182 ELEMENTS OF MAMMALIAN ANATOMY. 
will not exceed a millimeter in width. The roof is formed 
much in the same manner as that of the fourth ventricle, 
by a reflection of the pia mater lined with epithelium (Figs. 
95 and 97). A sagittal section of the brain placed in a 
pan of water will show the fold of pia mater called velum 
interpositum extending craniad from the pineal gland. 
Two folds of the pia mater hanging on either side from 
near the median line form the choroid plexus as in the 
fourth ventricle. The body of the fornix lies dorsad of 
the membranous roof of the ventricle. In the floor lie the 
corpora albicantia, the infundibulum, the tuber cinereum, 
and the optic commissure. Craniad the third ventricle 
communicates with the lateral ventricles by slit-like aper- 
tures, the foramina of Monro, passing laterad and ventrad 
of the anterior pillars of the fornix (Figs. 95 and 96). 
The lateral ventricles are found in the cerebral hemi- 
spheres ventrad to the corpus callosum. They are the 
largest ventricles of the brain. In order to view them satis- 
factorily, the entire dorsal portion of the brain down to the 
corpus callosum must be cut away, and a hole cut through 
the corpus callosum. A number of cross-sections of the 
cerebral hemispheres should also be studied. 
Each ventricle is composed of a body from which pro- 
jects an anterior cornu and a middle or descending cornu. 
The former extends into the frontal lobe and thence into 
the olfactory lobe, and the latter descends into the temporal 
lobe. The roof of the body of the lateral ventricle is 
formed by the corpus callosum (Figs. 95, 96 and 97) and 
the medial wall by the septum lucidum, a mass of gray 
matter lying between the fornix and the corpus callosum. 
A fringe of pia mater projects through the foramen of 
Monro into the lateral ventricle, where it forms the 
choroid plexus. The lateral ventricles are lined with the 
epithelial layer common to the other ventricles. 
