*The Cerebrum.*—The cerebrum comprises about seven eighths of the entire brain, and occupies all the front, middle, back, and upper portions of the cranial cavity, spreading over and concealing, to a large extent, the parts beneath. The surface layer of the cerebrum is called the _cortex_. This is made up largely of cell-bodies, and has a grayish appearance.(99) The cortex is greatly increased in area by the presence everywhere of ridge-like _convolutions_, between which are deep but narrow depressions, called _fissures_. The interior of the cerebrum consists mainly of nerve fibers, or axons, which give it a whitish appearance. These fibers connect with the cell-bodies in the cortex (Fig. 131). The cerebrum is a double organ, consisting of two similar divisions, called the _cerebral hemispheres_. These are separated by a deep groove, extending from the front to the back of the brain, known as the _median fissure_. The hemispheres, however, are closely connected by a great band of underlying nerve fibers, called the _corpus callosum_. [Fig. 131] Fig. 131—*Microscope drawing* of a neuron from cerebral cortex. _a._ Short segment of the axis cylinder with collateral branches. At the base of the cerebrum three large masses of cell-bodies are to be found. One of these, a double mass, occupies a central position between the hemispheres, and is called the _optic thalami_. The other two occupy front central positions at the base of either hemisphere, and are known as the _corpora striata_, or the striate bodies. *The Midbrain* is a short, rounded, and compact body that lies immediately beneath the cerebrum, and connects it with the hindbrain. On account of the great size of the cerebrum, the midbrain is entirely concealed from view when the other parts occupy their normal positions. However, if the cerebrum is pulled away from the hindbrain, it is brought into view somewhat as in Fig. 130. The midbrain carries upon its back and upper surface four small rounded masses of cell-bodies, called the _corpora quadrigemina_. The upper two of these bodies are connected with the eyes; the lower two appear to have some connection with the organs of hearing. On the front and under surface, the midbrain separates slightly as if to form two pillars, which are called the _crura cerebri_, or cerebral peduncles. These contain the great bundles of nerve fibers that connect the cerebrum with the parts of the nervous system below. *The Hindbrain* lies beneath the back portion of the cerebrum, and occupies the enlargement at the base of the skull. It forms about one eighth of the entire brain, and is composed of three parts—the cerebellum, the pons, and the bulb.