Anterior Compartment
of the Arm Anatomy
The anterior compartment of the arm
contains three muscles (biceps brachii, coracobrachialis and brachialis), the
brachial artery with its venae comitantes and three nerves (the median, ulnar
and musculocutaneous nerves).
The three muscles of the compartment
are supplied by the muscu- locutaneous nerve. The two heads of biceps separate
proximally (Fig.
3.21) and have tendinous
attachments to the scapula. The short head lies medially and attaches to the
tip of the coracoid process. The tendon of the long head attaches to the
supraglenoid tubercle, leaves the shoulder joint deep to the transverse humeral
ligament and continues distally in the intertubercular sulcus deep to the
tendon of pectoralis major. The muscle bellies fuse and are attached by a
tendon (Fig. 3.27) to the tuberosity of the radius and by the bicipital
aponeurosis, which fuses with deep fascia on the medial side of the forearm.
Biceps is a strong flexor of the elbow and supinator of the forearm at the
radioulnar joints, and a weak flexor of the shoulder joint.
Coracobrachialis attaches to the
coracoid process with the short head of biceps. The muscle attaches distally to
the medial side of the shaft of the humerus near its midpoint (Fig. 3.23).
Coracobrachialis is pierced by the musculocutaneous nerve (Fig.
3.22) and functions as a
weak flexor and adductor of the shoulder joint.
Brachialis lies deeply and has an
extensive attachment to the anterior surface of the distal half of the shaft of
the humerus (Fig. 3.23) and adjacent intermuscular septa. The muscle passes to the coronoid
process of the ulna (Fig. 3.34) and acts as a powerful flexor of the elbow
joint.
 |
Fig.
3.24 Brachialis,
the brachial artery and the nerves of the compartment. Biceps brachii and most
veins have been excised.
Vessels
The brachial artery is the
continuation of the axillary artery distal to teres major. In the upper part of
the arm, the brachial artery with its venae comitantes is accompanied by the
median and ulnar nerves (Fig. 3.24) and the medial cutaneous nerve of the forearm.
The artery passes distally and laterally, lying medial to biceps and anterior
to coracobrachialis and brachialis. An important branch, the profunda brachii
artery (p. 107), supplies the posterior compartment of the arm. Division of the
bra- chial artery into its terminal radial and ulnar branches usually occurs in
the cubital fossa but may occur more proximally. The brachial artery is vulnerable
to injury in fractures just proximal to the humeral epicondyles, with risk of
subsequent fibrosis in forearm muscles (Volkmann’s ischaemic contracture).
Venae comitantes, ascending from the
cubital fossa, accompany the brachial artery and are joined by the basilic vein
after it has pierced the deep fascia at about midarm level. At the lower border
of the axilla the venous channels usually combine to form a single axillary
vein.
The musculocutaneous nerve (Fig. 3.22)
is a terminal branch of the lateral cord. It pierces coracobrachialis and lies
between biceps and brachialis, supplying each of these muscles. The nerve
continues distally as the lateral cutaneous nerve of the forearm, which pierces
the deep fascia between biceps and brachioradialis to lie superficially over
the cubital fossa (Fig. 3.26).
The median and ulnar nerves traverse
the entire length of the arm, but neither gives any branches above the elbow
joint. The median nerve arises by lateral and medial heads, which are terminal
branches of the lateral and medial cords. In the upper part of the arm the
nerve lies lateral to the brachial artery but at midarm level it crosses
anterior to the vessels and finally lies medial to the artery (Fig. 3.24), a
position retained in the cubital fossa. The ulnar nerve is a terminal branch
of the medial cord and, together with the medial cutaneous nerve of forearm,
initially lies medial to the brachial artery but leaves the artery at midarm
level (Fig. 3.24). It then pierces the medial intermuscular septum and enters
the posterior compartment to lie between the septum and the medial head of
triceps.
