76 THE NERVOUS SYSTEM OF THE FROG. 
covered by a very vascular triangular membrane, 
beneath which lies the fourth ventricle. 
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the internal cavities. 
Fig. 12. A horizontal section through the brain of the frog, to show 
(From Kcker.) 
Aq, ventricles of the optic lobes, and Sylvian aqueduct: Dv, third ven- 
tricle: MF’, foramen of Monro: Sv, lateral ventricle; Vv, fourth ventricle. 
b. The cavities of the brain. 
Slice off the upper surface of the brain horizontally so as to 
expose the several cavities or ventricles, which are merely parts of, 
or outgrowths of, the original central canal of the neural tube of 
the embryo. 
i. The lateral ventricles: extend through the whole 
length of the cerebral hemispheres and a short 
way into the olfactory lobes. 
ii, The third ventricle: situated in the thalamen- 
cephalon : it opens in front through the foramina 
of Monro into the lateral ventricles: the stalk 
