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, e). It is 

 a fringe-like fold lying on the fimbria and extending from the 

 foramen of Monro (at h) 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, hi[)pocampus, 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, f), 

 on which it lies. 



The interventricular foramen (or foramen of Monro). — - 

 If the choroid plexus of the lateral ventricle (Fig. 148, c) is 

 followed to its medial end, it is found to pass through a foramen 

 (at //) 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 

 I^.Ionro). 1 he 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 



