thumb|Drawing of the right-sided triceps brachii, depicting its three heads; posterior view. 1, Lateral head. — 2, Long or scapular head. — 3, Medial or short head. — 4, Olecranon (ulna). — 5, Radius. — 6, Capsular ligament of the shoulder joint. (After Hughes.)
The triceps, or triceps brachii (Latin for "three-headed muscle of the arm"), is a large muscle on the back of the upper limb of many vertebrates. It consists of three parts: the medial, lateral, and long head. All three heads cross the elbow joint. However, the long head also crosses the shoulder joint. The triceps muscle contracts when the elbow is straightened and expands when the elbow is bent. The long head gets a further contraction when the arm is behind the torso due to how it crosses the shoulder joint. It is the muscle principally responsible for extension of the elbow joint (straightening of the arm).
Structure
- The long head arises from the infraglenoid tubercle of the scapula. It extends distally anterior to the teres minor and posterior to the teres major.
- The medial head arises proximally in the humerus, just inferior to the groove of the radial nerve; from the dorsal (back) surface of the humerus; from the medial intermuscular septum; and its distal part also arises from the lateral intermuscular septum. The medial head is mostly covered by the lateral and long heads and is only visible distally on the humerus. It has been suggested that each fascicle "may be considered an independent muscle with specific functional roles." and to the posterior wall of the capsule of the elbow joint where bursae (cushion sacks) are often found. Parts of the common tendon radiate into the fascia of the forearm and can almost cover the anconeus muscle. However, more recent studies observed that in around 14% of individuals, the long head of the triceps brachii was innervated by the axillary nerve, and in 3% it received dual innervation from both the radial nerve and axillary nerve.
Variation
A tendinous arch is frequently the origin of the long head and the tendon of latissimus dorsi. In rare cases, the long head can originate from the lateral margin of the scapula and from the capsule of the shoulder joint.
Clinical significance
The triceps reflex, elicited by hitting the triceps, is often used to test the function of the nerves of the arm. This tests spinal nerves C6 and C7, predominantly C7. This reflex can be tested by abducting the Elbow and Shoulder to 90 degrees, then tapping the tendon with a Reflex hammer near to the Olecranon.
Triceps tendinitis can occur when the muscles are overloaded. It typically takes the form of chronic posterior elbow pain and it worsens with active extension. the condition is normally seen in Men between the ages of 30 and 40 who are throwing Athlete<nowiki/>s. It is commonly treated with rest and pain control.
Many mammals, such as dogs, cattle, and pigs, have a fourth head, the accessory head. It lies between the lateral and medial heads. In humans, the anconeus is sometimes loosely called "the fourth head of the triceps brachii".
Additional images
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File:Triceps brachii muscle - animation02.gif|Animation. Close up.
File:Triceps brachii muscle10.png|Still image. Anterior view.
File:Gray412.png|Muscles on the dorsum of the scapula, and the triceps brachii.
File:Animation triceps biceps.gif|Movement of biceps and triceps when the arm is flexing
File:Braus 1921 170.png|Inner surface of the Musculus triceps brachii after the humerus was taken away. Note the atypical insertion of the Caput mediale at the Epicondylus medialis humeri.
</gallery>
See also
- Biceps