368 NERVOUS SYSTEM. 
dorsal view, but after the hemispheres have covered the 
thalami the fissure appears to be in the lower face of the hemi- 
sphere. Through this fissure the pia mater extends upward 
into the lateral ventricles as a fold separated from the ventricle 
by the thin brain-wall. This fold is vascular and is known as 
the choroid plexus of the lateral ventricle (Fig. 148, ¢). It is 
a fringe-like fold lying on the fimbria and extending from the 
foramen of Monroe (at 2) nearly to the end of the inferior horn 
of the lateral ventricle. 
The lateral ventricles are the extensions into the hemi- 
spheres of the originally unpaired cavity of the forebrain. 
Each consists of a portion parallel to the basis cranii and lying 
at the side of the septum pellucidum between the corpus cal- 
losum dorsad and the corpus striatum, hippocampus, and fornix 
ventrad, and of two horns, an anterior and an inferior horn. 
The inferior horn (Fig. 152, c) is a narrow cleft, crescent- 
shaped in cross-section, which follows the dorsal surface of the 
hippocampus (Fig. 148, d; Fig. 152, d) along the temporal 
lobe to its end almost against the tuber cinereum. It thus 
passes first caudad and then ventrad and finally mediad. The 
choroid plexus and hippocampus project into it from its floor. 
The anterior horn extends ventrad and then slightly caudad 
in the frontal lobe, following the corpus striatum (Fig. 148, /),, 
on which it lies. 
The interventricular foramen (or foramen of Monroe).— 
If the choroid plexus of the lateral ventricle (Fig. 148, ¢) is 
followed to its medial end, it is found to pass through a foramen 
(at 4) in which it becomes continuous with the opposite plexus 
or roof of the third ventricle. This foramen leads from one 
lateral ventricle to the other and is connected ventrally by a 
median opening with the third ventricle. It is thus Y-shaped 
or T-shaped and is the interventricular foramen (or foramen of 
Monroe). The foramen lies opposite the middle of the corpus 
striatum’ and caudad of the pillars of the fornix. 
Membranes of the Brain.—The membranes of the brain 
are the dura mater, the arachnoid, and the pia mater. 
The dura mater is a strong fibrous membrane lining the 
cranial cavity and covering the brain. It is strongly attached 
