The breast on the anterior thoracic wall has a number of significant relationships with the axilla and upper limb. It overlies the pectoralis major muscle, which forms most of the anterior wall of the axilla and attaches the humerus to the chest wall (Fig. 7.13). Often, part of the breast known as the axillary process extends around the lateral margin of the pectoralis major into the axilla.
of the breast is predominantly into lymph nodes in the axilla. Several arteries and veins that supply or drain the gland also originate from, or drain into, major axillary vessels.
Innervation of the upper limb is by the brachial plexus, which is formed by the anterior rami of cervical spinal nerves C5 to C8, and T1 (Fig. 7.14). This plexus is initially formed in the neck and then continues through the axillary inlet into the axilla. Major nerves that ultimately innervate the arm, forearm, and hand originate from the brachial plexus in the axilla.
As a consequence of this innervation pattern, clinical testing of lower cervical and T1 nerves is carried out by examining dermatomes, myotomes, and tendon reflexes in the upper limb. Another consequence is that the clinical signs of problems related to lower cervical nerves—pain; pins-and-needles sensations, or paresthesia; and muscle twitching—appear in the upper limb.
Dermatomes of the upper limb (Fig. 7.15A) are often tested for sensation. Areas where overlap of dermatomes is minimal include the: upper lateral region of the arm for spinal cord level C5, palmar pad of the thumb for spinal cord level C6, pad of the index finger for spinal cord level C7, pad of the little finger for spinal cord level C8, and skin on the medial aspect of the elbow for spinal cord level T1.
Selected joint movements are used to test myotomes (Fig. 7.15B):
Abduction of the arm at the glenohumeral joint is controlled predominantly by C5.
Flexion of the forearm at the elbow joint is controlled primarily by C6.
Extension of the forearm at the elbow joint is controlled mainly by C7.
Flexion of the fingers is controlled mainly by C8.
Abduction and adduction of the index, middle, and ring fingers is controlled predominantly by T1.
In an unconscious patient, both somatic sensory and motor functions of spinal cord levels can be tested using tendon reflexes:
A tap on the tendon of the biceps in the cubital fossa tests mainly for spinal cord level C6.
A tap on the tendon of the triceps posterior to the elbow tests mainly for C7.
The major spinal cord level associated with innervation of the diaphragm, C4, is immediately above the spinal cord levels associated with the upper limb.
Evaluation of dermatomes and myotomes in the upper limb can provide important information about potential breathing problems that might develop as complications of damage to the spinal cord in regions just below the C4 spinal level.
Each of the major muscle compartments in the arm and forearm and each of the intrinsic muscles of the hand is innervated predominantly by one of the major nerves that originate from the brachial plexus in the axilla (Fig. 7.16A):
All muscles in the anterior compartment of the arm are innervated by the musculocutaneous nerve.
The median nerve innervates the muscles in the anterior compartment of the forearm, with two exceptions—one flexor of the wrist (the flexor carpi ulnaris muscle) and part of one flexor of the fingers (the medial half of the flexor digitorum profundus muscle) are innervated by the ulnar nerve.
Most intrinsic muscles of the hand are innervated by the ulnar nerve, except for the thenar muscles and two lateral lumbrical muscles, which are innervated by the median nerve.