The complex muscles thus tilt the head backwards and the face upwards, but where are the muscles which serve as their opponents or antagonists and reverse the movement?  In a previous chapter it has been shown that every muscle has to work against an opponent or antagonist muscle.  Here we seem to come across a defect in the human machine, for the greater straight muscles in the front of the neck, which serve as opposing muscles, are not only much smaller but at a further disadvantage by being yoked to the pre-fulcral end of the lever, very close to the cup on which the head rocks.  However, if the greater straight muscles lose power by working on a very short lever, they gain, in speed; we set them quickly and easily into action when we give a nod of recognition.  All the strength or power is yoked to the post-fulcral end of the head; the pre-fulcral end of its lever is poorly guarded.  Japanese wrestlers know this fact very well, and seek to gain victory by pressing up the poorly guarded pre-fulcral lever of the head, thus producing a deadly lock at the fulcral joint.  Indeed, it will be found that those who use the jiu-jitsu method of fighting have discovered a great deal about the construction and weaknesses of the levers of the human body.

Merely to poise the head on the atlas may seem to you as easy a matter as balancing the beam of a pair of scales on an upright support.  I am now going to show that a great number of difficulties had to be overcome before our heads could be safely poised on our necks.  The head had to be balanced in such a way that through the pivot or joint on which it rests a safe passageway could be secured for one of the most delicate and most important of all the parts or structures of the human machine.  We have never found a good English name for this structure, so we use its clumsy Latin one—­Medulla oblongata—­or medulla for short.  In the medulla are placed offices or centres which regulate the vital operations carried on by the heart and by the lungs.  It has also to serve as a passageway for thousands of delicate gossamer-like nerve fibres passing from the brain, which fills the whole chamber of the skull, to the spinal cord, situated in the canal of the backbone.  By means of these delicate fibres the brain dispatches messages which control the muscular engines of the limbs and trunk.  Through it, too, ascend countless fibres along which messages pass from the limbs and trunk to the brain.  In creating a movable joint for the head, then, a safe passage had to be obtained for the medulla—­that part of the great nerve stem which joins the brain to the spinal cord.  The medulla is part of the brain stem.