MUSCLES OF THE SHOULDER AND ARMPIT.
The _deltoid_: its _form_, _thickness_, _actions_.--Muscular
form of the shoulder as a whole.--The deeper muscles
(_supraspinatus_ and _subscapularis_).--_Serratus magnus_
muscle: its relation to the armpit, its costal digitations; its
action in movements of the arm.--Forms of the region of the
armpit; prominence of the _coraco-brachialis_ muscle; relations
of the _biceps_ and _triceps_ to the armpit.
The upper and outer part of the prominence of the shoulder is formed
by a single powerful muscle, called the _deltoid_. Beneath this
are several deep muscles which fill up the fossæ of the scapula
(_supraspinatus_, _infraspinatus_, and _subscapularis_). But when the
arm is raised and maintained in the horizontal position, the surface
below the root of the arm dips into a cavity or pit, corresponding
to the external prominence of the shoulder; this cavity, called the
_armpit_, or _axilla_, has for a roof the skeleton of the shoulder
covered by the deltoid, and for its walls--in front, the pectoral
muscles, which adjoin the anterior border of the deltoid, and behind,
the subscapularis and the latissimus dorsi, separated from the
posterior border of the deltoid by the teres major muscle. On the inner
side is a muscle applied to the wall of the thorax, the _serratus
magnus_, and on the outer side is the humerus, covered by the _biceps_
and _coraco-brachialis_ muscles. Of the muscles which we have named,
some have already been studied in relation to the muscular structure of
the trunk (great pectoral and latissimus dorsi); of the others, we will
study the _deltoid_ and _serratus magnus_, in relation to the region of
the shoulder and axilla.
_Deltoid muscle._--This muscle is so called because it resembles the
Greek letter delta, and is triangular in form; it is short, broad,
thick, and shaped like half a cone to clasp the shoulder-joint. It
arises above from the outer third of the anterior border of the
clavicle (12, Fig. 68), from the acromio-clavicular articulation, from
the convex border of the acromion, and from the entire extent of the
posterior border of the spine of the scapula (18, Fig. 73). From this
origin its fibres are directed downwards, the middle fibres vertically,
the anterior or clavicular fibres downwards and backwards, and the
posterior fibres obliquely forwards, to be inserted into the external
surface of the humerus on a rough =V=-shaped groove, called the deltoid
impression (page 68).
[Illustration: FIG. 74.
THE SUPERFICIAL MUSCLES OF THE SHOULDER AND THE LATERAL PORTION
OF THE TRUNK.--1, the latissimus dorsi;--2, the lumbo-sacral
aponeurosis;--3, 4, 4, the iliac and costal origins of the
latissimus dorsi;--5, 6, 7, the upper portion of the latissimus
dorsi;--8, the teres major;--9, 10, 11, the trapezius;--12, the
infraspinatus;--13, the teres minor;--14, 15, 16, the external
oblique of the abdomen;--17, 18, the anterior and interior
border of the same muscle;--19, its inferior internal angle
rounded (see page 199);--20, 20, serratus magnus;--21, the
pectoralis major;--22, the gluteus maximus;--23, the tensor
of the _fascia lata_;--24, the deltoid;--25, the platysma
myoides;--26, the sterno-cleido-mastoid.]
This muscle is very thick, and is intersected by a vertical series of
strong fibrous bands or septa, which may be seen as depressions beneath
the skin in the contracted muscle. The action of the deltoid is to
elevate the arm, separating it from the trunk and supporting it in the
horizontal position; but while the middle fibres raise the arm directly
outwards, the anterior fibres elevate it and carry it forwards, and the
posterior fibres carry it backwards. It is necessary to remark that
this muscle is never at right angles to the lever which it moves, but
it always acts obliquely on the humerus. Hence, although very thick,
the deltoid cannot act with great power; therefore the horizontal
position of the arm produced by the action of the muscle is one which
requires great effort and quickly produces fatigue. In order to
understand the unfavourable arrangement of the deltoid with relation to
its humeral lever, it is sufficient to compare it with that which the
biceps presents relative to the forearm, and to see that the biceps,
which acts obliquely on the radius, becomes perpendicular to that bone
in proportion as flexion is carried on in the arm; and when the elbow
forms a right angle, the biceps muscle is found in the most favourable
condition to act with all possible force. In other words, the greatest
_momentum_ of a muscle occurs when it is perpendicular to its
lever; the deltoid muscle, therefore, does not possess much momentum.
In pointing out the relations which the three borders of the deltoid
present, we summarise the various details of the muscular contour of
the shoulder:--1st. The _superior border_ of the deltoid, by its origin
from the anterior border of the clavicle and the posterior border of
the spine of the scapula, repeats the insertions of the trapezius,
which is attached to the opposite lip and border of the same bones
(Fig. 74). The clavicle, acromion, and spine of the scapula form a
species of bony intersection between the trapezius and deltoid, and
when the muscles are contracted produce a deep groove between the
insertion of the trapezius and the origin of the deltoid. In animals
which have no clavicle, and in which the spine of the scapula is
not well developed, the fibres of the deltoid and trapezius are
directly continuous. We observe an arrangement of this nature in the
horse. 2nd. The _anterior border_ of the deltoid is separated from
the corresponding border of the great pectoral muscle by a linear
interval, very narrow below, but a little broader above, where it
forms a small triangle, of which the base corresponds to the middle of
the clavicle (Fig. 70). This interval, which becomes visible during
the contraction of the two muscles when we endeavour to raise the
arm upwards and forwards while it is held behind, as in the act of
drawing a load, gives passage to a vein called the _cephalic_, and
under such circumstances this vein becomes prominent and swollen. 3rd.
The _posterior border_ of the deltoid forms one of the sides of the
triangular space which we have studied in the region of the back, at
the level of the infra-spinous fossa (Figs. 73 and 74); and, under its
posterior border, pass successively on the one hand the infraspinatus
and teres minor muscles, which pass directly beneath the deltoid, and
on the other the teres major and latissimus dorsi muscles, which pass
more deeply, separated from the deltoid by the long head of the triceps
muscle (Fig. 75).
Two muscles of the shoulder remain to be mentioned which are not
visible on the model, but must at least be named in order to explain
how the fossæ of the shoulder-blade are filled up. These are:--1st. The
_supraspinatus_ muscle (11, Fig. 75), which occupies the supra-spinous
fossa of the scapula, passes beneath the coraco-acromial arch, and
is inserted into the uppermost facet of the great tuberosity of the
humerus. 2nd. The _infraspinatus_ muscle occupies the infra-spinous
fossa, and is partially concealed by the latissimus dorsi, trapezius,
and deltoid. It is also bound down on the back of the scapula by a
strong membrane derived from the deep fascia. It is inserted below the
supraspinatus into the back of the great tuberosity of the humerus.
3rd. The _subscapularis_ muscle (19, Fig. 68) occupies the subscapular
fossa, and is inserted into the lesser tuberosity of the humerus.
The _serratus magnus muscle_ (14, Fig. 68; 5, Fig. 70; 2, Fig. 71; 20,
Fig. 73).--This muscle, applied to the lateral part of the thorax, is
hidden throughout a great part of its extent by the scapula and the
muscles of the chest and shoulder; but it becomes superficial at its
lower part, in a series of prominent digitations and muscular bands,
which form very characteristic features in the contour of the lateral
region of the thorax. At the same time, as the muscle constitutes the
inner wall of the armpit, we must describe it here in detail.
The serratus magnus arises by nine fleshy slips from the outer surfaces
of the upper eight ribs (an additional slip arising between the first
and second ribs). From this origin the muscle, which forms a broad
sheet, sweeps backwards round the ribs, to be inserted into the whole
length of the vertebral border of the scapula. The five or six upper
digitations of the muscle are hidden by the great pectoral muscle (21,
Fig. 73), and only its three or four lowest digitations are visible on
the side of the thorax between the borders of the great pectoral muscle
in front and the latissimus dorsi behind; they interdigitate with the
upper slips of origin of the external oblique muscle of the abdomen
(20 and 16, Fig. 73). The alternations of origin of the serratus magnus
and external oblique are seen when the muscle is contracted, as when
the arm is thrust forward. When the arm is hanging loosely, or slightly
raised, we see only three digitations of the serratus magnus; but when
the arm is strongly elevated the great pectoral frequently leaves
another uncovered.
The action of this muscle is to fix the scapula, drawing this bone
downwards and forwards, while the rhomboid, on the other hand, draws
it upwards and backwards. Fixation of the scapula being necessary to
afford a fixed point for the contraction of the muscles of the arm
(particularly the biceps), it is easy to perceive that whenever the
upper limb accomplishes a powerful effort, the lower digitations of
the serratus magnus become clearly visible in the living model, as in
a sword-thrust, wrestling, lifting from the ground a heavy body, or in
pushing back an adversary, &c.
The serratus magnus muscle forms the inner wall of the armpit or
axilla, a cavity of which the anterior wall is represented by the
pectoral muscles, and the posterior wall by the subscapularis, teres
major, and latissimus dorsi. The cavity forms a triangular pyramid; its
summit, directed upwards, corresponds in the skeleton to the interval
between the first rib, the clavicle, and the upper border of the
scapula. In a dissected subject this cavity is open inferiorly, but
in the living model it is closed by the skin which forms the base of
the pyramid, and which, in passing from the outer border of the great
pectoral muscle to the border of the latissimus dorsi, is hollowed out
so as to ascend in the space, into which it is drawn by its attachment
to the strong, deep fascia of the axilla.
To complete the description of the axilla we must say a few words
concerning its boundaries, and the folds which correspond to the
lines of junction of its walls. There is nothing more to add with
regard to its anterior limit (formed by the great pectoral muscle) or
its posterior limit (formed by the latissimus dorsi and teres major
muscles clothing the axillary border of the scapula), but a word
must be said of the external limit of the space which corresponds
to the root of the arm. This is a comparatively thick surface, and
corresponds to the upper part of the shaft of the humerus. The bone
is covered by two muscles which descend from the scapula towards the
anterior surface of the arm,--the _biceps_ and _coraco-brachialis_,
which we will notice briefly. We will say, first of all, that the form
of the coraco-brachialis is clearly visible beneath the skin of the
armpit when the arm is strongly raised, as for example in a subject
crucified, when it raises the depressed skin in the armpit. The skin is
covered with hair more or less abundant, according to the individual,
and it is a classic habit to omit this part of the hairy system in
every representation of an elevated limb; but the artist should be
convinced by the study of anatomy that he should never conform to the
habit of tracing on the skin of the hollow of the armpit a fanciful
contour, since this skin is smooth and regularly depressed, and only
on its external part presents the fusiform muscular prominence of the
coraco-brachialis, on the inner side of the anterior surface of the
arm. The _biceps_ muscle escapes from under cover of the great pectoral
muscle, and lies external to the coraco-brachialis more on the front of
the limb.
The _triceps_ muscle of the arm, which, by its long head, takes origin
from the scapula, does not pass, like the biceps and coraco-brachialis,
through the armpit, but appears behind it on the back of the arm,
since, as we have already said, it passes between the teres minor
posteriorly and the teres major and latissimus dorsi anteriorly (Figs.
73 and 75).