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In anatomy, the arm is the upper limb of a bipedal mammal, specifically the segment between the shoulder and the elbow. Arm may also refer to an analogous structure, such as one of the paired forelimbs of a quadriped. Anatomically, the segment between the elbow and wrist is properly referred to as the forearm.
The humerus is the (upper) arm bone. It articulates with the scapula above and with the ulna and radius below. These two forearm bones in turn articulate with the carpal bones of the wrist. Beyond the wrist are the metacarpals and phalanges of the hand and fingers.
In primates the arms are richly adapted for both climbing and for more skilled, manipulative tasks. The ball and socket shoulder joint allows for movement of the arms in a wide circular plane, while the presence of two forearm bones which can rotate around eachother allows for additional range of motion at this level.
Anatomy of the human arm
The shoulder complex comprises five individual joints. The term "shoulder" often is used specifically to refer to the principle one of these, the glenohumeral joint , consisting of the proximal ball of the humerus and the associated ("glenoid") socket of the scapula. Above this lies the acromioclavicular , or "a-c" joint, between the clavicle and the acromial process of the scapula. The clavicle in turn articulates with the chest wall at the sternoclavicular joint, while the scapula slides across the posterior chest wall via the "scapulothoracic" joints.
The musculature of the shoulder region is similarly complex. The muscles which drive the arm include the pectoralis major (chest muscle), which pulls the arm forward; the trapezius, which elevates the shoulder; the deltoid, which elevates the arm to the side; and the latissimus dorsi, which pulls the arm down.
The clavicular part of the pectoralis major runs downward and outward from the inner half of the clavicle. The clavicular part of the deltoid attaches to the outer third of the clavicle. Between these two muscles is an elongated triangular gap with its base at the clavicle. Here, where the skin is somewhat depressed, the cephalic vein sinks between the two muscles to join the axillary vein .
The tip of the coracoid process is situated just under cover of the inner edge of the deltoid, one inch below the junction between the outer and middle thirds of the clavicle.
The deltoid muscle forms the prominence of the shoulder, and its convex outline is due to the presence of the head of the humerus deep within it. When the humerus is dislocated, the deltoid muscle's appearance becomes flattened.
The pectoralis major forms the anterior fold of the axilla or armpit, the posterior fold being formed by the latissimus dorsi and teres major muscles. The skin of the floor of this space is covered with hair in the adult, and contains many large sweat glands. The axillary vessels and brachial plexus of nerves traverse this region. Below the lateral edge of the pectoralis major, the swelling of the biceps begins to be visible. The course of this large muscle, which flexes the forearm, can easily be traced into its tendon of insertion, in the front of the elbow joint.
On the sides of the biceps lie the external and internal bicipital furrows , in the latter of which the brachial artery may be felt and compressed. The median nerve lies here, in close relation to the artery. At the bend of the elbow the two condyles of the humerus may be felt, on either side. The inner one projects beneath the skin, but the outer one is obscured by the rounded outline of the brachioradialis muscle. On the back of the arm lie the three heads of the triceps muscle, the external forming a marked oblique swelling when the forearm is forcibly extended and internally rotated. The triceps serves to "extend", or straighten, the elbow joint.
The elbow is composed of three joints, which allow the bones of the elbow to articulate with one another. These bones are the humerus (upper arm), the ulna (medial forearm) and the radius (lateral forearm). The major articulation of the elbow is the ulno-humeral joint, which enables arm flexion and extension. Interestingly, a person can perform most daily tasks with a range of flexion from 30 degrees to 70 degrees. Secondly, the head of the radius is dish-shaped and interacts with the capitellum (a spherical structure on the distal humerus) to form the radio-capitellar joint. This joint is responsible for adding stability to the elbow by transferring forces across the elbow joint. Thirdly, the radial head also spins against the ulna to form the proximal radio-ulnar joint, which is responsible for forearm rotation. The ulnohumeral, radiocapitellar and proximal radioulnar joints work together to position the hand in space.
On the back of the elbow, the olecranon process of the ulna is quite subcutaneous. During extension of the elbow it is in line with the two condyles. Between it and the inner condyle lies the ulnar nerve, here known popularly as the "funny bone". Striking the elbow in this area may send a shock wave of pain down the forearm.
The superficial veins at the bend of the elbow are very conspicuous. Their typical arrangement is an 'M', of which the radial and ulnar veins form the uprights, while the outer oblique bar is the median cephalic and the inner oblique the median basilic vein . At the divergence of these two the median vein comes up from the front of the forearm, while the two vertical limbs continue up the arm as the cephalic and basilic veins, on the outer and inner sides, respectively. In the upper part of the front of the forearm lies the antecubital fossa or triangle. Its outer boundary is the brachio-radialis, its inner the pronator radii teres , and its apex the point below at which they merge. In this space are three vertical structures: externally the tendon of the biceps, just internal to this the brachial artery, and still more internally the median nerve. Coming from the inner side of the biceps tendon the semi-lunar fascia may be felt. This passes deep to the median basilic vein and superficial to the brachial artery. In the past, this provided valuable protection to the artery when workers were injured and bleeding from the median basilic vein. Beyond the middle of the forearm the fleshy parts of the superficial flexor muscles cease, and only the tendons remain, so that the limb narrows rapidly.
The primary flexor of the elbow is the brachialis muscle which lies undernieth the biceps. The biceps muscle causes flexion as well as supination (outward forearm rotation). Pronation is caused by flexion of the pronator teres and pronator quadratus muscles of the forearm. The primary extensor of the elbow is the triceps muscle.
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