The brain has three major functional and anatomical parts--the forebrain, midbrain and hindbrain. The forebrain consists of the cerebral hemispheres, thalami, hypothalamus and limbic system. The top inch of the brain stem is the midbrain. The hindbrain includes the cerebellum, the pons and the medulla. Other parts of the brain's anatomy include: the cerebral cortex, a thin layer of nerve cells on the surface of the cerebral hemispheres; the cerebrum, composed of many millions of nerve fibers, is covered by the cerebral cortex; the corpus callosum, a C-shaped collection of fibers, forms a bridge between the left and right hemispheres; the thalamus, which relays and receives messages to and from other areas of the brain; the limbic system, concerned with memory and emotion; the cerebellum, the largest part of the hindbrain and responsible for balance and the fine control of muscle movements; and the hypothalamus, which lies below the third ventricle of the brain, and regulates many of the body's functions and the activity of the pituitary gland.