THE NERVOUS SYSTEM 215 



Craniad the fourth ventricle is continued as a small canal, 

 the iter, or aqueduct of cerebrum, which lies ventrad to the 

 corpora quadrigemina and opens into the third ventricle (Fig. 

 106). The latter is a narrow, vertical, cleft-like space between 

 the optic thalami. The two thai ami 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 ^^^ ios.-Diagram of 



reflection of the pia mater lined with Ventricles of the Brain 



. , „N . Viewed Dorsally. 



epithelmm (rigs. lOO and I05j. A ah. Anterior horn of the 



sagittal section of the brain placed in a ^^^^^^^f "^^ rrebrum- cc.' 



pan of water will show the fold of pia canalis centralis of the spinal 

 n 1 7 ' , . ', cord: 3 and 4, third and 



mater called velum tnterposttum ex- ^^^^^^^^ ventricles; w, fora- 

 tending craniad from the pineal gland, "^^n of Monro; mi, middle 



° . horn or cornu of the ven- 



Two folds of the pia mater hanging on tricie. 

 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 ven- 

 tricle. In the floor lie the corpora mamillaria, the infundibu- 

 lum, the tuber cinereum, and the optic commissure. Craniad 

 the third ventricle communicates with the lateral ventricles 

 by silt-like apertures, the interventricular foramina of Monro, 

 passing laterad and ventrad of the anterior pillars of the fornix 

 (Figs. 106 and 107). 



The lateral ventricles are found in the cerebral hemispheres 

 ventrad to the corpus callosum. They are the largest ven- 

 tricles of the brain. In order to view them satisfactorily, the 

 entire dorsal portion of the brain down to the corpus callosum 

 must be cut away, and a hole cut through the corpus callosum. 



