The shoulder blade belongs to the flat bones. It is joined by loose
connective tissue and
muscles, and lies on the rear chest wall (thorax). With its flat,
thin bone plate and two apophyses, it provides a large surface for the
attachment of muscles.
The shoulder blade is a three-sided bone. Its edges – a medial edge (margo
medialis), lateral edge (margo lateralis) and upper edge (margo
superior) are separated from each other by a lower shoulder blade angle
(angulus inferior), an upper angle (angulus superior) and a lateral
angle (angulus lateralis).
The front surface of the shoulder blade (facies costalis) is flat
with a slight cavity (fossa subscapularis). The rear surface is divided
by the scapula spine (spina scapulae) into a smaller supraspinous fossa
(fossa supraspinata) and a larger infraspinous fossa (fossa infraspinata).
The scapula spine extends with a triangular shape on the medial side
– increasing in height at the sides – to the free part, the acromion,
which ends at the head of the humerus. This is also where the articular
surface is for contact with the collarbone (facies articularis acromii).
At the transition to the lateral acromion edge there is a kink in the
scapula spine called the angulus acromialis. The hollow for the glenoid
cavity (cavitas glenoidalis) is located at the lateral angle of the
shoulder blade. At the upper edge there is a small tubercle (tuberculum
supraglenoidale), which forms the origin of the long head of the biceps.
Below the glenoid cavity there is another small tubercle (tuberculum
infraglenoidale), which forms the origin of the long head of the
triceps. The neck of the scapula (collum scapulae) is on the medial side
of the edge of the glenoid cavity. To the side of the insertion in the
upper edge of the shoulder blade (incisura scapulae), there is a
hook-shaped apophysis, which points upwards and is flattened off at the
end. This is called the coracoid process (processus coracoidus).
Please click on the image below for a 3D view of the shoulder blade:
The
muscles of the pectoral girdle
The muscles of the pectoral girdle consist of the front, rear
and lateral shoulder muscles. They move the whole shoulder
against the
chest and spinal column.
The rear shoulder muscles raise and lower the pectoral girdle
and turn and guide the arm backwards. The lower part of the
trapezius muscle (musculus trapecius) is responsible for holding
the pectoral girdle when pushing up on the arms (for example on
the parallel bars). Its middle section, which is the most
powerful, holds the pectoral girdle under load, e.g. when
carrying something heavy. Another rear shoulder muscle, the
broad back muscle (musculus latissimus dorsi), turns and guides
the arm to the back, for example when tying an apron behind the
back.
The lateral pectoral girdle muscles are responsible, among
other things, for arm movement. The serratus anterior muscle (musculus
serratus anterior) for example holds the pectoral girdle under
resistance, e.g. during press-ups. It can also pull the shoulder
blade forwards. This raises the shoulder joint upwards and the
arm is lifted over the horizontal plane.
The delta muscle (musculus deltoideus) is involved in all
movements of the shoulder joint (articulatio humeri). It moves
the arm backwards and forwards, and its middle part lifts the
arm sideways to the horizontal plane.
An important muscle in the lateral pectoral girdle area is
the large pectoral muscle (musculus pectoralis major). It allows
for the arms to be rolled inwards and crossed. It moves the arm
forwards, for example when throwing, swimming and boxing.
Together with the broad back muscle, it pulls the arm down with
great force, e.g. when chopping wood.
The supra- and infraspinatus muscles (musculus supraspinatus
and musculus infraspinatus) work together with the teres minor
muscle (musculus teres minor) to make the upper arm roll
outwards, as necessary when writing.
