The posterior surface of these two muscles consists below of a strong tendinous layer, from which a great part of their fleshy fibres arises ; it is common to the two as far as the middle of the lumbar region ; there it terminates on the sacro-lumbalis, but ascends much higher on the longissimus dorsi, separating into several distinct bands, between which vessels and nerves come out.
This tendon is not to be confounded with the fascia lumborum, which is much thinner and adheres to its posterior surface.
The spinalis dorsi* lies close along the spi nous ridge, arising from the two superior lum bar, and three inferior dorsal spines. It forms a thin muscle and is inserted into the nine superior dorsal spines. Below it is in contact with the longissimus; above it is separated from it by the next muscle.
The semi-spinalis dorsi arises from the trans verse processes of the dorsal vertebra from the eleventh to the sixth inclusive by so many distinct tendinous fasciculi which pass up, be come fleshy, unite and are inserted into the spines of the .four or five superior dorsal and two inferior cervical vertebra. The name of this muscle is intended to denote its attachment to the transverse as well as to the spinous pro cesses. It is at first concealed by the longis simus dorsi, then lies along the inner side of that muscle and the outer side of the spinalis dorsi, with which last it is often united in description.
These four muscles elevate the spine, and give it an inclination to their own side. The sacro-lumbalis will also depress the ribs slightly.
The cervicalis descendens looks like a con tinuation of the sacrolumbalis, between which and the longissimus dorsi it arises. Its origin is by tendinous slips from the angles of the second, third, fourth, fifth, and sixth ribs. These are at first blended with those of the sacro-lumbalis ; then they unite and form a slender muscle, which runs upwards, out wards, and forwards, to be inserted into the transverse processes of the third, fourth, fifth, and sixth cervical vertebra, between the trans versalis colli and the levator anguli scapula.
This muscle may elevate the ribs or extend the neck, turning it to one side. It is often considered as a portion of the sacro-lumbalis, and sometimes called musculus accessorius, or cervicalis ascendens. The name cervicalis descendens, that by which it is best known, was given to it by Diemerbroeck, who described it as descending from the neck to act on the ribs and elevate them.
The transversalis colli appears like a con tinuation of the longissimus dorsi, and as such is often described. It arises along its internal side by tendinous and fleshy slips from the transverse processes of the second, third, fourth, fifth, and sixth dorsal vertebra. These
unite, form a flat fleshy belly, which passes upwards, outwards, and forwards, to be in serted by similar slips into the transverse pro cesses of the cervical vertebra from the sixth to the second inclusive, between the cervicalis descendens and the complexus. The origin and insertion of this muscle are connected only to transverse processes—hence the name.
This muscle elevates the neck and inclines it to one side.
The trachelo-mastoideus lies to the inner side of the transversalis colli, by which it is in great measure concealed. It arises by ten dinous slips from the transverse processes of two or three superior dorsal, and of three or four cervical vertebrae. The slender muscle enlarges as it ascends, passes a little outwards, and is inserted into the posterior border of the mastoid process, underneath the splenius ca pitis. Its inner side rests on the complexus, then it covers the obliquus capitis inferior and superior, and the origin of the digastric, also the occipital artery. It is by some called the complexus minor, from the resemblance it bears to the complexus in its structure. Some anatomist's consider it as the cranial portion of the longissimus dorsi and transversalis colli. The origin of its name is obvious.
When in action, this muscle extends the neck, drawing the head back and to its own side.
The complexus is thicker and broader than the muscles we have been now describing in the cervical region. It arises from the trans verse and articulating processes of the four or five superior dorsal vertebrae, and from the transverse processes of the four inferior cervical, by tendinous slips : these are followed by fleshy and tendinous bundles. The muscle thus formed passes upwards and inwards, to be inserted into the os occipitis between its supe rior and inferior oblique ridges. The complexi are close to each other above, separated only by cellular tissue which is connected with the ligamentum nucliva ; lower down, however, there is some space between them. This mus cle is covered by the trapezius above, by the splenii in the middle, and by the trachelo mastoideus and longissimus dorsi at its lowest part. It rests on the spinalis colli, the obliqui and recti capitis. The name is derived from the complicated intermixture of tendinous and fleshy fibres of which it is composed. A su perficial portion of it is described by Albinus as the biventer cervicis, but it does not usually admit of subdivision.