The pathological changes in the osseous lesion are commonly one of structural derangement, deviation or complete displacement. The vertebral segments are of primary consideration owing to their important relations to the spinal nerves, spinal cord centers and sympathetics; the ribs owing to the close sympathetic and spinal nervous relations; and then other osseous tissues, as the innominata, clavicles, etc., depending upon their importance to contiguous vessels, nerves and organs. It should always be remembered and emphasized that mechanical changes of the anatomical structures is the primary essential in osteopathic etiology; this is the one great inception of pathological variations from the distinctively osteopathic conception, which the osseous lesion typifies. Consequently the osseous lesion factor is actually a luxation (complete, or partial, even to a very slight degree), or malalignment of the bony constituents, which by virtue of their physical malposition impinge or irritate contiguous tissues. The essential test is the functional one, movement. The degree of involvement may be one of many gradations ranging from a slight malposition or impaction to a marked deviation or firm anchorage. =Second= in importance from the static requirement of support is the =muscular lesion= though from the standpoints of movement and dynamics it is often of the first consideration etiologically. Many interosseous lesions are the result of spastic involvement of deep seated spinal muscles, of fibrotic changes and of tensions and weaknesses that either establish a rigidness of the segments, compromising nervous stimulus or vascular channel, or produce an imbalance of muscular tone and tension. In the latter instance some type of sidebending-rotation osseous lesion occurs, commonly anchored within the physiologic movements of the spine. The muscular lesion may be an actual dislocation of either muscle or tendon, but rarely. Commonly it is a contracted, or tensed, or contractured muscle. The muscle, also, may be diseased either from primary or secondary causes through nutritional and infectious sources and thus be an etiological feature. The muscular lesion is caused, (a) by direct or indirect violence the same as the osseous lesion; (b) by atmospheric influences; (c) by slumped posture, debilitating habits and various errors of living; (d) infections; (e) by reflex irritations; (f) by compensatory changes; (g) by disease causing hypertrophy or atrophy; and, (h) secondary to osseous lesions, being the result of impingement to the muscles’ nervous control. The tensed or stretched muscle may result from a separation of the points of origin and insertion. Herein the fundamental osteopathic concept is the resulting affection due to the physical encroachment, directly or indirectly, of the muscle tissue upon vascular channel or nerve fibre, or the effect upon the movement or alignment of the osseous tissue.