MUSCLES OF THE BACK.
_Trapezius_, attachments and surface form.--_Latissimus
dorsi._--_Deep muscles_ visible in relation to the trapezius
and latissimus dorsi--1, lateral region of neck (_splenius_ and
_complexus_ muscles); 2, region of the scapula (_rhomboid_,
_infraspinatus_, _teres major_ and _teres minor_ muscles).--Deep
muscles which are wholly concealed: _erector spinæ_, etc.
The most superficial muscles of the back are the trapezius and the
latissimus dorsi. They form two broad muscular sheets which cover over
the whole of the back and the posterior part of the neck, and extend
also to the shoulder and arm.
_Trapezius muscle._--The _trapezius_ muscle has the following
attachments. Its origin is from the middle line of the back, from the
inner third of the superior curved line of the occipital bone (13, Fig.
73), the fibrous band of the _ligamentum nuchæ_ between the occipital
protuberance and the spinous process of the seventh cervical vertebra,
and from the spinous processes of the seventh cervical and of all the
thoracic vertebræ. From these origins the muscular fibres are carried
outward and converge towards the shoulder, the intermediate fibres
passing transversely, the superior obliquely downwards (9, Fig. 74),
and the inferior fibres obliquely upwards. The muscle is inserted into
the shoulder girdle, to the superior border of the spine of the scapula
(Fig. 73), the inner edge of the acromion process, and the outer third
of the posterior border of the clavicle (19, Fig. 70; 7–13, Fig. 73).
With regard to external form the trapezius muscle presents this
remarkable fact, that in certain regions the muscular fibres are
replaced by aponeurotic fibres or tendons, so that in these regions the
muscle is thinner and in contraction shows slightly depressed surfaces.
These regions are three in number: 1. At the lower part of the neck
and the upper part of the back (10, Fig. 73), opposite the level of
the seventh cervical spine, the fibres of origin of the muscle are
aponeurotic, and form with those of the opposite side a lozenge-shaped
or elliptical surface with its long diameter vertical, in the centre of
which the spinous processes of the sixth and seventh cervical and first
thoracic vertebræ form more or less well-marked projections. 2. At the
lower part of the back, at the level of the last thoracic vertebræ,
the fibres of origin of the trapezius are also aponeurotic, in a small
triangular space; and in the living model, when the trapezius is
strongly contracted, its lower angle seems hollowed out, because at
that level the muscular fibres are wanting, and in consequence do not
swell up when the muscle is in action. 3. At the level of the root of
the spine of the scapula, the inferior fibres of the trapezius form
at their insertion a small triangular aponeurosis which glides on the
bone, and gives rise to a distinct depression at that spot during the
contraction of the muscle.
[Illustration: FIG. 73.
THE SUPERFICIAL MUSCLES OF THE BACK.--1, The lumbo-sacral
aponeurosis;--2, the latissimus dorsi;--3, its iliac
fasciculus;--4, the space which separates it from the external
oblique;--5, the upper portion of the latissimus dorsi;--6, 6,
the teres major muscle;--7, the lower portion of the trapezius,
with its aponeurotic portion (8) on a level with the spine
of the scapula;--9, the central portion of the trapezius,
with its aponeurosis (10);--11, 12, 13, the upper portion of
the trapezius;--15, the occipito-frontalis muscle;--16, the
sterno-cleido-mastoid;--17, the splenius capitis;--18, the
deltoid;--19, the infraspinatus;--20, the teres minor;--21, 21,
the external head, and 22, 22, internal head of the triceps
brachialis;--23, the posterior portion of the external oblique
of the abdomen;--24, 24, the gluteus maximus.]
_Action of the trapezius._--When the shoulder is strongly drawn
backwards the whole of the trapezius contracts, and in this case it
is the middle portion, of which the fibres are directed horizontally,
which shows most prominently beneath the skin. More frequently,
however, different parts of the muscle contract separately; thus the
superior fibres act, either by taking the shoulder as a fixed point,
and bending the head to the corresponding side (as when the face
is slightly turned to the opposite side), or by taking their fixed
point at the occipital and the cervical origin, and thus raising and
supporting the scapula, as when a burden is carried on the shoulder.
Under those circumstances the cervical border of the trapezius, that
which proceeds from the occipital bone to the clavicle (Fig. 74),
becomes prominent in a line nearly parallel to that of the outer border
of the sterno-cleido-mastoid (26, Fig. 74); and between these two
prominent lines appears a groove (the _posterior triangle_ of the
neck), to which we will return in dealing with the deep muscles of this
region. Lastly, if the inferior fibres of the trapezius contract alone,
they draw the shoulder downwards, and thus we see them become prominent
whenever the model causes a dragging with his upper limbs from above
downward, _e.g._, in the case of a bell-ringer who pulls violently
and with all his weight on a rope.
The two trapezius muscles (right and left) form together (Fig. 73) a
rhomboidal figure with its apex below, which resembles the contour of
a monk’s hood. Hence the trapezius has been called by anatomists and
artists the _cucullary muscle_ (_cucullus_, a hood).
_Latissimus dorsi muscle_ (1, 2, 3, 5, Fig. 73).--This forms a large,
somewhat triangular muscular sheet extending from the region of the
loins to the upper part of the arm. It arises by means of a broad
triangular aponeurosis (_vertebral aponeurosis_, 1, Fig. 73) from the
spinous processes of the lower six thoracic vertebræ, from the spinous
processes of the lumbar and sacral vertebræ and the posterior third
of the crest of the ilium (3, Fig. 73); the muscular fibres converge
from this origin towards the armpit, and the muscle obtains additional
origins at its borders. At the outer border of the muscle, three or
four fleshy bands join it which arise from the outer surfaces of the
last three or four ribs by slips which interdigitate with the fibres
of the external oblique muscle of the abdomen (4, 4, Fig. 74). At
its upper border, as it crosses the inferior angle of the scapula,
the muscle receives a narrow slip of origin from this part of the
shoulder-blade. From these origins the muscular fibres are directed
upwards and outwards, and converge so as to form a fleshy layer (5,
Fig. 73) which, passing over the inferior angle of the scapula and
along its axillary border, winds round the _teres major muscle_ (see
later), and over the posterior wall of the armpit to reach the upper
part of the shaft of the humerus, in which it is inserted by a broad
tendon, attached to the floor of the bicipital groove of the humerus
(page 61).
This muscle acts first of all like the lower part of the trapezius,
but with more energy. It, further, not only depresses the scapula, but
also acts on the humerus. It is the contraction of the latissimus
dorsi which enables us to bring the arm with force to the side,
carrying the upper limb slightly backwards, so that if the contraction
is carried very far the arms become crossed behind the back. Indeed,
the latissimus dorsi may be called the swimmer’s or golfer’s muscle.
It draws the humerus backwards behind the trunk, and at the same time
rotates the arm-bone and the whole limb inwards. The prominence formed
by the outer border of the latissimus dorsi during contraction (Fig.
74) is principally shown when the muscle accomplishes a powerful
effort, such as dragging, or pulling from above downwards, as in
pulling on a rope hanging vertically, or in hanging by the arms from a
horizontal bar. If in this situation--in the exercise of the trapeze,
for example--the model raises himself, and brings the trunk near the
bar, the latissimi dorsi muscles become very prominent, for then they
take their fixed points at the arms, and act on the trunk by carrying
it upwards and forwards.
Among the numerous deep muscles of the back there are not any which are
visible on the surface throughout their entire extent, but there are
many which appear in part in the spaces which limit the borders of the
trapezius, latissimus dorsi, and superficial muscles of the shoulder
and neck. These spaces are three in number, one at the side of the
neck, one at the level of the lower half of the scapula, and a third
over the iliac crest.
The _posterior triangle_ is the name given to the space on the
lateral surface of the neck, bounded (Figs. 73 and 74) behind by the
supero-anterior border of the trapezius, and in front by the posterior
border of the sterno-cleido-mastoid. This space forms a long and
superficial groove, extending from the occipital region to the middle
of the clavicle. It has a lower part (25, Fig. 74), covered over by the
_platysma myoides_ muscle of the neck (to which we will return later),
and a superior part, in which parts of one or two powerful muscles of
the neck are found. 1st. The muscular fibres which we see (17, Fig. 73)
directed obliquely from below upwards and outwards towards the mastoid
process belong to the _splenius capitis_ muscle, which arises from the
spinous processes of the last cervical and upper four or five thoracic
vertebræ, and ascends obliquely outwards to be attached to the mastoid
process of the temporal bone beneath the sterno-cleido-mastoid (16,
Fig. 73). 2nd. A small fleshy triangle, which may appear above the
splenius, at the apex of the posterior triangle, corresponds to a part
of a powerful muscle of the neck, called the _complexus_, which may or
may not be completely concealed by the trapezius muscle. It is always
responsible for the surface form of the neck, and, along with the other
deep muscles, appears as a prominent ridge on each side of the groove
formed by the edge of the ligamentum nuchæ.
The space situated in the back at the level of the lower part of the
scapula is triangular in form (Figs. 73 and 74). When the arm is
hanging beside the trunk the borders of the space are seen to be as
follows: Externally, the vertebral border of the scapula; above and
internally, the lower edge of the trapezius; and below, the upper edge
of the latissimus dorsi. In the floor of the space the greater rhomboid
muscle is seen; while external to the triangle are the prominences
of the muscles covering the lower part of the dorsal surface of the
scapula, _the infraspinatus_, _teres minor_, and _teres major_ muscles.
The _greater rhomboid muscle_ (rhomboideus major) arises from the
spinous processes of the seventh cervical and upper four or five
thoracic vertebræ; its fibres are directed obliquely downwards and
outwards, to be inserted into the vertebral border of the scapula. It
is only the lower fibres which become superficial, behind the vertebral
border of the scapula.
The _infraspinatus muscle_ (19, Fig. 73) rises from the infra-spinous
fossa of the scapula. From this origin its fibres ascend and,
converging beneath the deltoid (Fig. 74), are inserted by a short
tendon into the great tuberosity of the humerus.
The _teres minor muscle_ (20, Fig. 73) rises from the upper part of the
thick border of the infra-spinous fossa on the axillary border of the
scapula, and ascends along the edge of the infraspinatus and beneath
the deltoid, to be inserted into the lowest of three facets on the
great tuberosity of the humerus.
The _teres major_ (6, Fig. 73, and 8, Fig. 74) arises from the lower
part of the broad axillary border of the infra-spinous fossa; it
extends upwards and outwards like the preceding muscles, but it soon
leaves the teres minor (5, Fig. 75). Instead of remaining at the
posterior part of the shoulder it passes beneath the deltoid, and
accompanies the latissimus dorsi muscle (Figs. 17, 18; Fig. 68) to the
front of the long head of the triceps, and is finally inserted into the
inner lip of the bicipital groove of the humerus. The long head of the
triceps is found, therefore, passing between the teres minor, behind,
and the upper part of the teres major, in front (Fig. 73, between 20
and 6).
These muscles become more visible and prominent when the arm is raised
and arrives at the horizontal position (see the right side of Fig.
73). The space between the edge of the trapezius and the border of the
scapular spine above, and the latissimus dorsi below, becomes much
lengthened from within outwards, and the deltoid leaves uncovered
a greater extent of the infraspinatus, teres major and teres minor
muscles; at the same time the scapula, by the elevation of the arm
(page 64), being moved so that its inferior angle is drawn away from
the vertebral column, a greater part of the rhomboid muscle becomes
apparent between the outer border of the trapezius and the upper border
of the latissimus dorsi.
Although the other deep muscles of the back are not visible on the
subject stripped of its skin, we must not leave the region without
giving a few of the names of the powerful fleshy masses which occupy
the lumbar region, on each side of the spinous processes, and form
two powerful muscular columns, causing a prominence beneath the
aponeurosis of the latissimus dorsi (Fig. 73). This mass is formed by
muscles in the loin which are closely blended together below, and
constitute the _erector spinæ_. At the lower level of the thorax it
separates into an external muscle called the _ilio-costalis_, which
is attached by a series of tendons to the angles of the ribs, and an
internal muscle called the _longissimus dorsi_, which, by a series of
tendons, is attached to the ribs and to the transverse processes of the
thoracic vertebræ. These muscles are continued through the upper part
of the back and the neck by means of smaller slips which need not be
particularised. The action of the erector spinæ and its several parts
is to straighten the trunk, and maintain it when a burden is borne on
the shoulders or back. It is for this reason that their common mass in
the lower part of the back is so developed in men who usually carry
heavy loads on the shoulders, and forms that powerful muscular mass
of the loins, of which the prominence is visible beneath the skin and
the aponeurosis of the latissimus dorsi (1, Fig. 73). In the neck,
as already stated, the _complexus_ is the most important of the deep
muscles in giving rise to the form and contour of this region.