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6b. The Suboccipital Muscles
The suboccipital group (Fig. 389). comprises: Rectus capitis posterior major. Obliquus capitis inferior. Rectus capitis posterior minor. Obliquus capitis superior. The Rectus capitis posterior major (Rectus capitis posticus major) arises by a pointed tendon from the spinous process of the axis, and, becoming broader as it ascends, is inserted into the lateral part of the inferior nuchal line of the occipital bone and the surface of the bone immediately below the line. As the muscles of the two sides pass upward and lateralward, they leave between them a triangular space, in which the Recti capitis posteriores minores are seen. The Rectus capitis posterior minor (Rectus capitis posticus minor) arises by a narrow pointed tendon from the tubercle on the posterior arch of the atlas, and, widening as it ascends, is inserted into the medial part of the inferior nuchal line of the occipital bone and the surface between it and the foramen magnum. The Obliquus capitis inferior (Obliquus inferior), the larger of the two Oblique muscles, arises from the apex of the spinous process of the axis, and passes lateralward and slightly upward, to be inserted into the lower and back part of the transverse process of the atlas. The Obliquus capitis superior (Obliquus superior), narrow below, wide and expanded above, arises by tendinous fibers from the upper surface of the transverse process of the atlas, joining with the insertion of the preceding. It passes upward and medialward, and is inserted into the occipital bone, between the superior and inferior nuchal lines, lateral to the Semispinalis capitis. The Suboccipital Triangle.—Between the Obliqui and the Rectus capitis posterior major is the suboccipital triangle. It is bounded, above and medially, by the Rectus capitis posterior major; above and laterally, by the Obliquus capitis superior; below and laterally, by the Obliquus capitis inferior. It is covered by a layer of dense fibro-fatty tissue, situated beneath the Semispinalis capitis. The floor is formed by the posterior occipito-atlantal membrane, and the posterior arch of the atlas. In the deep groove on the upper surface of the posterior arch of the atlas are the vertebral artery and the first cervical or suboccipital nerve. Nerves.—The deep muscles of the back and the suboccipital muscles are supplied by the posterior primary divisions of the spinal nerves. Actions.—The Sacrospinalis and its upward continuations and the Spinales serve to maintain the vertebral column in the erect posture; they also serve to bend the trunk backward when it is required to counterbalance the influence of any weight at the front of the body—as, for instance, when a heavy weight is suspended from the neck, or when there is any great abdominal distension, as in pregnancy or dropsy; the peculiar gait under such circumstances depends upon the vertebral column being drawn backward, by the counterbalancing action of the Sacrospinales. The muscles which form the continuation of the Sacrospinales on to the head and neck steady those parts and fix them in the upright position. If the Iliocostalis lumborum and Longissimus dorsi of one side act, they serve to draw down the chest and vertebral column to the corresponding side. The Iliocostales cervicis, taking their fixed points from the cervical vertebræ, elevate those ribs to which they are attached; taking their fixed points from the ribs, both muscles help to extend the neck; while one muscle bends the neck to its own side. When both Longissimi cervicis act from below, they bend the neck backward. When both Longissimi capitis act from below, they bend the head backward; while, if only one muscle acts, the face is turned to the side on which the muscle is acting, and then the head is bent to the shoulder. The two Recti draw the head backward. The Rectus capitis posterior major, owing to its obliquity, rotates the skull, with the atlas, around the odontoid process, turning the face to the same side. The Multifidus acts successively upon the different parts of the column; thus, the sacrum furnishes a fixed point from which the fasciculi of this muscle acts upon the lumbar region; which in turn becomes the fixed point for the fasciculi moving the thoracic region, and so on throughout the entire length of the column. The Multifidus also serves to rotate the column, so that the front of the trunk is turned to the side opposite to that from which the muscle acts, this muscle being assisted in its action by the Obliquus externus abdominis. The Obliquus capitis superior draws the head backward and to its own side. The Obliquus inferior rotates the atlas, and with it the skull, around the odontoid process, turning the face to the same side. When the Semispinales of the two sides act together, they help to extend the vertebral column; when the muscles of only one side act, they rotate the thoracic and cervical parts of the column, turning the body to the opposite side. The Semispinales capitis draw the head directly backward; if one muscle acts, it draws the head to one side, and rotates it so that the face is turned to the opposite side. The Interspinales by approximating the spinous processes help to extend the column. The Intertransversarii approximate the transverse processes, and help to bend the column to one side. The Rotatores assist the Multifidus to rotate the vertebral column, so that the front of the trunk is turned to the side opposite to that from which the muscles act.
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