The Complete Bridgman Part 3

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The Complete Bridgman Part 3

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THE TORSO – Front View

The thorax, or chest, is composed of bones and cartilages. It is designed not only to protect the heart and lungs, which it contains, but also to allow the whole mass to be turned and twisted with the different movements of the body. This cage is formed, at the back, by the spinal column, on the sides by the ribs, and in front by the breastbone. It protects the heart and lungs as a baseball mask protects the face; its structure is yielding and elastic, so that it may serve as a bellows. The ribs are not complete circles, nor do they parallel each other; they incline downward from the spine and bend at an angle at the sides, to take a forward thrust toward the breastbone. The breastbone is called the sternum.

If each rib were rigid and circular, the chest would be immovable and no chest expansion could take place. According to Keill, the breastbone, with an easy inspiration, is thrust out one-tenth of an inch, allowing forty-two cubic inches of air to enter the lungs; and this may be increased, with effort, to seventy or even one hundred cubic inches.
The pelvis is the mechanical axis of the body. It is the fulcrum for trunk and legs, and is large in proportion. Its mass inclines a little forward and as compared with the trunk above is somewhat square. The ridge at the sides is called the iliac crest and this is the fulcrum for the lateral muscles; it flares out widely for this purpose, rather more widely in front than behind.

Masses of the Torso

The masses of the torso are the chest, the abdomen or pelvis, and between them the epigastrium; the first two comparatively stable, the middle one quite movable. A straight line marking the collar bones defines the top of the first mass; and paralleling it, a line through the base of the breast muscles and pit of the epigastrium forms its base.

Below this arch is the abdomen, the most movable part of the mobile portion. It is bounded below by a line passing approximately through the anterior points of the iliac crests. Its profile shows the lines of the cone of the thorax diverging downward, the lines of the wedge of the chest and shoulders converging downward, and the buttressing of the lateral muscles.

In the bending or turning of the body the central line of this portion bends always to the convex side, always paralleled by the borders of the rectus muscle. By this movement the straight wedge of the front is broken. It becomes not a bent wedge, but two wedges; one the upper half of the original wedge, prolonged but not completed downward; the other the lower half, prolonged upward to meet the one above. More unchanging than either of the above is the mass of the abdomen. The central groove is here shallow and may lose itself below. The long wedge ends in the symphysis pubis.

Planes of the Torso—Front View

From the front, the masses of the trunk may be divided into three distinct planes.

The first may be outlined by drawing lines from the inner third of each collar bone to the base of the breast muscles (the point where they take an upward direction to their insertion on the upper arm) and then joining them with a base line across the sixth ribs. The second is the epigastrium which forms the upper part of the abdominal region. For our purpose, it is a flattened plane bordering the breast muscles above and the stomach below. The third plane is more rounded and is bounded at the sides by the lower ribs and pelvic bones. It is placed in the lower cavity of the trunk —the abdomen.

Muscles of the Torso

1 Pectoralis: pertaining to the breast

2 The serrati: the deep muscles of the spine

3 Muscles that pull the arm down; pectoralis; latissimus dorsi

4 Abductors: draw the thigh toward the medium line

5 Tendons that pass through a loop or slit: omo-hyoid; digastric

6 Pulley: knee-cap, tendon and ligament

7 Rectus, upright: abdominis and femoris

8 Rhomboideus: rhomb-shaped, not right-angled; from the shoulder blade to the spine

9 Deltoid: delta-shaped, triangular, equilateral of the shoulder

10 Trapezius: table-shaped

11 Oblique, slanting


1 Pectoralis major

2 Deltoid

3 Rectus abdominis

4 Serratus magnus

5 External oblique
Rectus Abdominis: From symphysis pubis to cartilages of ribs, from fifth to seventh. Action: Flexes thorax.

Serratus Magnus: From eight upper ribs to scapula—spinal edge, under surface. Action: Draws shoulder blade forward, raises ribs.

External Oblique: From eight lower ribs to iliac crest and ligament to pubis. Action: Flexes thorax.

Cage of the Torso


The erect torso presents in profile the long curve of the front, broken by depressions at the border of the breast muscle and at the umbilicus or navel into three lesser curves, almost equal in length. The back presents the sharp anterior curve of the waist, opposite the umbilicus, bending into the long posterior curve of the chest, and the shorter curve of the buttocks. The curve of the chest is broken by the almost vertical shoulder blade and the slight bulge of the latissimus below it. In profile the torso presents three masses: that of the chest, that of the waist, and that of the pelvis and abdomen. The first and last are comparatively unchanging. Above, the mass of the chest is bounded by the line of the collar bones; below, by a line following the cartilages of the ribs, being perpendicular to the long diameter of the chest. This mass is widened by the expansion of the chest in breathing, and the shoulder moves freely over it, carrying the shoulder blade, collarbone, and muscles.

The chest is marked by the ridge of costal cartilages that forms its border, sloping up and forward, and by the ribs themselves, sloping down and forward, and by the “digitations” (finger marks) of the serratus magnus (big saw-toothed) muscle, little triangles in a row from the corner of the breast muscle, paralleling the cartilages of the ribs, disappearing under the latissimus. Below, the mass of the pelvis and abdomen slopes up and forward. It is marked by the iliac crest and hip. In front it may be flattened by contraction of the abdominal muscles. Over its surface the hip moves freely, changing the tilt of the pelvis. Between these the central mass contains the waist vertebrae, and is very changeable. Practically all of the movement of flexion and extension for the whole spine occurs here, and much of the side-bending. This mass is marked by a buttress of lateral muscles, slightly overhanging the pelvic brim and bearing inward against the side above. It changes greatly in different positions of the trunk.

1 Latissimus dorsi

2 External oblique

Latissimus dorsi: From spine, sixth dorsal, to sacrum and iliac crest; passes inside of humerus to fasten to front side near head. Action: Draws arm backward and inward.

External Oblique: From eight lower ribs to iliac crest and ligament to pubis. Action: Flexes thorax.


I. bones: (1) Scapula, (shoulder blade), a large flat bone triangular in shape. It articulates with the collar bone at the summit of the shoulder. (2) The serratus magnus muscles follow the ribs. See Muscles No. II. (3) The thorax or rib cage is the cavity enclosed by the ribs, attached to the spine behind and to the sternum in front. The upper ribs are quite short and grow longer till they reach the seventh rib, which is the longest and the last to fasten to the breastbone. The upper seven ribs are named the true ribs.

II. muscles: (t) The latissimus dorsi muscle covers the region of the loins to be inserted into the upper part of the arm at the lower border of the bicipital groove. It is a superficial muscle, a thin sheathing that finds attachments at the small of the back and at the crest of the ilium near the lumbar and last dorsal vertebrae. (2) The serratus magnus muscle is seen only at its lower parts as prominent digitations that show on the side of the thorax or rib cage below the armpit. A large portion of this muscle is covered by the pectoralis major and the latissimus dorsi muscles. (3) The external oblique is attached aboye to the lower eight ribs, where they interlock with the serratus magnus. From here they are carried downward to be attached to the iliac crest.

III. The serratus magnus draws the shoulder blade forward and raises the ribs. The latissimus dorsi draws the arm backward and inward. Its upper border curves backward at the level with the sixth or seventh dorsal vertebrae, as it passes over the lower angle of the shoulder blade. The serratus magnus muscle forms the inner wall of the armpit. Its insertion to the ribs above are not seen, while those below, three or four in number are plainly visible in the region between the great pectoral and the latissimus dorsi.

IV. In profile, the torso in front is marked by the ridge of the costal cartilage that forms its border. Sloping up and forward (with the ribs themselves sloping down and forward) the digitations of the serratus magnus meet the external oblique. In its attachment to the crest of the ilium, the external oblique forms a thick oblique roll, its base marking the iliac furrow. When one side of this muscle contracts, it gives the trunk a movement of rotation to the right or left side. When both sides pull, the oblique muscles draw the ribs downward, thus bending the body forward.

Rib Cage


1 The fulcrum or hinge on which the lever works

2 The ribs have to be lifted by muscular force

3 The front end of the rib is lowered and raised by muscular force. Whether ascending or descending, the muscles hold or balance the axis on which the ribs turn. They are worked by two muscular engines, one that raises and expands the chest and the one that pulls the cage down. These opposing muscles are known as elevators and depressors.

The enlargement and contraction of the chest depends on the mechanical contrivance of the bones which enclose it. The ribs articulate to the sides of the backbone from where they project obliquely downward. When they are pulled upward, they are at the same time being pulled outward, which brings them more to a right angle to the spine, causing the sternum or breastbone to which they are attached at the front, to be thrust forward. The muscular bands that enlarge and contract the chest pass upward obliquely from pelvis to the front and sides of the rib cage.

THE TORSO – Back View

The trapezius is a diamond-shaped muscle, with upper apex at the base of the skull, lower apex well below the shoulder blades, and corners at the shoulder girdle opposite the deltoid, as though it were a continuation of that muscle. From the sacrum the muscles diverge upward, while the lower ribs and lower corner of the shoulder blade diverge downward, making lesser diamonds of various definiteness of outline.

The ridge of the shoulder blade is always conspicuous, pointing diagonally toward the corner of the shoulder. It sets at a fixed angle with the spinal edge (more than a right angle) and at a right angle with the lower turned-out corner. In relaxation, both ridge and blade are ridges under the skin, and are converted into grooves by the muscles bulging in contraction.

Of these muscles, those on either side of the ridge are easily recognizable—the deltoid, below and outside, and trapezius, above and inside, but the trapezius also spreads from the inner end of the ridge to well down the spine. Under this, helping to form the bulge, are the rhomboidei, extending from the blade diagonally upward to the spine, and the levator anguli scapulae, from its upper corner almost vertically to the top of the neck.

1 Trapezius

2 Deltoid

3 Latissimus dorsi

Trapezius: From occipital bone, nape ligament and spine as far as twelfth dorsal, to clavicle, acromion and ridge of shoulder blade. Action: Extends head, elevates shoulder and rotates shoulder blade.

The back presents numerous depressions and prominences. This is due not only to its bony structure, but to the crossing and recrossing of a number of thin layers of muscles. It should be borne in mind that the superficial or outside layers manifest themselves only when in action. For this reason, under all changes of position, the spine, the shoulder blade with its acromion process, and the crest of the ilium, must be regarded as the landmarks of this region.

The spine is composed of twenty-four vertebrae. It extends the full length of the back, and its course is marked by a furrow. The vertebras are known as the cervical, dorsal and lumbar. The cervical vertebrae are seven in number, and the seventh is the most prominent in the whole of the spine. It is known as vertebra prominens. In the dorsal region the furrow is not so deep as below. Here there are twelve vertebrae. When the body is bent forward, the processes of the vertebrae in this section are plainly indicated.

The spinal furrow becomes deeper as it reaches the lumbar vertebrae, where it is marked by dimples and depressions. It widens out, too, in this part of the body, and as it passes over the surface of the sacrum to the coccyx it becomes flattened. The average length of the spine is about two feet three inches.

The outer corner of the shoulder girdle is the acromion process, which is the high outer extremity of a ridge rising from the shoulder blade. The shoulder blade or scapula (spade) is a flat plaque of bone fitting snugly against the cage of the thorax, having a long inner vertical edge, parallel to the spine; a sharp lower point; a long outer edge pointing to the armpit; and a short upper edge parallel with the slope of the shoulder. The ridge, or spine of the scapula, starts at the spinal edge, about a third of the way down, in a triangular thickening, and rises until it passes high over the outer upper corner, where the shoulder joint ties, then turns forward to join with the collarbone at the acromion. The prominent portions are this ridge and the spinal edge and the lower corner. The upper outer corner is thickened to form the socket for the head of the humerus, forming the shoulder joint proper.


Movement of flexion and extension occurs almost entirely in the waist or lumbar vertebrae. Movement of side-bending occurs throughout the whole length. Movement of rotation occurs in the lumbar vertebrae when the spine is erect, in the middle vertebrae when it is half flexed, in the upper vertebrae when the spine is fully bent. In the lumbar vertebrae, the axis of this rotation is behind the spine; in the middle vertebrae it is neutral; in the upper dorsals it is in front of the spine. Each vertebra moves a little, and the whole movement is the aggregate of the many little movements.

The shoulder blade slides against the surface of the cage of the thorax, in any direction, and may be lifted from it so that its point or its spinal edge becomes prominent under the skin. It produces easily fifty per cent of the whole movement of the shoulder.

Wedging of the Cage into the Hips

Mechanism of the Torso and Hips


The cage and the pelvic bones are connected by a portion of the spine called the lumbar region. Muscular power acts on these masses as levers and allows the body to move forwards and backwards or turn. The pelvis can be compared to a wheel with only two spokes; the hub is the hip joint and the spokes are the legs which swing back and forth as in walking or running. When force is applied to the long end of a lever, the power is increased. When speed is desired, the lever is shortened.

The muscular power of the human body can only pull upon and bend the levers at the joints, when the masses of the back and pelvis are bent backward or forward, or to the side. The movement of the back is limited to the extent that the bony structure of the spine allows. Each segment of the spine is a lever, upon which the masses of the rib cage and the pelvis bend or turn. From the rear, the torso presents a great wedge with its apex directed downward. The base of the wedge is at the shoulders. This wedge is driven in between the two buttresses of the hips. In movement these two masses turn or bend.

Shoulder Girdle


Shoulder blades are embedded rather than attached to the back. They move from their attachment at the summit of the blade to the collarbone and are raised, lowered, or twisted by muscular force. The movements of the collar bones and the shoulder blades are free except where the collarbone joins the sternum in front. These bones curve around the cone shaped thorax, and are known as the thoracic girdle.
This girdle, except at its attachment at the sternum, may be raised or lowered; thrown forward or twisted round the static rib cage without interfering in any way with the act of expiration or inspiration. There is a space between the borders of the shoulder blades at the back and in front and between the two ends of the collar bones. The muscles that raise the shoulders away from the rib cage, when set in motion, work against each other with perfect balance.


The deltoid muscle resembles a delta in shape. It arises from the outer third of the clavicle and the convex border of the acromion and runs the entire length of the spine of the shoulder blade. All three portions are directed downward. The middle portion is vertical and the inner and outer descend obliquely, to be inserted by a short tendon into the outer surface of the humerus. Nature allows these three portions to work in harmony. The deltoid, when all three portions are working, pulls the arm up vertically. The portions that pull diagonally from the collarbone, and from the crest of the shoulder blade, carry it forwards and backwards.

The pectoralis major muscle twists upon itself when the arm is down. When the arm is extended or raised above the head, its fibres are parallel. When drawing a pectoral seven points should be noted: (1) where the tendon leaves the arm (2) its attachment on the collarbone (3) where it meets at its step-down from clavicle to sternum (4) its descent down the sternum (5) its attachment to the seventh rib (6) where it crosses till it leaves the sixth rib (7) the location of the second and third ribs that are just below the pre-sternum.



In treating the shoulder as a mechanical device, one tries to discover its function, its leverage and its power. The shoulder must be looked upon as the foundation of the arm.

The large diagram on the opposite page shows the muscular arrangement of the shoulder blade. The arm is separated at a distance from the shoulder, showing the devices which nature has contrived in order that the arm may be pulled forward, inward or back. The origin of all the muscles shown are on the shoulder blade, while the insertions on the arm are on both the top, front and back of the humerus. They are so placed, that when pulling against one another, their contracting fibres cause a rotary movement of the arm. These muscles entirely or in part are seen only in the triangular space bound by the trapezius, latissimus dorsi and the deltoid.

o Supra-spinatus muscle

1 Infra-spinatus muscle

2 Teres minor muscle

3 Teres major muscle

4 Triceps muscle

5 Humerus bone


o Supra-spinatus muscle

1 Infra-spinatus muscle

2 Teres minor muscle

3 Teres major muscle

4 Triceps muscle

5 Humerus bone

6 Latissimus dorsi muscle

1 Levator anguli scapulae: the elevator of the scapula, raises the angle of the shoulder blade.

2 Rhomboideus: arises from the seventh cervicle to the fourth and fifth dorsal. It elevates and retracts the shoulder blade.

3 Serratus magnus: from the vertebral border of the shoulder blade; draws the shoulder blade forward.

Mechanism of the Scapular Region

1 The inner border of the shoulder blade parallels the spine when the arm is down.

2 When the arm is raised above, a right angle to the body, the greater tuberosity of the humerus presses the upper rim of the glenoid cavity. The shoulder blade then starts to revolve.

3 The horizontal bar represents the collarbone as it articulates with the sternum at the front, and with the acromion process of the shoulder blade at the summit of the shoulder.

4 The axis on which the shoulder blade turns (seen from the back) is where the collarbone and the crest of the shoulder blade meet.

5 The shoulder blade or scapula.

6 The humerus: arm bone.


The 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

The Arm—Flexed


Bones of the Arm

1 Humerus

2 Ulna (little finger side)

3 Radius (thumb side)

Muscles of the Upper Arm, front view

1 Coraco-brachialis

2 Biceps

3 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.

Turning of the Hand on the Forearm and the Forearm on the Arm

Muscles of the Upper Arm, outer view

1 Triceps

2 Supinator longus

3 Extensor carpi radialis v longus

4 Anconeus

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.

Anconeus: From back of external condyle to olecranon process and shaft of ulna. Action: Extends forearm.

THE ARM Front View

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.

THE ARM Back View

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.

THE ARM Outer View

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.

THE ARM Inner View

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.


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 relaxation.

Mechanism of the Arm


1 Biceps

2 Brachialis anticus

3 Coraco-brachialis
The muscles of the human body not only bend the body by muscular force, but also serve as brakes, slowing the reactions. For instance, the biceps and the brachialis anticus muscles are placed in the front of the upper arm and, by their contraction, they bend the elbow. If power ceased altogether, the forearm would drop down. But the opposing muscle slows the otherwise uncontrolled movement after the manner of a brake. This mechanism of slow motion pervades all the limbs and every movement of the body.

The Forearm


The muscles of the forearm move the wrist, the hand and fingers. They are muscular above and tendinous below. These tendons are strapped down to pass over and under the wrist and fingers. There is a great variety of formation and shape to the muscles of the forearm. There are muscles with tendons that are single and again double as they pass to the wrist and hand. The muscles act separately or in groups with quickness and precision as the occasion requires.

1 The front and inner side of the forearm is composed of muscles that arise from the internal condyle of the humerus by common tendons and terminate below by tendons that are two-thirds the length of the muscle. These tendons separate to be inserted into the wrist and fingers and are known as flexors.

2 The muscles of the back and outer side of the forearm as a group arise from the external condyle and adjacent ridge of the humerus. As a mass they are on a higher level than those of the inner side of the forearm. As to these muscles in general: they pass down the back of the forearm and divide into tendons as they approach the wrist where they are held in place by a band called the annular wrist ligament.

3 When the arm is bent to a right angle and the hand is directed toward the shoulder, the flexor muscles are set in motion by contraction. They swell to their muscular centers and their tendons pull the hand downward. When the hand is bent at the wrist in the direction toward the front of the forearm, it is flexion. The reverse is called extension.

4 The extension of the hand on the forearm shows the muscles and the tendons lying on the outer side and back of the forearm. These are held in place by the annular ligament. The rounded forearm is made up of the fleshy bodies of muscle that terminate mostly in long tendons that pass to and over the wrist and hand. Some of these muscles move the hand on the forearm or the different finger joints on each other. There are also deep muscles of the forearm from which the tendons emerge though the muscles are hidden.

FOREARM Front View


I. bones: (1) The upper arm bone, the humerus, is the longest bone of the upper limb. (2) At the lower end of the humerus there are two projections. The inner projection (the inner condyle) is quite prominent, always in evidence, and is used as a point of measurement. (3) The ulna hinges at the elbow, and articulates with the bone above by a beak-like process. It descends toward the little finger side of the hand, where it is seen as a knob-like eminence at the wrist. (4) The radius carries the thumb side of the wrist and hand at its lower extremity. At the upper end, the head is hollowed out to play freely on the radial head of the humerus.

II. muscles: (1) The pronator teres. From its origin on the internal condyle of the humerus, it is directed downward and outward and inserted into the outside of the radius about half way down the shaft. In contraction it turns the forearm and thumb side of the hand inward causing pronation. (2) There are four flexor muscles that arise from the internal condyle of the humerus. Their bodies are mostly fleshy, terminating at their lower half in long tendons. (3) The palmaris longus, also a flexor, shows a long slender tendon directed toward the middle of the wrist. It is inserted into the palmaris facia that stretches across the palm of the hand. (4) Flexor carpi ulnaris.

III. Muscles must lie above and below the joint they move. Muscles that bulge the forearm in front are flexors. They terminate as wires or strings that pull the wrist, hand and fingers together as they contract.
IV. The inner condyle of the humerus is a landmark when the forearm is seen from the front and the bones are parallel. In this position, the muscles and their tendons are directed downward to the wrist and hand.

The first, the pronator teres, passes obliquely to the middle of the radius. The second, the flexor carpi, radiates toward the outer side of the hand. The third, the palmaris longus, is toward the middle. The fourth, the flexor carpi ulnaris, is toward the inner border of the hand. The muscles just named are situated on the front and inner side of ihe forearm and all arise from the inner condyle of the humerus.


I. bones: (1) The humerus of the arm presents a shaft and two extremities. (2) Olecranon process of the ulna, elbow. (3) The ulna, from the elbow to the little finger side of wrist. (4) Radius, the thumb side of the forearm at the wrist. (5) The styloid process of the radius.

II. muscles: (1) The supinator longus arises from the outer border of the humerus about a third of the way up its shaft. It then enlarges as it descends to its greatest size at about the level of the external condyle. Below, its fibres are replaced by a long tendon that is inserted into the styloid process of the radius. (2) On the humerus, just below the supinator, arises the long extensor of the wrist. This muscle descends by a slender tendon to the index .finger and is named the extensor carpi radialis longus. (3) Anconeus, a small triangular muscle attached to the external condyle of the humerus and inserted into the ulna just below the elbow. (4) There are four extensors including the long extensor of the wrist just mentioned. Three of these arise from the external condyle of the humerus, descend as muscles about half way down and end as tendons that extend the wrist, the hand and the fingers. The fourth arises from the shaft of the humerus just above the external condyle. (5) Extensors of the thumb.

III. The muscles of the forearm are placed just below the elbow, moving the hand, the wrist and fingers by long slim tendons that are securely strapped down as they pass under or over the wrist. It is a fixed law that a muscle contracts toward its center. Its quickness and precision of movement depends upon its length and bulk. If the muscles of the forearm had been placed lower down, the beauty of the arm would have been destroyed.

IV. The muscles that lies on the outer side and back of the forearm are known as the supinator and the extensor group. They emerge from between the biceps and the triceps at about a third of the distance up the arm as a fleshy mass. These wedge-shaped muscles are placed on a higher level than the pronator or flexor group, as they arise some distance above the outer condyle of the humerus. The extensor group take their origin from the condyle below. The extensor tendons are on the back of the arm and always point to the outer condyle of the humerus. The extensor muscles are the direct antagonists of the pronators and flexors in front. The chief action of the supinator longus is that of a flexor but acts as in supination as well.

Masses of the Shoulder and Arms

The masses of the shoulder, arm, forearm and hand do not join directly end to end with each other, but overlap and lie at various angles. They are joined by wedges and wedging movements.

Constructing these masses first as blocks, we will have the mass of the shoulder, or deltoid muscle, with its long diameter sloping down and out, beveled off at the end; its broad side facing up and out; its narrow edge straight forward.

This mass lies diagonally across and overlaps the mass of the arm, whose long diameter is vertical, its broad side outward, its narrow edge forward.

The mass of the forearm begins behind the end of the arm and passes across it at an angle forward and out. It is made of two squares. The upper half of the forearm is a block whose broad side is forward, its narrow edge sideways. The lower half, smaller than the upper, has its narrow edge forward, and its broad side facing out (with the hand held thumb up).

These blocks are joined by wedges and wedging movements, and to the straight lines are wedded the curved lines of the contour of the muscles. The deltoid is itself a wedge, whose apex sinks into the outer groove of the arm half way down. The mass of the biceps ends in a wedge which turns outward as it enters the cubital fossa.

The mass of the forearm overlaps the end of the arm on the outside by a wedge (supinator longus) that arises a third of the way up the arm, reaches a broad apex at the broadest part of the forearm and tapers to the wrist, pointing always to the thumb; and on the inside by a wedge that rises back of the arm and points to the little finger (flexor-pronator muscles).

In the lower half of the forearm, the thin edge of the mass, toward the thumb, is made by a continuation of this wedge from the outside. The thin edge toward the little finger is made by the end of the wedge from the inside.

When the elbow is straight and the hand turned in, the inner line of the forearm is straight with that of the arm. When the hand is turned out, this line is set out at an angle that corresponds with the width of the wrist. The little finger side (ulna) is the hub of its movement.

The flexor tendons on the front of the forearm point always to the inner condyle; the extensor tendons on the back point always to the outer condyle.

The breadth of the hand corresponds with that of the lower mass, not joining it directly, but with a step-down toward the front.

In the back view of the arm, the mass of the shoulder sits across its top as in the front view. The back edge of this mass is seen to be a truncated wedge arising under the deltoid and focusing on the elbow. The upper end resolves itself into the three heads of the triceps; the lower or truncated end is the triceps tendon, to which is to be added the tiny wedge of the anconeus (donkey’s foot) muscle bridging from outer condyle to ulna.

Wedging of the Arm into the Forearm Back View

Wedging of Arm into the Forearm at the Elbow

1 Biceps

2 Triceps

3 Supinator longus

4 Flexors

5 Extensors

Pronator and Supinator

The two muscular forces that rotate or turn the forearm, by crossing one bone over the other, are the supinator and the pronator.

1 The supinator extends from the wrist to about a third way up the bone of the upper arm. It is a long muscle. The lower third is tendinous. It rises above the outer condyle of the humerus. The upper portion is the large fleshy mass that lies on the outer and upper third of the forearm. In action it flexes as well as supinates.

2 The opposing muscle to the supinator is the short round pronator teres, which passes obliquely downward across the forearm. It arises from the inner condyle of the humerus to be inserted near the middle of the outer border of the radius.

These two muscles pull the radius with a wheel-like motion over the ulna and back again carrying the thumb side of the hand toward or away from the body. The supinator is the force that turns the doorknob and the screwdriver away from the body. It is the only flexor of the forearm that can be seen on the surface of its entire length.


1 The upper extremity of the elbow as seen from the front. The inner surface of the coranoid process of the ulna is curved so as to clasp the pulleylike trochlea of the humerus.

2 The lower extremity of the humerus is somewhat flat. Projecting from each side are the internal and external condyles. Between the two is the rounded groove that receives the lip of the ulna.

3 Here the bones of the arm and forearm are connected. This is a view from the front. The humerus above shows the two condyles with a notch that receives the coranoid process of the ulna, when the arm is bent. The ulna at the elbow swings hinge-like on the bone of the upper arm. It moves backward and forward in one plane only. Just below the outer condyle of the humerus is a small and rounded bursa, called the radial head of the humerus, on the surface of which rolls the head of the radius.

Elbow—Front View

The large bone, which carries the forearm, may be swinging upon its hinge at the elbow, at the same time that the lesser bone which carries the hand may be turning round it. Both these bones of the forearm, the radius and ulna, have prominent ridges and grooves. They are directed obliquely from above, downward and inward. The radius turns round the ulna in these grooves and on the tubercles at the heads of both bones.

The lower extremity of the humerus gives a key to the movements of the elbow joint. Above, the shaft of the humerus is completely covered by the muscles of the upper arm. Below, the inner and outer condyles come to the surface near the elbow. The inner condyle is more in evidence. The outer one is hidden by muscle, when the arm is straightened out. When the arm is bent, it becomes more prominent and easier to locate.

Elbow—Back View

1 The humerus at the elbow is flattened in front and back, terminating in two condyles. Between these is placed the trochlea, a rounded spool-like form that is clasped by the olecranon process of the ulna.

2 This is a diagram of the spool-like form of the trochlea with the embracing condyles at the sides.

3 From the back, the olecranon process of the ulna is lodged into the hollowed-out portion of the back of the humerus, forming the elbow point.

4 This shows the bony structure of the hinge joint at the elbow.

Elbow—Side View

1 The ulna swings on the pulley of the humerus. The articulation is known as a hinge joint.

2 Shows the mechanical device used in straightening the forearm, on the arm, at the elbow. The common tendon of the triceps grasps the olecranon of the ulna, which in turn clasps round the spool-like trochlea of the humerus.

3 When the forearm is flexed on the arm, the ulna hooks round the pulleylike device of the humerus. The triceps in this position is opposed by the biceps and brachialis anticus in front, which becomes the power that raises the forearm upward. The triceps in reverse is inert and somewhat flattened out.


The hollow of the arm, filled with its friction hairs, is made into a deep pit great breast muscle (pectoralis major) in front, and the greater latis-dorsi behind.
Its floor slopes forward, downward and outward, following the slope of the chest wall.
Its rear wall is deeper, since the latissimus attaches farther down the back; thicker because made of two muscles (latissimus and teres major); and rounder because its fibres turn on themselves.

The front wall is longer because the pectoral muscle attaches farther down the arm.

Into this pit the biceps and triceps muscles plunge, with the coraco-brachialis between them.

The bottom of the pit may, when the arm is fully raised, be bulged by the head of the arm bone and the lymph glands that lie there.