Coracobrachialis muscle.
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The coracobrachialis muscle is a thin, elongated bilateral flexor muscle that extends between the coracoid process of the scapula and the humerus bone. It is the shortest of the three muscles that attach to the coracoid process, the others being the pectoralis minor muscle and the tendon of the short head of the biceps brachii muscle. The coracobrachialis muscle attaches by way of a tendon into the middle third of the medial surface of humerus between the origins of the triceps brachii and brachialis. Its tendon mixes with the tendon of the pectoralis minor.
The coracobrachialis id one of the three muscles contained in the anterior compartment (flexor compartment) of the arm, the other two being the brachialis and the biceps brachii.
The coracobrachialis muscle helps to flex and adduct the arm as well as to stabilize the shoulder joint, helping prevent dislocation. It receives innervation from the musculocutaneous nerve (C5-C7). This nerve, as it continues distally pierces the muscle and appears on its anterior aspect coursing inferiorly, The muscle is used when you reach with your hand and forearm to the contralateral aspect of your body, as in reaching to scratch your opposite ear, or doing a bench press.
It is found deep to the pectoralis major and anterior to the axillary artery and the brachial plexus. Along with the humerus and the short head of the biceps brachii, the coracobrachialis muscle forms the lateral wall of the axilla.
The coracobrachialis is one of the 17 muscles that attach to the scapula.
Note: The side image modified from the original in "Gray's Anatomy" by Henry VanDyke Carter, MD. Public domain. Animated image below by Wikimedia Commons - Anatomography [CC BY-SA 2.1 following Creative Commons attributes.
Sources:
1. “Gray’s Anatomy” Henry Gray, 1918
2. "Tratado de Anatomia Humana" Testut et Latarjet 8th Ed. 1931 Salvat Editores, Spain
3. "Gray's Anatomy" 42nd British Ed. Churchill Livingstone 2021
4. “An Illustrated Atlas of the Skeletal Muscles” Bowden, B. 4th Ed. Morton Publishing. 2015
5. "Trail Guide to The Body" 4th. Ed. Biel, A. Books of Discovery. 2010