This structure is one mass, called anteriorly the corpus striatum, or striated body, and posteriorly the optic thalamus or bed of the optic nerve, though the optic nerve has its principal origin in another part, called the optic lobes. The thalamus and corpus striatum are called together, the _great inferior ganglion_ of the brain. They are masses of gray substance, with white fibres from below passing through them, and white fibres originating in them to ascend and spread, so that their entire masses of fibres, ascending and spreading out like a fan, constitute an extensive structure which folds together toward the median line somewhat like a nervous sac, inclosing the cavity of the ventricle and sending its representative fibres across the median line,--which are called the corpus callosum. This will be more fully explained when we consider the genesis of the brain as it grows in the unborn infant. As the reader now understands the principal parts around the ventricles, let him look lower down to complete the survey and understand the plan of the brain, though not its anatomical minutiae. The optic thalamus is indicated in the engraving, but the corpus striatum, being more exterior and anterior, does not appear. Practically they may be regarded as one body. Where the thalami come together and touch or unite on the median line, the junction is called a commissure (commiss. med.) and the space between them where they do not touch is called the third ventricle (ventric. III), which, like the lateral ventricles, may also hold a little serum. It is unnecessary to consider the small parts above the thalami, the choroid plexus of blood vessels, the fornix or strip of nerve membrane, and the septum lucidum or delicate fibres under the corpus callosum. Beginning at the bottom of the figure, we observe the medulla oblongata rising from the spinal cord to reach the cerebrum. Behind this we see the cerebellum divided on the median line, and thus presenting where it is divided the appearance called _arbor vitae_, from its resemblance to the leaf of that evergreen. As the fibres of the medulla oblongata ascend they pass between the cerebellum and the _pons Varolii_ (bridge of Varolius) mingling with its substance. The pons or bridge (for if the brain were laid on its upper surface the pons would appear like a bridge over the river represented by the medulla oblongata) is the commissure or connecting body of the cerebellum, as the corpus callosum is of the cerebrum. When the head is held erect the fibres of the pons arch forward from the interior of the cerebellum on one side across the median line to the other side, so that a straight line through from the right to the left ear would pierce its lower portion. It looks toward the front, corresponding with the upper jaw, just below the nostrils, through which region it may be reached for experiment.