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line of the centre of gravity the flexion of the joints ceases, the muscles relax, and the limb gradually returns to the ground. The other limb then passes through the same cycle, the weight of the body now resting on the limb which is in contact with the ground. As the foot reaches the ground it, as it were, rolls over it; the heel touches it first, then the sole, and lastly, as the foot leaves the ground again, only the toes. In running, the previous events are all exaggerated. The time of the event is diminished, while the force and distance are increased. Both feet are off the ground at one time; the action of flexors and extensors alternately is much more powerful, so that on the one hand the knees are drawn upwards to a greater extent in the forward movement, and not the whole foot, but only the toes reach the ground in the extension of the limb. The attempt is made to bring the foot to the ground in front of the line of the centre of gravity. At the same time the trunk is sloped forwards much more than in walking. In leaping, the actions of the limbs are still more exaggerated. The movements of flexion of the limb are still more marked, and the foot reaches the ground still farther in front of the line of the centre of gravity.

AXIAL MUSCLES.

THE FASCIA AND MUSCLES OF THE BACK.

THE FASCIA OF THE BACK.

The general fascial investments of the back have been described along with the superficial muscles associated with the shoulder-girdle (p. 365). The latissimus dorsi muscle has been described as arising in large part from the posterior layer of the lumbo-dorsal fascia. This is a strong fibrous lamina which conceals the sacrospinalis muscle. In the loin it extends from the spines of the lumbar vertebræ, laterally, to the interval between

sacrospinalis. The layer can be followed upwards

ABDOMINIS
TRANSVERSUS

the last rib and ABDOMINIS

Fascia transversalis
Peritoneum

the iliac crest,
where it joins the
middle layer.
Below the loin
the posterior layer
of the lumbo-
dorsal fascia is
attached to the Extraperitoneal
iliac crest, and
more mediallyl Kidney
blends with the Lumbo-dorsal fascia
subjacent tendin-

tissue

ous origin of the

RECTUS ABDOMINIS

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Psoas fascia

Second lumbar vertebra

PSOAS MAJOR

Anterior layer of lumbar fascia

MULTIFIDUS

Middle layer of lumbar fascia

ILIOCOSTALIS

SEMISPINALIS

over the sacrospinalis in the region of the thorax, where it is attached laterally to the ribs and is continuous with the intercostal aponeuroses. In the lower part of the thorax it is replaced by the muscular slips of the serratus posterior inferior; in the upper part of the thorax it passes beneath the serratus posterior superior and blends with the deep cervical fascia.

Fascia Lumbodorsalis. The lumbo-dorsal fascia consists of three fascial strata, called respectively the posterior layer, just described; the middle, and the anterior layers. They unite at the lateral margin of the sacrospinalis muscle to

DORSI

Vertebral aponeurosis

FIG. 390.-TRANSVERSE SECTION THROUGH THE ABDOMEN, OPPOSITE THE
SECOND LUMBAR VERTEBRA.

LONGISSIMUS DORSI

form a narrow ligamentous band which connects the last rib to the iliac crest between the muscles of the back on the one hand and those of the abdominal wall on the other. The middle layer is a fascia which stretches laterally from the ends of the transverse processes of the lumbar vertebræ, between the sacrospinalis behind and the quadratus lumborum muscle in front. The anterior layer is attached to the lumbar vertebræ near the bases of their transverse processes. It covers the anterior surface of the quadratus lumborum muscle, and separates it from the psoas major. The psoas fascia is continuous at the lateral border of the psoas major muscle with the anterior layer of the lumbo-dorsal fascia. At the lateral borders of the quadratus lumborum and sacrospinalis muscles the three layers blend together, and give partial origin to the obliquus internus and transversus abdominis muscles.

LONGISSIMUS
CAPITIS

LONGISSIMUS
CERVICIS

ILIOCOSTALIS.
CERVICIS

ILIOCOSTALIS

DORSI

ILIOCOSTALIS
LUMBORUM

SEMI-
-SPINALIS

CAPITIS

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The first series of muscles of the back, connecting the axial skeleton to the upper limb, have already been described. They are arranged in two layers: (1) trapezius and latissimus dorsi superficially; (2) levator scapulæ, and rhomboidei, deep to the trapezius (p. 368).

The remaining muscles are almost entirely axial, and may be divided into four groups: (1) serrati posteriores, superior and inferior; splenius capitis and splenius cervicis; (2) sacrospinalis and semispinalis capitis; (3) semispinalis dorsi and cervicis (transversospinales); and (4) the small deep muscles (rotatores, interspinales, intertransversarii, and suboccipital muscles). They extend from the sacrum

OF THE LEFT SACROSPINALIS MUSCLE.

FIG. 391.-SCHEMATIC REPRESENTATION OF THE PARTS to the head, forming a cylindrical column in the loin, filling up the vertebral groove in the thorax, and giving rise to the muscular mass at the back of the neck.

First Group.

M. Serratus Posterior Superior.-The serratus posterior superior has a membranous origin from the ligamentum nucha and the spines of the last cervical

and upper three or four thoracic vertebræ. It is directed obliquely downwards and laterally, to be inserted, by separate slips, into the second, third, fourth, and fifth ribs. The muscle is concealed by the vertebro-scapular muscles, and crosses obliquely over the splenius, sacrospinalis and semispinalis capitis. It lies superficial to the lumbo-dorsal fascia.

Nerve-Supply. Posterior rami of upper thoracic nerves.

Actions. It is an accessory muscle of inspiration and an extensor of the vertebral column. Acting on the vertebral column, from the costal attachment, it assists in lateral movement of the column.

M. Serratus Posterior Inferior. The serratus posterior inferior has a membranous origin, through the medium of the lumbo-dorsal fascia, from the last two thoracic and first two lumbar spinous processes.

It forms four muscular bands which pass almost horizontally to an insertion into the last four ribs. The muscular slips overlap one another from below upwards. The muscle is on the same plane as the posterior layer of the lumbodorsal fascia, and is concealed by the latissimus dorsi.

Nerve-Supply.--Posterior rami of the lower thoracic nerves.

Actions. The muscle is an extensor of the vertebral column and an accessory muscle of inspiration, raising, everting, and fixing the lower four ribs.

M. Splenius. The splenius muscle is a broad, flattened band which occupies the back of the neck and the upper part of the thoracic region. It arises from the ligamentum nucha (from the level of the fourth cervical vertebra downwards) and from the spinous processes of the last cervical and higher (four to six) thoracic vertebræ.

Its fibres extend upwards and laterally into the neck, separating in their course into an upper and a lower part. The upper part forms the splenius capitis, which is inserted into the mastoid portion of the temporal bone and the lateral part of the superior nuchal line of the occipital bone (Fig. 396, p. 444). The lower part forms the splenius cervicis, which is inserted into the posterior tubercles of the transverse processes of the upper three or four cervical vertebræ, behind the origin of the levator scapulæ.

The muscle is partially concealed by the trapezius and sterno-mastoid, and appears between them in the floor of the posterior triangle of the neck (splenius capitis). It is covered by the rhomboid muscles, levator scapulae, and serratus posterior superior.

Nerve-Supply. Posterior rami of cervical and upper thoracic nerves.

Actions. The splenius cervicis extends the spine, and assists in lateral movement and rotation. The splenius capitis helps in the movements of raising the head, and also of lateral flexion and rotation.

Second Group.

M. Sacrospinalis.-The sacrospinalis (O.T. erector spina) possesses vertebral, vertebro-cranial, and vertebro-costal attachments. It consists of an elongated mass composed of separated slips extending from the sacrum to the skull. Simple at its origin, it becomes more and more complex as it is traced upwards towards the head.

It arises (1) by fleshy fibres from the iliac crest; (2) from the posterior sacroiliac ligament; and (3) by tendinous fibres continuous with the former from the iliac crest, the dorsum of the sacrum, and the spines of the upper sacral and all the lumbar vertebræ. Its fibres extend upwards through the loin, enclosed between the posterior and middle layers of the lumbo-dorsal fascia, and separate into two columns a lateral portion derived from the lateral fleshy origin, the iliocostalis, and a medial portion comprising the remaining larger part of the muscle, the longissimus.

M. Iliocostalis.-The iliocostalis lumborum is inserted by six slender slips into the lower six ribs.

Medial to the insertion of each of these slips is the origin of the iliocostalis dorsi (O.T. accessorius), which, arising from the lower six ribs medial to the

iliocostalis lumborum,, is inserted in line with it by similar slips into the upper

six ribs.

The iliocostalis cervicis (O.T. cervicalis ascendens) arises in the same way by six slips from the upper six ribs, medial to the insertions of the previous muscle. It forms a narrow band, which, extending into the neck, is inserted into the posterior tubercles of the transverse processes of the fourth, fifth, and sixth cervical vertebræ, behind the scalenus posterior. The iliocostales, lumborum, dorsi, and cervicis form together a continuous muscular column, and constitute the most lateral group of the component elements of the sacrospinalis.

M. Longissimus.-The longissimus is the largest element in the sacrospinalis muscle. The longissimus dorsi forms the middle column of the muscle. It is continued up into the neck as the longissimus cervicis and longissimus capitis. Mostly tendinous, on the surface, at its origin, it becomes fleshy in the upper part of the loin. It is thickest in the loin, and becomes thinner as it passes upwards in

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FIG. 392.-SCHEME OF MUSCULAR-ATTACHMENTS TO THE TRANSVERSE AND ARTICULAR
PROCESSES OF THE CERVICAL VERTEBRÆ.

the back between the column formed by the iliocostalis and its upward continuations laterally, and the spinalis dorsi medially.

It is inserted by two series of slips, medial and lateral, laterally into nearly all the ribs, and medially into the transverse processes of the thoracic and the accessory processes of the upper lumbar vertebræ. It is prolonged upwards into the neck by its association with the common origin of the longissimus cervicis and the longissimus capitis.

The longissimus cervicis (transversalis cervicis) has an origin from the transverse processes of the upper six thoracic vertebræ, medial to the insertions of the longissimus dorsi.

Extending upwards into the neck, it is inserted into the posterior tubercles of the transverse processes of the second, third, fourth, fifth, and sixth cervical vertebræ. It is concealed in the neck by the iliocostalis cervicis and splenius cervicis muscles.

The longissimus capitis (trachelo-mastoid) arises, partly by an origin common to it and the previous muscle, from the transverse processes of the upper six thoracic vertebræ, and partly by an additional origin from the articular processes of the lower four cervical vertebræ.

Separating from the longissimus cervicis, the muscle ascends through the neck as a narrow band which is inserted into the mastoid portion of the temporal bone,

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