Given a subluxated vertebra causing nerve impingement and thus interruption of the normal action of the reflex arc, irritation may result in greater contraction upon the _opposite_ side than upon the side of the irritation. This is an abnormal condition and accounts for the _increase_ of previously existing subluxations under pain or peripheral irritation. But in every instance trauma must and does precede and cause subluxation. Reaction of Secondary Causes Once produced, however, a subluxation may not cause noticeable effect until it has been _increased_ in degree by the reaction of forces within the body such as poisons, general fever, etc. Thus germs, dietetic errors, exposure to sudden temperature changes, waste of energy through abnormal mental activities, as hate, fear, worry, etc., or through physical excess--in fact, all the _secondary_ causes of disease may _appear_ to have produced a subluxation. In fact, they have merely accentuated that which already existed and have done so through the muscular contractions which they induced. General thinning of intervertebral substance through a condition of disturbed metabolism itself produced through the agency of some _one_ serious subluxation, may narrow all the foramina and increase impingement of nerves at any point where a slight subluxation previously existed. An irritated nerve may become swollen and the nerve impinged at the foramen. Law Governing Location So definite is the law governing the effect of force applied to a given portion of the body upon an associated vertebral segment that the skilled Chiropractor who has studied vertebrate segmentation thoroughly may determine, from the history of a fall or injury, the vertebra which would tend to be subluxated by that injury and the tissues controlled from that part. The rule is this: _Force applied to any body segment tends to subluxate the segmentally associated vertebra. This subluxation tends to produce disease throughout the area of distribution of the subjacent pair of spinal nerves._ The task of explaining this law seems hopeless unless the student is familiar with human embryology and the life history of the vertebrata, as well all the details of human anatomy. To such a student the law will be self-evident, so interwoven with the threads of higher organization as practically to form its pattern. In simple terms we might offer this general statement. Any force applied to the body with sufficient violence will produce subluxation of the vertebra above the spinal nerves supplying the injured area. Thus, the brachial plexus controls the arm and shoulder and connects with the spine by way of the 5, 6, 7, 8, Cervical and 1 Dorsal nerves. Any force striking the arm or shoulder tends to produce subluxation of the sixth or seventh Cervical or first Dorsal vertebra so that all permanent disease conditions resulting will be found in the arm or shoulder or nearby tissues of the neck.