The first section we encounter as we pass down is that of the _corpus callosum_, a body of white fibre firmer than the external surface of the brain, and therefore called the corpus callosum or callous body, which consists of white nerve fibres gathered in from nearly all parts of the brain on each side and crossing the median line. We may regard it as a mass of representative fibres rooted in the soft substance of the convolutions or gray matter of the brain generally, and thus connecting across the median line the corresponding parts of the right and left brain. [Illustration] It must be borne in mind that the brain like the body is double, and that every organ is fully developed in each brain, so that no amount of injury or paralysis of organs would deprive us of any faculty, unless corresponding parts were destroyed in each hemisphere. The left brain governs the right half of the body, and the right brain governs the left half, the connecting fibres having their crossing (called decussation) in the spinal cord. Hence the left brain is usually more fully developed in the occipital and basilar regions than the right, in right handed people, as may frequently be detected by a careful examination of the head, or an inspection of the interior the skull. The left brain, also, seems to have a general ascendency over the right; so that paralysis of speech is most generally produced by disease in the region of language on the left side. Whatever occurs on one side of the body is in relation to the opposite side of the head. Paralysis, if not dependent on the spinal cord, is dependent on the basilar region of the opposite side of they brain; and conditions of the right eye affect the lower margin of the left front lobe, in which the perceptive organs are situated. If we thrust our fingers into the brain immediately under the corpus callosum, pushing away the delicate little structure called the _septum lucidum_ (or translucent septum), and pressing down fornix (which is a thin, horizontal nerve membrane) we find that our fingers enter a cavity by pressing its walls apart, of which the corpus callosum is the vault or roof,--a cavity which may be explored back and forth, far into the interior of the occipital lobe within an inch of the surface, and far into the front lobe, near the surface of the frontal convolutions, as well as downwards and forwards into the bottom of the middle lobe (the part called temporo-sphenoidal). These extensions of this great cavity or ventricle are called the anterior and posterior horns (_cornua_) and the descending horn (_cornu_).