Location and Structure
The brachialis muscle (also known as Casserio's muscle) belongs to the deep muscle layer of the anterior arm. It occupies the lower two-thirds of the arm and is covered by deep fascia.
- Origin (Origo) — Anterior surface of the distal half of the humerus, as well as the medial and lateral intermuscular septa (septum intermusculare mediale et laterale).
- Insertion (Insertio) — Ulnar tuberosity (tuberositas ulnae) and coronoid process (processus coronoideus). Some muscle fibers blend into the anterior wall of the elbow joint capsule.
Function
The muscle is the main flexor of the forearm at the elbow joint. Part of its fibers inserts into the joint capsule, protecting it from pinching during flexion.
Blood Supply and Innervation
- Blood supply is provided by branches of the brachial artery (a. brachialis), superior and inferior ulnar collateral arteries (aa. collaterales ulnaris superior et inferior), and the radial recurrent artery (a. recurrens radialis).
- Innervation is supplied by the musculocutaneous nerve (n. musculocutaneus, C5–C7). The lateral part frequently receives additional branches from the radial nerve (n. radialis).
Topography and Related Structures
The brachialis muscle serves as an important anatomical landmark in the upper extremity:
- In the arm: Located in the deep (subfascial) layer, superficially covered by the biceps brachii (m. biceps brachii). The m. brachialis itself overlies the deepest layer—the elbow joint and its capsule.
- Neurovascular bundle: The brachial artery (a. brachialis) runs along the surface of the muscle within the medial bicipital groove. The lateral cutaneous nerve of the forearm (n. cutaneus antebrachii lateralis) emerges from the interval between the biceps and brachialis muscles.
- In the cubital fossa (fossa cubiti): The brachialis muscle, together with the supinator (m. supinator), forms its floor. Here, the brachial artery lies in the groove between the m. brachialis and the pronator teres (m. pronator teres), deep to the bicipital aponeurosis (aponeurosis m. bicipitis brachii).
- Lateral groove: Deeply positioned between the brachialis and brachioradialis (m. brachioradialis) muscles are the radial nerve (n. radialis) and the radial collateral artery. The radial recurrent artery courses superiorly and laterally between m. brachialis and m. brachioradialis.
Clinical Significance
Forearm flexion may be impaired in compression neuropathies of the musculocutaneous nerve. The primary site of nerve entrapment is where it pierces the coracobrachialis muscle. This condition occurs in strength athletes, including armwrestlers and weightlifters, due to muscle hypertrophy or repetitive movements involving elbow flexion with forearm pronation.