The anterior and posterior vesicles undergo much more considerable change. The walls of the posterior vesicle thicken enormously in their foremost portion and form the _cerebellum_ on top (_Cb_ in all the figures) and the _pons Varolii_ below (_P.V._ in Fig. 33). In its hindmost portions the posterior vesicle thickens below into the medulla oblongata (_Mo_ in all the figures), whilst on top its walls thin out and melt, so that one can pass a probe into the cavity without breaking through any truly nervous tissue. The cavity which one thus enters from without is named the fourth ventricle (4 in Figs. 32 and 33). One can run the probe forward through it, passing first under the cerebellum and then under a thin sheet of nervous tissue (the _valve of Vieussens_) just anterior thereto, as far as the _aqueduct of Silvius_. Passing through this, the probe emerges forward into what was once the cavity of the anterior vesicle. But the covering has melted away at this place, and the cavity now forms a deep compressed pit or groove between the two walls of the vesicle, and is called the _third ventricle_ (3 in Figs. 32 and 33). The 'aqueduct of Sylvius' is in consequence of this connection often called the _iter a tertio ad quartum ventriculum_. The walls of the vesicle form the _optic thalami_ (_Th_ in all the figures). [Illustration: FIG. 33 (after Huxley).] From the anterior vesicle just in front of the thalami there buds out on either side an enlargement, into which the cavity of the vesicle continues, and which becomes the _hemisphere_ of that side. In man its walls thicken enormously and form folds, the so-called _convolutions_, on their surface. At the same time they grow backwards rather than forwards of their starting-point just in front of the thalamus, arching over the latter; and growing fastest along their top circumference, they end by bending downwards and forwards again when they have passed the rear end of the thalamus. When fully developed in man, they overlay and cover in all the other parts of the brain. Their cavities form the _lateral ventricles_, easier to understand by a dissection than by a description. A probe can be passed into either of them from the third ventricle at its anterior end; and like the third ventricle, their wall is melted down along a certain line, forming a long cleft through which they can be entered without rupturing the nervous tissue. This cleft, on account of the growth of the hemisphere outwards, backwards, and then downwards from its starting point, has got rolled in and tucked away beneath the apparent surface.[29]