. 



Between the diverging posterior pillars a portion of the 

 splenium with transverse markings is exposed ; this is known 

 as the lyra . 



The foramen of Monro is the interval between the anterior 

 pillars of the fornix and the optic thalami. It is Y-shaped, 

 the two limbs anteriorly communicating with the lateral 

 ventricles on either side, and the junction of the two opening 

 into the 3rd ventricle. 



The velum interposition is a triangular process of pia mater 

 prolonged through the transverse fissure and lying over the 

 3rd ventricle and upper surfaces of the optic thalami. Its 

 apex reaches the foramen of Monro, and the fornix lies upon 

 its upper surface. 



Choroid plexuses : 



Of the lateral ventricles are fringed vascular processes extend- 

 ing from the foramen of Monro to commencement of descend- 

 ing cornu, and attached along the lateral margins of the 

 velum interpositum ; thence each passes into the descending 

 cornu, resting on the taenia hippocampi and hippocampus 

 major, forming the choroid plexus of the descending cornu. 



Of the yd, ventricle: from the under surface of the velum 

 interpositum there depend two vascular fringes, diverging 

 behind, and forming the choroid plexus of 3rd ventricle. 



Veins of Galen : two veins formed by junction of the vein 

 of the corpus striatum and choroidal vein of each side, and 

 running backwards between the layers of the velum inter- 

 positum, and uniting posteriorly into one trunk, which opens 

 into the straight sinus. 



The corpora striata, so called from their appearance on 

 section, which displays alternate white and grey bands, are 

 two in number, one in each hemisphere, and are sometimes 

 termed the ganglia of the hemisphere. Each consists of an 



