This theme presents a magnificent field for individual study and research but is, per se, beyond the limitations set for this work. Effect of Subluxations Slight subluxations may exist, because of the adaptation of surrounding parts and the slight play within the intervertebral foramen, without producing noticeable effect. They always, however, evidence a _tendency_ to disease. The majority of subluxations do produce disease, to some degree, and do so by _impinging nerves_. Impingement may be either by pressure _against_ a nerve or ganglion or by _constriction_ of a nerve where it passes through an intervertebral foramen; the former occurs in the case of the Cervical sympathetic, the sub-occipital nerves, and the sacral nerves; the latter is the commoner form in Dorsal and Lumbar regions of the spine. Probably the most positive constriction of a nerve which can occur within the body is to be found in rotation of Lumbar vertebrae; the body of the rotated vertebra encroaches upon the inferior nerve on the side opposite to the direction taken by the spinous process. Either variety of impingement produces disease, morbid structure or function, by irritation of the nerve: light impingement irritates, heavy impingement partially or completely paralyzes, the nerve. VARIETIES OF SUBLUXATION According to the abnormal relations between vertebrae subluxations may be variously described as rotated, tipped, anteriorly, posteriorly, or laterally displaced. They commonly combine two or more of these forms, so that the purely rotary or the entirely lateral subluxation is uncommon. Rotation Every vertebra has a vertical axis around which it tends to rotate. This axis is not always the center of mass but depends upon the arrangement of mass, the fixity of cartilages, ligaments, and muscles, which tend to hold some parts of the vertebra more fixed than others, and the apposition of articular processes, which tends to prevent movement in certain directions. The axis of rotation of the first Cervical is the center of the odontoid process of the second Cervical, which articulates with the transverse ligament and anterior arch of the first. A frequent subluxation of the Atlas is a rotation around this process so that the one transverse is permanently posterior to its normal position and the other correspondingly anterior. The axis of rotation of the Cervicals below the Atlas is in the extreme anterior portion of their bodies. This part remains relatively fixed in rotatory subluxation while the tip of the spinous process describes the greatest arc. In the Dorsals the axis of rotation lies in the posterior portion of the centrum near the neural canal. When the spinous process appears laterally displaced in rotation the anterior portion of the body is slightly displaced in the opposite direction, twisting and straining the fibres of the intervertebral disk.