Each of these segments is further divided into two compartments which are formed by deep fascia – tough connective tissue septa (walls).

The lateral intermuscular septum extends from the lower part of the crest of the greater tubercle of the humerus, along the lateral supracondylar ridge, to the lateral epicondyle; it is blended with the tendon of the deltoid muscle, gives attachment to the triceps brachii behind, and to the brachialis, brachioradialis, and extensor carpi radialis longus muscles in front.

It is perforated by the radial nerve and profunda branch of the brachial artery.

It extends from the lower part of the crest of the lesser tubercle of the humerus below the teres major, and passes along the medial supracondylar ridge to the medial epicondyle; it is blended with the tendon of the coracobrachialis, and gives attachment to the triceps brachii behind and the brachialis in front.

The triceps brachii is a large muscle containing three heads a lateral, medial, and middle.