Ahe arm has its base in the shoulder girdle. Its one bone, called the humerus, is cylindrical, slightly curved, with a spherical head fitting into the cup-shaped cavity of the shoulder blade. Its ball-and-socket joint is covered with a lubricating capsule and held together by strong braces of membranes and ligaments. These, crossing at different angles, brace the arm as well as allow great freedom of movement. The lower part of the arm ends at the elbow in a hinge joint, on the inner and outer sides of which are two prominences, called inner and outer condyles.
Both prominences show on the surface. The inner condyle is used as a point of measurement and is more conspicuous than the outer one.
The forearm has two bones. One, called the ulna, is notched to fit around the rounded surface between the two condyles of the arm, at the elbow. The extremity of the lower end of this shaft has the shape of a knob which shows plainly above the wrist on the little finger side. The other bone, called the radius, joins the wrist on the thumb side of the hand. Here it is wide, curving upward to its head, which is small and cup-shaped, a ring of ligament holding it in place below the outer condyle of the arm bone, or humerus.
The radius, on the thumb side of the wrist, radiates around the ulna on the little finger side. At the elbow, the arm and forearm act as a hinge joint.
The mass of the shoulder descends as a wedge, sinking into the flattened outer arm half way down.
At this point, from the front, the arm wedges downward to enter the forearm below the elbow. When the thumb is turned away from the body, the mass of the forearm is oval, becoming round when the bones of the forearm cross.
The mass corresponding to the wrist is twice as wide as it is thick and enters the forearm half way up, as a flattened wedge.
From the back, the shoulder enters the arm on the side. Beneath it there is a truncated wedge from the center of which, in a line from elbow to shoulder, is the plane of the tendon of the elbow. The forearm is rounded or oval, depending upon whether or not the bones of the forearm are crossed. The wrist is twice as wide as it is thick.
The Arm—Back View
Bones of the Arm:
2 Ulna (little finger side)
3 Radius (thumb side)
Muscles of the Upper Arm, front view:
7 Brachialis anticus
4 Pronator radii teres
5 Flexors, grouped
6 Supinator longus
Coraco-brachialis: From coracoid process, to humerus, inner side, half way down. Action: Draws humerus forward, rotates humerus outward.
Biceps: Long head from glenoid cavity (under acromion) through groove in head of humerus; short head from coracoid process; to radius. Action: Depresses shoulder blade; flexes forearm; rotates radius outward.
Muscles of the Upper Arm, outer view:
2 Supinator longus
3 Extensor carpi radialis longus
5 Extensors, grouped
Extensor Digitorum Communis: From external condyle to second and third phalanges of all fingers. Action: Extends fingers.
Extensor Minimi Digiti: From external condyle to second and third phalanges of little finger. Action: Extends little finger.
Extensor Carpi Ulnaris: From external condyle and back of ulna to base of little finger. Action: Extends wrist and bends down. 3 Anconeus: From back of external condyle to olecranon process and shaft of ulna. Action: Extends forearm.
I. bones: (1) The coracoid process is a part of the shoulder blade that extends beyond and above the rim of the cup that holds the head of the humerus. (2) The head of the humerus is rounded and covered with cartilage. It contacts with the glenoid cavity of the shoulder blade. (3) The humerus is one of the long bones of the body. It is composed of a shaft and two large extremities; the upper articulates at the shoulder and the lower at the elbow. (4) The shaft of the humerus at the elbow is flattened from front to back ending in two projections: one on the inner, the other on the outer side, called the inner and outer condyles. The inner side is the more prominent.
II. muscles: (1) The coraco-brachialis is a small round muscle placed on the inner surface of the arm lying next to the short head of the biceps. (2) The biceps is so called because it is divided into two parts: the long and the short. The long head ascends in the bicipital groove of the humerus to be inserted just above the upper margin of the glenoid cavity of the shoulder blade. The short head has its attachment to the coracoid process. The biceps descends as a tendon to the radius below the elbow. (3) The brachialis anticus muscle lies beneath the biceps. It stretches across the lower half of the humerus to the ulna.
III. Both the biceps and brachialis muscles are placed in front of the arm. When they contract they bend the elbow. Every muscle is provided with an adversary; as an example: the finger is not bent or straightened without the contraction of two muscles taking place. The biceps and brachialis anticus are the direct antagonists of the triceps. The brachialis anticus muscle covers the lower half of the humerus in front and is inserted into the ulna just below the elbow. Its attachment to the ulna is so short that it is at a great disadvantage as to power, but what is lost in strength is gained in speed by its short leverage.
IV. The mass of the shoulder descends as a wedge on the outer surface of the arm half way down. The biceps is seen as a flattened mass when not in contraction as it wedges downward to enter the forearm below the elbow. There are great changes in the form of the arm above the elbow as a mass; the biceps is lengthened in repose, but becomes short and globular during contraction.
I. bones: (1) The great tuberosity of the humerus is situated on the outer side of the bicipital groove. At its upper extremity it is a prominent bony point of the shoulder. Though covered by the deltoid, it materially influences the surface form. (2) The shaft of the humerus is cylindrical. (3) The olecranon of the ulna forms the point of the elbow.
II. muscles: (1) Long head. (2) External portion. (3) Internal portion of the triceps. (4) Common tendon of the triceps. The triceps muscle has been so named because it is composed of three portions or heads, one of which is central and two lateral. The long head arises from the border of the shoulder blade immediately below the glenoid cavity and terminates in a broad flat tendon, which is also the termination of the internal and external portions. The external head arises from the upper and outer part of the humerus. The inner head is also on the humerus, but on the inner side. Both muscles are attached to the common tendon, which is inserted into the olecranon process of the ulna. (5) The anconeus muscle, small and triangular in shape, is attached in the external condyle of the humerus above, and below to the ulna, a continuation of the triceps.
III. Muscles act only by contraction. When exertion ceases they relax. The muscles that are placed on the front part of the arm, by their contraction bend the elbow; and extend and straighten the limb. The triceps (the opposing muscle) is brought into play with no less than that which bent it. The elbow joint that these muscles move is a hinge joint that moves in one plane only either forward or backward.
IV. The back of the arm is covered by the large muscular form of the triceps, which extends the entire length of the humerus. This muscle is narrow above, widening below to the furrow of the outer head of the triceps. From here the common tendon of the triceps follows the humerus as a flattened plane to the olecranon process of the ulna. The common tendon of the triceps receives the muscular fibres from all three heads of the triceps. The direction of this broad flat tendon is in line with the humerus.
I. bones: (1) Acromion process of the shoulder blade. (2) Head of the humerus. (3) Shaft of the humerus. (4) The external condyle.
II. muscles: (1) The triceps is a three-headed muscle. By contraction, it extends the forearm. (2) The biceps is a two-headed muscle. By contraction, it depresses the shoulder blade, flexes the forearm and rotates the radius outward. (3) Brachi-alis anticus (brachialis, pertaining to arm; anticus, in front): By contraction, it flexes the forearm. (4) Supinator longus. (5) Extensor carpi radialis longus; extensor (extender); carpi (carpus, the wrist); radialis (radiates); longus, (long) is responsible for the action that extends the wrist.
III. Muscles with their tendons are the instruments of motion as much as the wires and strings that give the movements to a puppet. In the upper arm, the wires that raise or lower the forearm are placed in directions which parallel the bones. All the muscles of the body are in opposing pairs. When a muscle pulls, the opposing one yields with just sufficient resistance to balance the one that is pulling. The forearm is the lever on which both the biceps and the triceps flex and straighten out the arm at the elbow. The muscles just mentioned parallel the arm to swing the forearm backward and forward. Another contrivance is needed to give rotary motion to the thumb side of the hand. In order to do this, the power is attached to the lower third of the humerus above the outer condyle and extends to near the end of the radius at the wrist. It is this muscle that aids in turning the doorknob and the screwdriver.
IV. In looking at the arm from the outer side it is seen that the deltoid descends as a wedge sinking into an outer groove of the arm. The masses of the biceps and triceps lie on either side. There is as well an outer wedge, the supinator longus. These different forms denote entirely different functions. Mechanism has always in view one of two purposes; either to move a great weight slowly, or a lighter weight with speed. The wedge at the shoulder creates power; lower down on the arm, speed. This mechanism allows the wrist and hand to move up and down as well as circularly, with a certain firmness and flexibility compared to the comparatively slow motion with which the arm can be raised.
I. bones: (1) The bone of the upper arm, the humerus, consists of a long strong cylinder. As it is not flexible, it can turn only on joints, one at the shoulder to raise the arm and one at the elbow to bend it. The upper extremity seen from the inner side consists of a round smooth ball that is covered over by a layer of cartilage and is known as the head of the humerus. It glides in the cup-shaped cavity of the shoulder blade, the glenoid cavity. (2) The cylindrical shaft of the humerus. (3) The inner condyle of the humerus is larger and more prominent than the outer one. It is the origin of the flexors of the forearm as well as a muscle that pulls the thumb side of the forearm toward the body, the pronator teres.
II. muscles: (1) Coraco-brachialis: from coracoid process to humerus, inner side half way down. It draws forward and rotates the humerus outward. (2) Biceps: the long head from upper margin of the glenoid cavity, the short head from-coracoid process to radius. It flexes the forearm and rotates the radius outward. (3) Triceps: the middle or long head; the external head; the internal or short head. It extends the forearm. (4) Brachialis anticus: from front of the humerus and the lower half to the ulna. It flexes the forearm. (5) Pronator radii teres: extends from the internal condyle to the radius on the outer side and half way down. It pronates the hand and flexes the forearm. (6) Supinator longus: the external condyloid ridge of the humerus to the end of the radius. It supinates the forearm.
III. The arm and forearm are pivoted or jointed at the elbow. The elbow is the fulcrum. The power that moves the lever is a muscular engine. When the forearm is raised the power is exerted by the biceps and brachialis anticus. When this action takes place, the triceps are inert.
IV. The arm, seen from the inner side, presents the greatest width at the fleshy region of the deltoid, two-thirds of the way above the elbow, then diminishes as a hollow groove, bordered by its common tendon. The inner view of the arm, the side that lies next the body, has a number of muscles that point this way and that way, as well as up and down, to pull and draw the joint in the direction to which it is attached. The crossing at different angles braces the arm as well as allowing great freedom of movement.
TRICEPS AND BICEPS
1 The triceps straightens out the flexed arm.
2 The biceps bends the elbow and flexes the forearm on the arm.
A finger is not bent or straightened without the contraction of two muscles taking place. A muscle acts only by contraction. In the same way a finger is bent, the forearm is bent. The muscles on the front part of the arm by their contraction, bend the elbow; those on the back extend and straighten the arm. The lever of the forearm is pivoted or jointed at the elbow which acts as its fulcrum. To straighten the arm, the heavy three-headed triceps plays against its antagonist, the two-headed biceps. When the exertion of either of these two muscles ceases, they relax to their former state.
The arm consists of a strong cylinder of bone which turns on the joint at the shoulder to raise the arm, and another joint at the elbow to bend it. These joints are made to slip on one another and are pulled as they contract or relax, thus changing the surface forms while undergoing action or re-
Was this article helpful?
You can have significantly bigger arms in only 31 days. How much bigger? That depends on a lot of factors. You werent able to select your parents so youre stuck with your genetic potential to build muscles. You may have a good potential or you may be like may of the rest of us who have averages Potential. Download this great free ebook and start learns how to build your muscles up.