Section 119. The lower figure of (5) shows, in a diagrammatic manner, the derivation of the adult brain from this primitive state. From the fore-brain vesicle, a hollow outgrowth on either side gives rises to the (paired) cerebral hemisphere (c.h.), which is prolonged forward as the olfactory lobe (o.l.). From the fore-brain the retina of the eye and the optic nerve also originate as an, at first, hollow outgrowth (op.). The roof of the mid-brain is also thickened, and bulges up to form two pairs of thickenings, the corpora quadrigemina, (c.q.). The hind-brain sends up in front a median outgrowth, which develops lateral wings, the cerebellum (cbm.), behind which the remainder of the hind-brain is called the medulla oblongata, and passes without any very definite demarcation into the spinal cord. Section 120. Figure 1 is a corresponding figure of the actual state of affairs in the adult. The brain is seen in median vertical section. (ch.) is the right cerebral hemisphere, an inflated vesicle, which, in the mammal-- but not in our lower types-- reaches back over the rest of the fore-brain, and also over the mid-brain, and hides these and the pineal gland in the dorsal view of the brain (Figure 2). The hollow of the hemisphere on either side communicates with the third ventricle, the original cavity of the fore-brain (1 in Figure 5), by an aperture (the foramen of Monro), indicated by a black arrow (f.M.). Besides their original communication through the intermediation of the fore-brain, the hemispheres are also united above its roof by a broad bridge of fibre, the corpus callosum (c.c.), which is distinctive of the mammalian animals. The original fore-brain vesicle has its lateral walls thickened to form the optic thalami (o.th.), between which a middle commissure, (m.c.), absent in lower types, stretches like a great beam across the third ventricle. The original fore-brain is often called the thalamencephalon, the hemisphere, the prosencephalon, the olfactory lobes, the rhinencephalon. Section 121. The parts of mid-brain (mesencephalon) will be easily recognised. Its cavity is in the adult mammal called the iter; its floor is differentiated into bundles of fibres, the crura cerebri (c.cb.), figured also in Figure 4. Section 122. The cerebellum (metencephalon) consists of a central mass, the vermis (v.cbm.), and it also has lateral lobes (l.l.), prolonged into flocculi (f.cbm.), which lastare -em-bedded in pits, [in] the periotic bone, and on that account render the extraction of the brain from the cranium far more difficult than it would otherwise be. The roof of the hind-brain, before and behind the cerebellum, consists of extremely thin plates of nervous matter. Its floor is greatly thickened to form the mass of the medulla, and in front a great transverse track of fibres is specialized, the pons Varolii (p.V.). Its cavity is called, the fourth ventricle.