Sacral and Lumbar Regions.
Cervical Region.
Erector Spinae.
Cervicalis ascendens.
Dorsal Region.
Transversalis cervicis.
Sacro-lumbalis.
Trachelo-mastoid.
Musculus accessorius ad sacro-lumbalem.
Complexus.
Longissimus dorsi.
Biventer cervicis.
Spinalis dorsi.
Spinalis cervicis.

Dissection. To expose the muscles of the fourth layer, the Serrati and vertebral aponeurosis should be entirely removed. The Splenius may then be detached by separating its attachments to the spinous processes, and reflecting it outwards.

The Erector Spinte (fig. 142), and its prolongations in the dorsal and cervical regions, fill up the vertebral groove on each side of the spine. They are covered in the lumbar region by the lumbar aponeurosis; in the dorsal region, by the Serrati muscles and the vertebral aponeurosis; and in the cervical region, by a layer of cervical fascia continued beneath the Trapezius. This large muscular and tendinous mass varies in size and structure at different parts of the spine. In the sacral region, the Erector gpinae is narrow and pointed, and its origin chiefly tendinous in structure. In the lumbar region, it becomes enlarged, and forms a large fleshy mass. In the dorsal region, it subdivides into two parts, which gradually diminish in size as they ascend to be inserted into the vertebra; and ribs, and are gradually lost in the cervical region, where a number of special muscles are superadded, which are continued upwards to the head, which they support upon the spine.

The Erector spinae arises from the sacro-iliac groove, and from the anterior surface of a very broad and thick tendon, which is attached, internally, to the spines of the sacrum, to the spinous processes of the lumbar and three lower dorsal vertebrae, and the supra-spinous ligament; externally, to the back part of the inner lip of the crest of the ilium, and to the series of eminences on the posterior part of the sacrum, representing the transverse processes, where it blends with the great sacro-sciatic ligament. The muscular fibres thus arising form a single large muscular mass, bounded in front by the transverse processes of the lumbar vertebra;, and by the middle lamella of the fascia of the Transversalis muscle. Opposite the last rib, this mass divides into two parts, one external, the Sacro-lumbalis, the other internal and larger, the Longissimus dorsi.

The Sacro-Lumbalis, the external and smaller portion of the Erector spinae, is inserted, by a series of separate tendons, into the angles of the six lower ribs. If this muscle is reflected outwards, it will be seen to be reinforced by a series of muscular slips, which arise from the angles of the ribs; by means of these the Sacro-lumbalis is continued upwards, to be connected with the upper ribs, and with the cervical portion of the spine, forming two additional muscles, the Mus-culus accessorius and the Cervicalis ascendens.

The Musculus Accessorius ad Sacro-Lumbaletn arises by separate flattened tendons, from the upper margins of the angles of the six lower ribs; these become muscular, and are finally inserted, by separate tendons, into the angles of the six upper ribs.

The Cervicalis Ascendens is the continuation of the Sacro-lumbalis upwards into the neck: it is situated on the inner side of the tendons of the Accessorius, arising from the angles of the four or five upper ribs, and is inserted, by a series of slender tendons, into the posterior tubercles of the transverse processes of the fourth, fifth, and sixth cervical vertebras.

Longissimus Dorsi. The inner portion of the Erector spinse, the larger and longer of the two, has received the name ' Longissimus dorsi.' It arises, with the Sacro-lumbalis, from the common origin already mentioned. In the lumbar region, where it is as yet blended with the Sacro-lumbalis, some of the fibres are directed forwards to be inserted into the posterior surface of the transverse processes of the lumbar vertebrse their whole length, into the tubercles at the back of tho articular processes, and into the layer of lumbar fascia connected with the apices of the transverse processes. In the dorsal region, the Longissimus dorsi is inserted, by long and thin tendons, into the extremities of the transverse processes of all the dorsal vertebra;, and into from seven to eleven ribs between their tubercles and angles.

This muscle is continued upwards to the cranium and cervical portion of the spine, by means of two additional slender fasciculi, the Transversalis oolli, and Trachelo-mastoid.

The Transversalis Colli, placed on the inner side of the Longissimus dorsi, arises, by long thin tendons, from the summit of the transverse processes of the third, fourth, fifth, and sixth dorsal vertebra, and is inserted, by similar tendons, into the posterior tubercles of the transverse processes of the five lower cervical.

The Trachelo-Mastoid lies on the inner side of the preceding, between it and the Complexus muscle, and may be regarded as the continuation of the Longissimus dorsi upwards to the head. It arises, by four tendons, from the transverse processes of the third, fourth, fifth, and sixth dorsal vertebras, and from the articular processes of the three or four lower cervical; these joining form a small muscle, which ascends to be inserted into the posterior margin of the mastoid process, beneath the Splenius and Sterno-mastoid muscles. This small muscle is almost always crossed by a tendinous intersection near its insertion into the mastoid process.

The spinous processes of the upper lumbar and the dorsal vertebras are connected together by a series of muscular and tendinous slips, which are intimately connected with the Longissimus dorsi, forming, in fact, part of this muscle; it is called the Spinalis dorsi.

The Spinalis Dorsi is situated at the inner side of the Longissimus dorsi. It arises, by three or four tendons, from the spinous processes of the two upper lumbar and the two lower dorsal vertebra?: these uniting, form a small muscle, which is inserted, by separate tendons, into the spinous processes of all the upper dorsal vertebrae, the number varying from four to eight. It is intimately united with the Semi-spinalis dorsi, which lies beneath it.

The Spinalis Cervicis is a small muscle, connecting together the spinous processes of the cervical vertebra?, and analogous to the Spinalis dorsi in the dorsal region. This muscle varies considerably in its size, and in its extent of attachment to the vertebrae, not only in different bodies, but on the two sides of the same body. It usually arises by fleshy or tendinous fibres, varying from two to four in number, from the spinous processes of the fifth and sixth cervical vertebra;, and occasionally from the first and second dorsal, and is inserted into the spinous process of the axis, and occasionally into the spinous processes of the two vertebrae below it. This muscle has been found absent in five cases out of twenty-four.

The Complexus is a broad thick muscle, situated at the upper and back part of the neck, lying beneath the Splenius, the direction of which it crosses obliquely from without inwards. It arises, by a series of tendons, about seven in number, from the posterior and upper part of the transverse processes of the three upper dorsal and seventh cervical, and from the articular processes of the three cervical above this. The tendons uniting form a broad muscle, which is directed obliquely upwards and inwards, and is inserted into the innermost depression between the two curved lines of the occipital bone. This muscle, about its middle, is traversed by a transverse tendinous intersection.

The Biventer Cervicis, is a small fasciculus, situated on the inner side of the preceding muscle, and in the majority of cases blended with it; it has received its name from presenting a tendon of considerable length with two fleshy bellies. It is sometimes described as a separate muscle, arising, by from two to four tendinous slips, from the transverse processes of as manv upper dorsal vertebrae, and is inserted, on.the inner side of the Complexus, into the superior curved line of the occipital bone.

Relations. By their superficial surface, with the Trapezius and Splenius. By their deep surface, with the Semi-spinalis dorsi and colli and the Recti and Obliqui. The Biventer cervicis is separated from its fellow of the opposite side by the liga-mentum nuchae, and the Complexus from the Semi-spinalis colli by the profunda cervicis artery, the princeps cervicis branch of the occipital, and by the posterior cervical plexus of nerves.

Nerves. The Erector spin* and its subdivisions in the dorsal region are supplied by the external posterior branches of the lumbar and dorsal nerves. The Cervicalis ascendens, Transversalis colli, Trachclo-mastoid, and Spinalis cervicis, by the external posterior branches of the cervical nerves; the Complexus, by the internal posterior branches of the cervical nerves, the sub-occipital and great occipital.