The |brain| is the primary component of the nervous system, occupying the cranial cavity. Without its outermost protective membrane, the |dura mater|, the |brain| weighs an average of three pounds (about 1.4 kilograms), comprising about 97% of the entire central nervous system. The |brain| is connected to the upper end of the |spinal cord| (which connects through the |foramen magnum| of the |skull|) and is responsible for issuing ~nerve~ impulses, processing ~nerve~ impulse data, and engaging in the higher order thought processes. The |brain| is divided into three parts: the large |cerebrum|, the smaller |cerebellum|, and the brainstem leading to the |spinal cord|. The brainstem is also descriptively divided into the |medulla| oblongata, the midbrain, and the |pons|.
The |cerebrum|, or forebrain, forms the bulk of the |brain|, formed of a large mass of white and grey neural |fiber| in the upper cranium. It is responsible for the higher thought processes (memory, judgement, reason), processing sensory data, and with initiating willful motor processes, such as voluntary muscle flexion. The |cerebrum| is composed of two lateral halves, or ~hemispheres~, which feature a number of folds (gyri) and furrows (sulci) and which are connected in the middle at the |medulla|. The |cerebrum| is descriptively divided into four section, or |lobes|, named for the cranial bones which they are nearest: the frontal |lobe|, the occipital |lobe|, the parietal |lobe|, and the temporal |lobe|. Cerebrospinal fluid, used to support the |brain| and buffer it, is transmitted to these |lobes| by means of |lateral ventricles| which project branches, or horns (cornu), into the frontal, occipital, and temporal |lobes|. The functions of each |lobe| are coordinated by connecting, or commissural, |fibers|. The largest and densest of these is the |corpus callosum| which connects the two ~hemispheres~ and connects through branches to the rest of the surface, or |cortex|, of the |cerebrum|. The two smaller commissural |fibers| are the ~anterior~ commissure, which contains olfactory |fibers| as well as other temporal connections, and the hippocampal commissure which runs transversely below the rear of the |corpus callosum| and is specifically related to the olfactory centers of the |brain|. Containing about a trillion neurons, the human |brain| is the most complex mechanism known, and its many functions continue to be both a source of amazement and a focus of much research.
