Impingement of Nerves When a vertebra has lost its normal articular relations with its fellows and occupies an abnormal position as a consequence in regard to _all_ surrounding or adjacent tissues it may impinge nerve tissue in two ways, by _tension_ or by _constriction_. By the displacement of one vertebra of a pair the size and shape of the intervertebral foramen may be altered (occlusion) constricting the nerve which passes through the opening. That this change in the size and shape of the foramina does frequently occur is shown by the frequency with which alterations in the shape of vertebrae appear in dry spines, by post-mortems which have demonstrated the altered foramina in the cadaver and by permanent occlusion of the foramina in ankylosed spines so that the occlusion may be preserved. Adding cartilage changes in the intervertebral disks to alterations in bone shape and position, especially the latter, we find full and sufficient reason for all the pathological phenomena which follow the subluxation. Explain it as you will, these morbid results _do_ follow subluxation and can be experimentally produced in animals. Moreover, the disease may be directed to a desired organ or region by selection of the particular vertebra to be displaced. The suboccipital, sacral, and coccygeal nerves cannot be constricted as they pass through the foramina because they do not emerge through complete rings formed of separate and movable bones. But these nerves may be _pressed upon_ or stretched by displaced bone, as may also the great gangliated cord of the sympathetic, especially the Cervical portion of it. _Tension_ of the Cervical sympathetic cord by subluxation of vertebrae is a very common occurrence. Whether the impingement be by constriction or by tension the effect is much the same depending upon the degree to which the molecular continuity of the nerve substance is impaired--interference with the function of the organ connected with the nerve and sometimes swelling and pain in the nerve itself followed by degeneration. The effects are chiefly noticeable in peripheral tissues. S. Weir Mitchell says (1872), “A continuous pressure upon a nerve results in the degeneration of the nerve and a disturbance of function of the parts innervated by that nerve.” No clearer statement can be made. It must not be understood that all nerve impingement is due directly to subluxation of a vertebra. A dislocated shoulder would produce a similar effect of nerve tension. But dislocated shoulders are seldom met with as permanent conditions. Likewise there may be secondary impingement from new growths, themselves due to some primary subluxation. Aneurism of the thoracic aorta often produces hoarseness by impingement of the recurrent laryngeal.