The Spinal Column. - The back is characterized by the spinal column, which constitutes the long axis of the body.  This column consists of twenty-four movable vertebrae, the sacrum and coccyx.  The movable bony segments are separated from each other by fibro-cartilaginous discs.  Each vertebra is characterized by a body and an arch which extends from the posterior lateral portions of the body.  The body serves to give strength, stability and weight-carrying capacity to the column.  The arches serve to form an incomplete bony canal for the protection of the spinal cord and its membranes.   Although these arches form a fairly complete protection to the contents of the canal in the upper dorsal region, the approximation of the laminae is not nearly so perfect in the lumbar region.  It is through the gaps between the laminae in the lumbar region that puncture can most easily be made.

    Spinal Ligaments. - The discs of fibro-cartilage are very strongly attached to the bodies of the vertebrae and the fibrous tissue of these discs interweaves with the fibers of the common ligaments which extend from end to end of the spinal column on the anterior and posterior surfaces of the bodies.  After cutting the neural arches, at their junction with the bodies, we have left a strong column of bony segments, separated by fibro-cartilaginous discs which are strongly adherent.  Both bones and cartilages are very strongly bound together by the anterior and posterior common ligaments.

    Flexibility. - This column is characterized by moderate flexibility and certain curves.  The elasticity is due tothe structure of the fibro-cartilaginous discs.  The center of the discs is a very soft mass of fibro-cartilage, thicker than the margins and containing the remains of the chorda dorsalis of the embryo.

    Normal Spinal Curves. - The curves are due to the unequal thickness of the anterior and posterior portions of the discs.  The discs are thicker anteriorly in the cervical and lumbar regions, thicker posteriorly in the dorsal region, thus producing anterior curves in the cervical and lumbar, and a posterior curve in the dorsal.  The bodies of the vertebrae also vary slightly in thickness anteriorly and posteriorly, i. e., the anterior depth of the bodies is less than the posterior so that without the discs the whole column presents a posterior curve with a loss of the anterior curves in the cervical and lumbar regions.  These curves and the characteristics of the centers of the inter-vertebral discs give the column its resilience.  In bending this column to the side, rotation of the vertebral bodies is inevitable.

    Limitation of Flexibility. - The anterior and posterior common ligaments of this column of vertebral bodies and inter-vertebral discs tend, by their inelastic fibrous tissue, to limit flexibility.  By adding the neural arches with their ligaments the flexibility of the column is still further limited.  Besides the common ligaments binding the bodies there is one other ligament which extends the whole length of the column, i. e., the supraspinal ligament, consisting of inelastic fibrous tissue extending over the spinous processes from the sacrum to the seventh cervical, where its structure changes to yellow elastic fiber and is known as the ligamentum nuchae through its continuation to the cervical spinous processes and the occipital bone.  This ligament limits flexibility in the dorsal and lumbar regions.  The remainder of the spinal ligaments are intervertebral, i. e., extend between two vertebrae.  They are inelastic with one exception, the ligaments subflava connecting the laminae of the neural arches.  There are many other things which are factors in limiting the inherent flexibility of the spinal column, viz., the articulation with the ribs to form the thorax, the articulation with pelvic bones, the contents of abdomen and thorax and the bulk of the soft tissues which round out the body.

    Articular Processes. - Although the spinal ligaments and other structures limit the flexibility of the spinal column, the character of its evident flexibility is largely governed in the various regions by the shape and position of articular processes, which form a series of gliding joints between the neural arches.  The articular processes are located at the junctions of the pedicles and laminae.  They consist of two superior and two inferior for each vertebra.

    Cervical Region. - In the cervical region the articular processes are placed very obliquely.  The surfaces of the superior look upward and backward and are somewhat arched to fit the slight concavity of the interior which look downward and forward.  This arrangement permits flexion, extension and side bending accompanied by slight rotation.  These are recognized as the physiological movements for this region of the column.  The highly specialized articulations between the occipital bone and atlas, the axis and atlas are worthy of more detailed consideration. (See Chap.  XIV.)

    Dorsal Region. - The surfaces of the dorsal articular processes are vertical, the superior facing backward, the inferior forward.  The surfaces are slightly curved from side to side thus forming parts of the surface of a theoretical cylinder having its axis located in front of the body.  The movements permitted by this structural arrangement are the physiological ones known as flexion, extension and side bending rotation.  Rotation is the most characteristic of the movements in this region.  It is greatest in the upper dorsal articulations and decreases as the articular processes begin to take on lumbar characteristics.  The eleventh, sometimes the tenth, dorsal verterbra marks the limit of this characteristic dorsal movement.  All movements are limited in the dorsal region, not only by the inherent form of the vertebral articulations but also by the attachment of the ribs.  The typical costovertebral articulation is characterized by the head of the rib articulating with the bodies of two vertebrae while the tubercle of the rib unites with the articular facet on the transverse process of the lower of these two vertebrae.  The first, eleventh and twelfth costovertebral articulations are exceptions. The  junction of the upper ribs with the sternum serves still further to give stability to the thorax and limit movement in the upper portion of the dorsal division of the column.

    Lumbar Region. - The articular processes in the lumbar region are vertical.  The surfaces of the superior and inferior face almost directly inward and outward, respectively.  These surfaces are curved in the opposite direction from those in the dorsal region, so that they would form parts of a theoretical cylinder having its axis posterior to the spinous process.

    Flexion and Extension. - Since the arrangement of the articular processes is an index to the character of movement normally permitted between the vertebrae in the various regions of the spinal column, it is advisable that we call attention to a few points concerning them.  It is readily seen that flexion is a fairly free movement in all portions of the column, with the exception of that portion of the dorsal which articulates with the seven true ribs.  Extension is likewise free in these same sections, i. e., where flexion is free it is met by fairly free extension.

    Side Bending Rotation. - Side bending, of a column having antero-posterior curves, is characterized by rotation.  This inherent rotation of the segments of the spinal column will naturally take place with a center of rotation theoretically located on a line extending directly from end to end.  This line would pass on the concave side of the curves, i. e., be posterior to the bodies in the cervical region, anterior to the bodies in the dorsal and posterior to the bodies in the lumbar.  A study of the articular processes will show how this action is favored by the facing of their articular surfaces.  Lines drawn perpendicular to the surfaces of the superior articular processes of a typical cervical, i. e., the fourth or fifth cervical, will meet at a point behind the spinous processes and about three inches above the level of the body of the vertebra.  These lines incline backward, upward and inward.

    Best Position for Freest Movement. - The range of movement in any joint is favored by relaxation of its ligaments, therefore any characteristic movement will be greater when the relation of the joint surfaces to each other is least limited by the ligaments.  This position will be practically attained when the surfaces are in their normal position for weight-carrying, i. e., balance.  The weight of the head upon the neck is balanced by the muscles governing the movement in the arthrodial articulations so that there is no sense of strain.  This erect position favors rotation. The extent of rotation diminishes as the neck is flexed.  It also diminishes as the neck is extended.  In either flexion or extension, a series of ligaments becomes tense and hence limits the extent of another movement, which requires freedom of this tense ligament.  The erect position of the neck signifies ligamentous relaxation and is therefore the position of election for reducing subluxations in the cervical region.  The seventh cervical marks the change in direction of the facing of the articular processes.  Its superior surfaces adhere to cervical characteristics while its inferior become more vertical and lines perpendicular to them meet at a point in front of the column.  There is frequently a gradual change in the facing of the cervical articular processes which begins at the sixth cervical.  The inferior processes of the sixth may face so as to bring their perpendicular lines together in front of. and below the body.

    Rotation in the Dorsal Region. - The articular processes in the dorsal are characteristically vertical and theoretically move in line with the surface of a cylinder having its axis anterior to the bodies of the vertebrae.  Thus rotation in the dorsal appears to move on a fixed point, just anterior to the bodies of the vertebrae and hence the spinous processes make an actually as well as apparently greater excursion to right or left.  The same rule with relation to freedom of movement being greatest in the normal poised position, applies here.  Rotation is greater in the upper dorsal and decreases downward, disappearing at the variable point where lumbar characteristics begin to influence the form of the articular processes.  This variable point is found from the ninth to eleventh dorsal.  Rotation with the axis of movement anterior to the vertebral body usually ceases at the articulation between the eighth and ninth dorsal.  The lateral flexion between the ninth and tenth, tenth and eleventh and eleventh and twelfth, is characterized by very little rotation of either the dorsal or lumbar type.  In this short region of the dorsal we have an almost pure lateral flexion.  Rotation in the upper dorsal is decreased in the flexed or extended position, for the same reasons given for the cervical.  All movements in the upper dorsal are lessened by the costovertebral articulations.  Since the head of a rib articulates with the bodies of two vertebrae and their interovertebral disc, it is apparent that this would tend to block the movement of one vertebra on the other and hence greatly limit rotation.  Although a study of the mechanics of this portion of the column seems to show a very solid and unyielding construction, the fact exists that we have a considerable amount of movement in the upper dorsal articulations.  Rotation is probably the most pronounced of the upper dorsal movements and it is in this region of the column lateral subluxations are found.  Flexion and extension are readily demonstrated from first to fourth and from eighth to twelfth dorsal, i. e., in these regions they are more pronounced than in the mid-dorsal.

    Characteristic Movement in the Lumbar Region. - The lumbar articular processes are vertical and face so that they move in line with the surface of a theoretical cylinder having its axis running in the tips of the spinous processes.  There is much variation in the form of the lumbar articular surfaces.  Since they have much greater weight for their bearing surfaces to support they are heavily developed.  Exaggeration of the normal lumbar curve during the developing period causes them to take on a greater weight carrying function than normal and hence changes the bearing movable surface so as to decrease the range of movement.  The more nearly the bodies of the vertebrae tend to support the superincumbent weight the greater freedom of movement will naturally exist in the arthrodials between the articular processes.  The characteristic form of the lumbar articular surfaces is not conducive to rotation, as a well defined movement, such as we find in the upper dorsal and cervical, but nevertheless, side bending in this region is characterized by rotation having its center in a line drawn vertically through the spinous processes.  Thus we 'note that rotation in the three regions of the column places the center of movement on the concave side of the curve.  Any corrective movements made with reference to any portion, or the column as a whole, must be made with reference to these points of normal rotation.  As in the other regions of the column, rotation in the lumbar is lessened proportionally by flexion or extension.  Flexion is a greater check in this region than extension.

    Rotation Toward Concavity of a Curve. - It is readily noted that, in each region of the column, movement toward the concavity of the curve is less of a check on rotation than the reverse.  Movement in the opposite direction compresses the intervertebral disks and hence lessens their resilience.

    Adaptability of Position to Body Weight. - Flexion and extension in the lumbar are normally quite free, hence there is great adaptability to the position of the body weight.  A decided deviation of a single spinous process is seldom found in this region.  The direction of the articular surfaces tends to prevent such deviation.