The triceps brachii is a large, thick muscle on the dorsal part of the upper arm. It often appears as the shape of a horseshoe on the posterior aspect of the arm. The main function of the triceps is the extension of the elbow joint. It is composed of three heads (tri = three, cep = head): a long head, a lateral head, and a medial head. The tendons all have different origins, but the three heads combine to form a single tendon distally. The long head originates from the infraglenoid tubercle of the scapula while both the lateral head and the medial head both originate from the humerus. The three heads converge into a single tendon, and this tendon attaches to the proximal portion of the olecranon process (the bony prominence of the elbow) located on the upper portion of the ulna.

The triceps' primary function is extending the forearm at the elbow joint, which opposes the action of the flexors such as the biceps brachii. At rest, with the arm slightly bent with flexion, the biceps brachii overpowers the triceps brachii. Along with extending the forearm at the elbow joint, the triceps can also stabilize the elbow joint when the forearm and hand are doing fine movements such as writing.[1][2]

The triceps brachii is located in the dorsal compartment of the arm. The lateral intermuscular septum is what separates the dorsal part from the arm from the ventral part which is where the flexors of the arm are (biceps, brachialis, and brachioradialis). 

Long head—The origin of the long head is the infra-glenoid tubercle of the scapula. Because it attaches the scapula, the long head not only extends the elbow, but will also have a small action on the glenohumeral, or shoulder joint. With the arm adducted, the triceps muscle acts to hold the head of the humerus in the glenoid cavity. This action helps prevent any displacement of the humerus. The long head also assists with the extension and adduction of the arm at the shoulder joint. The lateral head is also active during extension forearm at the elbow joint when the forearm is supinated or pronated. 

Medial head—The origin of the medial head is at the dorsal humerus, inferior to the radial groove, and connecting to the intermuscular septum. The medial head does not attach to the scapula and therefore has no action on the glenohumeral joint whether that be with stabilization or movement. The medial head is active, however during extension of the forearm at the elbow joint when the forearm is supinated or pronated.  

Lateral head—The lateral head originates at the dorsal humerus as well, but unlike the medial head, it is superior to the radial groove where it fuses to the lateral intermuscular septum. This head is considered to be the strongest head of the three. It is active during extension of the forearm at the elbow joint when the forearm is supinated or pronated.

The mesoderm develops during the third week following the gastrulation process. At the third week, the paraxial mesodermal layer begins to organize into segments called somitomeres. These consist of concentric layers of mesodermal cells formed with cranial to caudal progression. Subsequently, they organize in somites. The somites appear with a frequency of about three pairs a day so that at the fifth week are recognizable 42-44 pairs of somites of which: four occipital pairs, seven cervical, twelve thoracic, five lumbar, five sacral and between eight and ten coccyx pairs.

The triceps brachii begins to develop during the 5th week of embryogenesis. The muscle originates from the dorsal muscle mass of the upper limb bud and is derived from the paraxial mesodermal leaflet.[3]

The arterial supply to the triceps is provided by the deep brachial artery, which is a branch of the brachial artery and is supplied by ulnar collateral arteries. The venous drainage is the brachial vein with runs with the deep brachial artery. 

Lymphatic vessels, like lymph nodes, are divided into superficial and deep. The superficial lymphatic collectors are the cutaneous lymphatic capillaries, especially of the hand, from which they course proximally in the forearm to form three groups: medial, lateral, and anterior collectors.  In the arm then they continue to get to the armpit to the axilla.

The provision of nerve supply to the triceps is by the radial nerve (root C6, C7, and C8). However, according to the cadaveric study, it was found that the medial head of the triceps brachii could be innervated partially by the ulnar nerve. Following that line of investigation, other research also reveals that the long head of the triceps brachii also can be innervated partially by the axillary nerve.

The radial nerve originates from the posterior cord of the brachial plexus in the anterior axilla. The nerve then courses along with the profunda brachii artery (the artery that supplies blood to the posterior arm) travels through the arm inferolaterally. It passes behind the humerus and through the radial groove between both the lateral and medial heads of the triceps brachii.[4]