The biceps is a two-headed muscle and is one of the chief flexors of the forearm. Here is the left side, seen from the front.
|Origin||Short head: coracoid process of the scapula.
Long head: supraglenoid tubercle
|Insertion||Radial tuberosity and bicipital aponeurosis into deep fascia on medial part of forearm|
|Nerve||Musculocutaneous nerve (C5–C7)|
|Antagonist||Triceps brachii muscle|
|Latin||musculus biceps brachii|
|Anatomical terms of muscle|
In human anatomy, the biceps, also biceps brachii (/ /), is a two-headed muscle that lies on the upper arm between the shoulder and the elbow. Both heads arise on the scapula and join to form a single muscle belly which is attached to the upper forearm. While the biceps crosses both the shoulder and elbow joints, its main function is at the elbow where it flexes the forearm and supinates the forearm. Both these movements are used when opening a bottle with a corkscrew: first biceps unscrews the cork (supination), then it pulls the cork out (flexion).
The biceps is a muscle in the upper arm with two heads, called the short head and the long head, which converge into a single mass. It is one of the three muscles in the anterior compartment of the arm, the others being the brachialis muscle and the coracobrachialis muscle, with which the biceps shares a nerve supply. Both heads of the biceps arise from the scapula in the shoulder, and the muscle attaches to the radius (bone), in the forearm. Specifically, the short head of the biceps originates from the coracoid process at the top of the scapula. The long head originates from the supraglenoid tubercle just above the shoulder joint. From here, the long head has a tendon that passes up along the intertubercular groove of the humerus into the joint capsule of the shoulder joint. The tendon of the short head runs adjacent to the tendon of the coracobrachialis and likewise attaches to the coracoid process.
Both heads join on the middle of the humerus, to form a single muscle mass usually near the insertion of the deltoid, to form a common muscle belly. Distally, biceps ends in two tendons: the stronger attaches to the radial tuberosity on the radius, while the other, the bicipital aponeurosis, radiates into the ulnar part of the antebrachial fascia.
The tendon that attaches to the radial tuberosity is partially or completely surrounded by a bursa; the bicipitoradial bursa, which ensures frictionless motion between the biceps tendon and the proximal radius during pronation and supination of the forearm.
Two muscles lie underneath the biceps brachii. These are the coracobrachialis muscle, which like the biceps attaches to the coracoid process of the scapula, and the brachialis muscle which connects to the ulna and along the mid-shaft of the humerus. Besides those, the brachioradialis muscle is adjacent to the biceps and also inserts on the radius bone, though more distally.
Traditionally described as a two-headed muscle, biceps brachii is one of the most variable muscles of the human body and has a third head arising from the humerus in 10% of cases (normal variation) — most commonly originating near the insertion of the coracobrachialis and joining the short head — but four, five, and even seven supernumerary heads have been reported in rare cases. 
The biceps shares its nerve supply with the other two muscles of the anterior compartment. The muscles are supplied by the musculocutaneous nerve. Fibers of the fifth, sixth and seventh cervical nerves make up the components of the musculocutaneous nerve which supply the biceps.
The proximal tendons of the biceps brachii are commonly involved in pathological processes and are a frequent cause of anterior shoulder pain. Disorders of the distal biceps brachii tendon typically result from partial and complete tears of the muscle. Partial tears are usually characterized by enlargement and abnormal contour of the tendon. Complete tears generate a soft-tissue mass in the anterior aspect of the arm, the so-called Popeye sign, which paradoxically leads to a decreased strength during flexion and supination of the forearm. Tears of the biceps brachii occur in athletic activities and corrective surgery repairs biceps brachii tendon tears. Proximal ruptures of the long head of the biceps tendon can be surgically repaired by two different techniques. Biceps tenodesis is resurfacing the tendon by screw fixation on the humerus and biceps tenotomy is trimming the long head of the biceps tendon promoting the muscle origination from the coracoid process. Preexisting degeneration in the tendon can cause partial tears called lesions and are rarely associated with a traumatic event. The most common symptom of a biceps tear is pain. It will be the most severe in the muscle, but may stretch to the shoulders and elbows as well. Treatment of a biceps tear depends on the severity of the injury. In most cases, the muscle will heal over time with no corrective surgery. Applying cold pressure and using anti-inflammatory medications will ease pain and reduce swelling. More severe injuries require surgery and post-op physical therapy to regain strength and functionality in the muscle. Corrective surgeries of this nature are typically reserved for elite athletes who rely on a complete recovery.
The biceps are usually attributed as representative of strength within a variety of worldwide cultures.
The biceps brachii muscle is the one that gave all muscles their name: it comes from the Latin musculus, "little mouse", because the appearance of the flexed biceps resembles the back of a mouse. The same phenomenon occurred in Greek, in which μῦς, mȳs, means both "mouse" and "muscle".
The term biceps brachii is a Latin phrase meaning "two-headed [muscle] of the arm", in reference to the fact that the muscle consists of two bundles of muscle, each with its own origin, sharing a common insertion point near the elbow joint. The proper plural form of the Latin adjective biceps is bicipites, a form not in general English use. Instead, biceps is used in both singular and plural (i.e., when referring to both arms).
The English form bicep [sic], attested from 1939, is a back formation derived from interpreting the s of biceps as the English plural marker -s. While common even in professional contexts, it is often considered incorrect.
To isolate the biceps brachii in elbow flexion, place the shoulder in hyperextension.
In training the biceps brachii, it is important to distinguish between the long head and the short head of the biceps. The long head is the outer portion of the muscle. The short head is the inner portion of the muscle.
There is much debate over the best biceps workouts for targeting each of these heads.
The first theory for targeting is based on the proximity of the arms in relation to the body. It is said that when the elbows are pulled back behind the body, this targets the long head more. To target the short head, the elbows should be in front of the body.
The second theory uses grip placement and angle as the primary factor in targeting each head. For instance, to target the long head when using dumbbells or cables, the grip should be semi-supinated (hammer) grip where the palms face each other. If using a barbell (EZ grip or straight), the grip should be inside of shoulder width. To target the short head when using dumbbells or cables, grip should be supinated, where the palms are facing up completely. If using a barbell (EZ grip or straight), grip should be outside of shoulder width.
Leonardo da Vinci expressed the original idea of the biceps acting as a supinator in a series of annotated drawings made between 1505 and 1510; in which the principle of the biceps as a supinator, as well as its role as a flexor to the elbow were devised. However, this function remained undiscovered by the medical community as da Vinci was not regarded as a teacher of anatomy, nor were his results publicly released. It was not until 1713 that this movement was re-discovered by William Cheselden and subsequently recorded for the medical community. It was rewritten several times by different authors wishing to present information to different audiences. The most notable recent expansion upon Cheselden's recordings was written by Guillaume Duchenne in 1867, in a journal named Physiology of Motion. To this day it remains one of the major references on supination action of the biceps brachii.
In Neanderthals, the radial bicipital tuberosities were larger than in modern humans, which suggests they were probably able to use their biceps for supination over a wider range of pronation-supination. It is possible that they relied more on their biceps for forceful supination without the assistance of the supinator muscle like in modern humans, and thus that they used a different movement when throwing.
In the horse, the biceps' function is to extend the shoulder and flex the elbow. It is composed of two short-fibred heads separated longitudinally by a thick internal tendon which stretches from the origin on the supraglenoid tubercle to the insertion on the medial radial tuberosity. This tendon can withstand very large forces when the biceps is stretched. From this internal tendon a strip of tendon, the lacertus fibrosus, connects the muscle with the extensor carpi radialis -- an important feature in the horse's stay apparatus (through which the horse can rest and sleep whilst standing.) 
Biceps and triceps.
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