Semi-spinalis Dorsi.
Semi-spinalis Colli.
Multifldus Spinae.
Rectus Capitis Posticus Major.
Rectus Capitis Posticus Minor.
Rotatores Spinae.
Supra-spinales.
Extensor Coceygis.
Inter-transversales.
Obliquus Superior.
Inter-spinales.
Obliquus Inferior.

Dissection. The muscles of the preceding layer must be removed by dividing and turning aside the Complexus, then detach the Spinalis and Longissimus dorsi from their attachments, and divide the Erector spinse at its connection below to the sacral and lumbar spines, and turn it outwards. The muscles tilling up the interval between thespiiious and transverse processes are then exposed.

The Semi-spinales Muscles connect together the transverse and spinous processes of the vertebra;, extending from the lower part of the dorsal region to the upper part of the cervical.

The Semi-spinalis Dorsi consists of a thin, narrow, fleshy fasciculus, interposed between tendons of considerable length. It arises by a series of small tendons from the transverse processes of the lower dorsal vertebras, from the tenth or eleventh to the fifth or sixth; these uniting form a small muscular fasciculus, which subdividing into five or six tendons, is inserted into the spinous processes of the four upper dorsal and two lower cervical.

The Semi-spinalis Colli, thicker than the preceding, arises by a series of tendinous and fleshy points from the transverse processes of the four upper dorsal vertebrae, and is inserted into the spinous processes of the four upper cervical vertebrae, from the axis to the fifth cervical. The fasciculus connected with the axis is the largest, and chiefly muscular in structure.

Relations. By their superficial surface, from below upwards with the Longis-simus dorsi, Spinalis dorsi, Splenius, Complexus, the profunda cervicis and princeps cervicis arteries, and the posterior cervical plexus of nerves. By their deep surface, with the Multifidus spinse.

The Multifidus Spinae consists of a number of fleshy and tendinous fasciculi, which fill up the groove on either side of the spinous processes of the vertebra! from the sacrum to the axis. In the sacral region, these fasciculi arise from the sacral groove, as low down as the fourth sacral foramen, being connected with the aponeurosis of origin of the Erector spinae. In the iliac region, from the inner surface of the posterior superior spine, and posterior sacro-iliac ligaments. In the lumbar and cervical regions they arise from the articular processes, and in the dorsal region, from the transverse processes. Each fasciculus, ascending obliquely upwards and inwards, is inserted into the lamina and whole length of the spinous process of the vertebra above. These fasciculi vary in length; the most superficial, the longest, pass from one vertebra to the third or fourth above; those next in order pass from one vertebra to the second or third above; whilst the deepest connect two contiguous vertebra?.

Relations. By its superficial surface, with the Longissimus dorsi, Spinalis dorsi, Semi-spinalis dorsi, and Semi-spinalis colli. By its deep surface, with the lamina; and spinous processes of the vertebrae, and with the Rotatores spinse in the dorsal region.

The Rotatores Spinte arc found only in the dorsal region of the spine, beneath the Multifidus spinre, they are eleven in number on each side. Each muscle, which is small and somewhat quadrilateral in form, arises from the upper and back part of the transverse process, and is inserted into the lower border and outer surface of the lamina of the vertebra above, the fibres extending as far inwards as the root of the spinous process. The first is found between the first and second dorsal, the last, between the eleventh and twelfth. Sometimes the number of these muscles is diminished by the absence of one or more from the upper or lower end.

The Supra Spinules consist of a series of fleshy bands, which lie on the spi-nous processes in the cervical region of the spine. The Inter-Spinales are short muscular fasciculi, placed in pairs between the spinous processes of the contiguous vertebrae. In the cervical region they are most distinct, and consist of six pairs, the first being situated between the axis and third vertebra, and the last between the last cervical and the first dorsal. In the dorsal region they are found above, between the first and second vertebrae, and occasionally between the second and third; and below, between the eleventh and twelfth. In the lumbar region there are four pairs of these muscles in the intervals between the five lumbar vertebrae. There is also occasionally one in the inter-spinous space, between the last dorsal and first lumbar, and between the fifth lumbar and the sacrum.

The Extensor Coccygis is a slender muscular fasciculus, occasionally present, which extends over the lower part of the posterior surface of the sacrum and coccyx. It arises by tendinous fibres from the last bone of the sacrum, or first piece of the coccyx, and passes downwards to be inserted into the lower part of the coccyx. It is a rudiment of the Extensor muscle of the caudal vertebrae present in some animals.

The Inter-Transversales are small muscles placed between the transverse processes of the vertebrae. In the cervical region they are most developed, consisting of two rounded muscular and tendinous fasciculi, which pass between the anterior and posterior tubercles of the transverse processes of two contiguous vertebras, being separated from one another by the anterior branch of a cervical nerve, which lies in the groove between them, and by the vertebral artery and vein. In this region there are seven pairs of these muscles, the first being between the atlas and axis, and the last between the seventh cervical and first dorsal vertebra1. In the dorsal region the Inter-transversales are least developed, consisting chiefly of rounded tendinous cords in the inter-transverse spaces of the upper dorsal vertebras; but between the transverse processes of the three lower dorsal vertebrae and the first lumbar, they are muscular in structure. In the lumbar region they are four in number, and consist of a single muscular layer, which occupies the entire interspace between the transverse processes of the lowest lumbar vertebrae, whilst those between the transverse processes of the upper lumbar, ar"e not attached to more than half the breadth of the process.

The Rectus Capitis Posticus Major, the larger of the two Eecti, arises by a pointed tendinous origin from the spinous process of the axis, and becoming broader as it ascends, is inserted into the inferior curved line of the occipital bone and the surface of bone immediately beneath it. As the muscles of the two sides ascend upwards and outwards, they leave between them a triangular space, in which are seen the Recti capitis postici minores muscles.

Relations. By its superficial surface, with the Complexus, and at, its insertion, with the Superior oblique. By its deep surface, with the posterior arch of the atlas, the posterior occipito-atloid ligament, and part of the occipital bone.

The Rectus Capitis Posticus Minor, the smallest of the four muscles in this region, is of a triangular shape, it arises by a narrow, pointed tendon from the tubercle on the posterior arch of the atlas, and becoming broader as it ascends, is inserted into the rough surface beneath the inferior curved line, nearly as far as the foramen magnum, nearer to the middle line than the preceding.

Relations. By its superficial surface, with the Complexus. By its deep surface, with the posterior occipito-atloid ligament.

The Obliquus Inferior, the largest of the two oblique muscles, arises from the apex of the spinous process of the axis, and forms a thick cylindrical muscle, which passes almost horizontally outwards, to be inserted into the apex of the transverse process of the atlas.

Relations. By its superficial surface, with the Complexus, and is crossed by the posterior branch of the second cervical nerve. By its deep surface, with the vertebral artery and posterior occipito-atloid ligament.

The Obliquus Superior, narrow below, wide and expanded above, arises by tendinous fibres from the upppr part of the extremity of the transverse process of the atlas, joining with the insertion of the Inferior oblique muscle, and passing obliquely upwards and inwards, is inserted into the occipital bone, between the two curved lines, external to the Complexus. Between the two oblique muscles and the Rectus posticus major, a triangular interval exists, in which is seen the vertebral artery and the posterior branch of the sub-occipital nerve.

Relations. By its superficial surface, with the Complexus and Trachelo-mas-toid. By its deep surface, with the posterior occipito-atloid ligament.

Nerves. The Semi-spinalis dorsi and Rotatores spinae are supplied by the internal posterior branches of the dorsal nerves. The Scmi-spinalis colli, Supra-spinales, and Inter-spinales, by the internal posterior branches of the cervic.-il nerves. The Inter-transversales, by the internal posterior branches of the cervical, dorsal, and lumbar nerves. And the Multifidus spinae, by the same, with the addition of the internal posterior branches of the sacral nerves. The Recti and Obliqui muscles are all supplied by the sub-occipital and great occipital nerves.

Actions. The Erector gpina;, comprising the Sacre-lumbalis, with its accessory muscle, the Longissimus dorsi and Spinalis dorsi, serves, as its name implies, to maintain the spine in the erect posture; it also serves to bond the trunk backwards, when it is required to counter-balance 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 development, as in pregnant women or in abdominal dropsy; the peculiar gait under such circumstances depends upon the spine being drawn backwards, by the counter-balancing action of the Erector spinae muscles. The continuation of these muscles upwards to the neck and head, steady and preserve the upright position of these several parts. If the Sacro-lumbalis and Longissimus dorsi of one side act, they serve to draw down the chest and spine to the corresponding side. The Musculus accessor! us, taking its fixed point from the cervical vertebras, elevates those ribs to which it is attached. The Multifidus spinae act successively upon the different segments of the spine; thus the lateral parts of the sacrum furnish a fixed point from which the fasciculi of this muscle act upon the lumbar region; these then become the fixed points for the fasciculi moving the dorsal region, and so on throughout the entire length of the spine; it is by the successive contraction and relaxation of the separate fasciculi of this and other muscles, that the spine preserves the erect posture without the fatigue that would necessarily have existed had this movement been accomplished by the action of a single muscle. The Multifidus spinae, besides preserving the erect position of the spine, serves to rotate it, 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 Complex!, the analogues of the Multifidus spinae in the neck, draw the head directly backwards; 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 Rectus capitis posticus minor and the Superior oblique draw the head backwards, and the latter from the obliquity in the direction of its fibres, may turn the face to the opposite side. The Rectus capitis posticus major and the Obliquus inferior, rotate the atlas, and with it the cranium around the odontoid process, and turn the face to the same side.