Picture copyright 2006 Martin S. Spiller, D.M.D.

In the image above, the patient had an abscess of an upper front tooth.  Infections in upper front teeth are within the area of the face known as the "dangerous triangle".  The dangerous triangle is visualized by imagining a triangle with the top point about at the bridge of the nose and the two lower points on either corner of the mouth.  If your mother ever told you not to pick a pimple on your face or else you would get a brain infection, this is what it would look like if it actually happened

The head is, of course, located higher than the heart.  This means that the blood pressure in the head and face is lower than the blood pressure in areas of your body located below the level of the heart.  There are a lot of blood vessels and nerves in the face and the anterior of the mouth and nose reflecting the extreme sensitivity of structures in this area.  The blood pressure in the arteries that feed this area are at relatively low pressure, but the venous system that drains the area must be at an even lower pressure for the system to work as designed.  Normally, the blood flowing in the veins drains into large vessels in the neck, so if you get an infection in which bacteria are released into the bloodstream, the germs are swept into general circulation and are dealt with by the general bodily defenses.

The brain's venous flow empties into a fairly large cistern located on either side of the center of the base of the skull, called the cavernous sinus.  Venous blood from the cavernous sinus empties into the same vessels that carry venous return from the dangerous triangle area.  Normally, the system works as planned, but occasionally, the blood pressure in the cavernous sinus may drop below the blood pressure in the adjoining venous vessels and the blood flow may temporarily reverse allowing blood from the dangerous triangle area to flow backwards into the cavernous sinus.  If this blood contains bacteria from an infected area of the face, nose or the anterior upper teeth, the infected blood may cause clotting in the cavernous sinus.  If this happens, venous blood may "back up" in the vessels of the brain causing serious repercussions.  The cavernous sinus contains a large number of vital nerves and blood vessels and also surrounds the pituitary gland.  Without modern antibiotics, this condition can lead to death.  See the diagram below to visualize the location of the cavernous sinus and the proximity of the eye sockets.  The sinus itself is a paired organ, and is actually seen on either side of the sella tursica, which is the bowl shaped area in the center of the floor of the skull in which the pituitary gland sits.  In this view you are looking down into the base of the skull from the top of the head.  The brain would normally be sitting in the various depressions in the base of the skull.