THE NERVOUS SYSTEM. 193 



aspect of the pia mater. Some nervous matter in addi- 

 tion to the pia mater and epithelium forms the superior 

 velum. Two longitudinal vascular fringes, hanging from 

 the roof of the ventricle on either side of the mid-line, 

 form the choroid plexus, which is merely a network of 

 blood-vessels carried by a reflected portion of the pia 

 mater. 



Cephalad the fourth ventricle is continued as a small 

 canal, the iter, or aqueduct of Sylvius, which lies ventrad 

 to the corpora quadrigemina and opens into the third 

 ventricle (Fig. 92). The latter is a narrow, vertical, 

 cleft-like space between the optic thalami. The two 

 thalami are united by the soft or middle commissure, 

 better designated as the massa intermedia, extending 

 through the ventricle. Unless this ventricle has been 

 injected with a starch mass through the infundibulum 

 before the brain was hardened, its cavity will not exceed 

 a millimeter in width. The roof is formed much in the 

 same manner as that of the fourth ventricle, by a re- 

 flection of the pia mater lined with epithelium (Figs. 92 

 and 94). A sagittal section of the brain placed in a pan 

 of water will show the fold of pia mater called velum 

 interpositum extending cephalad 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. Cephalad the 

 third ventricle communicates with the lateral ventricles 

 by slit-like apertures, the foramina of Monro, passing 

 laterad and ventrad of the anterior pillars of the fornix. 



The lateral ventricles are found in the cerebral hemi- 

 spheres ventrad to the corpus callosum. They are the 



