ARM, MUSCLES OF THE.—The mus cles which clothe the os humeri are part of the deltoid, the biceps, coraco-brachialis, brachixus anticus, the origin of the supinator longus in front, and the triceps behind.
The deltoid belongs to the shoulder, and will be described with the other muscles of that part. (See SCAPULAR REGION.) 1. Coruco-brachialis (coraco-humeral). The coraco-brachialis arises from the point of the coracoid process, in common with the short head of the biceps,tendinous in front and fleshy behind ; it separates from the biceps at its middle third, passes inwards, and is in serted tendinous into the internal surface of the humerus a little above its middle between the triceps and brachius anticus.
This muscle has in front of it the deltoid and pectoralis major, which cover and conceal from view its upper part; behind it the tendon of the subscapularis, the tendons of the latissi mus dorsi and teres major, the axillary artery, the median and the external cutaneous nerves. The latter nerve perforates the muscle about its middle, and passes through its substance to reach the outer side of the arm ; hence the epithet perjbratus has been applied to this muscle. The coraco-brachialis can carry the arm forwards and inwards ; when the humerus is fixed, it can act upon the scapula, and by depressing its coracoid angle, elevate the in ferior angle and separate it from the ribs.
2. Biceps flexor cubiti ( scapulo-coraco-ra dial).—This is a long muscle swollen in the centre, divided above into two portions called heads, one internal short, the other external long. The internal or short head arises from the coracoid process of the scapula in common with the coraco-brachialis. The long head is attached by a long slender flattened tendon to the upper part of the margin of the glenoid cavity, and is united by a dense cellular tissue to the glenoid ligament. This tendon passes over the head of the humerus, and enters the groove between the two tuberosities in which it is bound down by the fibres of the capsular ligament of the shoulder joint ; a pro longation of the synovial membrane also lines the groove, and forms a synovial sheath for the tendon ; the tendon terminates in a fleshy. belly which unites with the short head to form the large belly of the biceps ; the muscle ter minates below in a tendon, which, passing over the brachizeus anticus and the front of the elbow-joint, sinks into a triangular hollow between the pronator teres and supina tor longus to be inserted into the back part of the tubercle of the radius ; but before it sinks into this triangular space, it sends off from its internal side an aponeurosis (the semilunar fascia of the biceps), which is inserted into the internal condyle, and the fascia which covers the muscle at the inner side of the bend of the elbow.
The biceps is covered by the deltoid, the pectoralis major, the fascia of the arm and integuments in front ; behind it lies on the humerus, coraco-brachialis, brachiwus anticus, and the external cutaneous nerve ; internal to it lie the coraco-brachialis and brachial artery. It bends the elbow and makes tense the fascia of the fore-arm ; it is also a very powerful supinator of the hand by virtue of its insertion into the radius. If the fore-arm be extended and fixed, it will depress the scapula on the humerus.
3. BrachitTus unticus ( B. internus, hume rocubital).—When the biceps has been raised from its situation, we observe the brachius anticus deeply situated on the front of the arm ; it arises by two fleshy tongues, one on each side of the insertion of the deltoid ; from the whole of the anterior surface of the humerus, and the internal intermuscular ligament which separates it from the triceps, its fleshy fibres pass downwards in front of the elbow, and end in a broad tendon which is inserted into a triangular roughness on the anterior surface of the coronoid process of the ulna. This muscle is covered in front by the biceps, supi nator longus, the fascia of the arm and integu ments, the musculo-cutaneous and median nerves, the brachial artery, and the pronator teres; behind it covers the front of the lower part of the humerus and the elbow-joint. This muscle is the most powerful flexor of the fore arm upon the arm. As Bichat remarks, flexion of the fore-arm takes place directly if the bra chius combines its action with that of the biceps ; if either acts alone, the flexion is in the direction inwards or outwards; inwards when the biceps acts alone, outwards when the brachiwus.