The human brain is a highly sophisticated organ, and is responsible for control of the rest of the body.  It receives 20% of the blood flow of the body.  The brain consists of two hemispheres, a right and a left, and the two halves are connected by a band of tissue known as the corpus callosum.  The "top" and the "bottom" of the brain are separated by a fibrous membrane known as the tentorium.  Above the tentorium (supratentorial) , the brain is divided into several lobes, and below the tentorium (infratentorial)  lie the cerebellum and the brainstem.  Above the tentorium, the lobes are named frontal, parietal, occipital and temporal.    Each lobe is responsible for a different function, and information is sent to the spinal cord by way of "relay stations" known as the basal ganglia, located deep within the brain.  There are other deep structures and paths within the brain which are responsible for the sending information from one portion of the brain to the other, and modifying and enhancing thought and movement processing. 

Bathing the brain is the cererbrospinal fluid, which is a clear, colorless fluid which protects and cushions the brain.  Most of the spinal fluid is located within the deep portions of the brain known as the ventricles.  Some of it percolates around the surface of the brain in what is known as the the subarachnoid spaces. 

The brain derives its blood supply from four arteries, two internal carotid arteries and two vertebral arteries.  There is some degree of redundancy of blood flow supplied by the circle of Willis, which is a ring of blood vessels at the base of the brain.  This helps to prevent stroke should one of the feeding arteries become occluded.  The brain also has a very elaborate venous drainage system consisting of veins which empty into venous sinuses, which are large venous channels created by folds of dura (the tough covering of the brain).

There are 12 cranial nerves which enter the brain.  These supply ones ability for smell, vision, eye movement, facial sensation, hearing, facial movement, taste, tongue movement, shoulder shrug and head turning. 

The spinal cord enters the skull through a hole in the skull bas known as the foramen magnum.  It merges with the lowest portion of the brainstem known as the medulla oblongata.