1 82 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. 



