SKELETAL SYSTEM2

The Thoracic Cage

The thoracic cage consists of the thoracic vertebrae, sternum, and ribs. It forms a more or less conical enclosure for the lungs and heart and provides attachment for the pectoral girdle and upper limb. It has a broad base and a somewhat narrower superior apex; it is rhythmically expanded by the respiratory muscles to create a vacuum that draws air into the lungs.

The inferior border of the thoracic cage is formed by a downward arc of the ribs called the costal margin. The ribs protect not only the thoracic organs but also the spleen, most of the liver, and to some extent the kidneys.

SternumThe sternum (breastbone) is a bony plate anterior to the heart. It is subdivided into three regions: the manubrium, body, and xiphoid process. The manubrium is the broad superior portion.

It has a superomedial suprasternal notch (jugular notch), which you can easily palpate between your clavicles (collarbones), and right and left clavicular notches, where it articulates with the clavicles.

The body, or gladiolus is the longest part of the sternum. It joins the manubrium at the sternal angle, which can be palpated as a transverse ridge at the point where the sternum projects farthest forward. In some people, however, it is rounded or concave.

The manubrium and body have scalloped lateral margins where cartilages of the ribs are attached. At the inferior end of the sternum is a small, pointed xiphoid process that provides attachment for some of the abdominal muscles.

Ribs

There are 12 pairs of ribs, with no difference between the sexes. Each is attached at its posterior (proximal) end to the vertebral column. A strip of hyaline cartilage called the costal cartilage extends from the anterior (distal) ends of ribs 1 to 7 to the sternum. Ribs 1 to 7 are thus called true ribs.

Ribs 8 to 10 attach to the costal cartilage of rib 7, and ribs 11 and 12 do not attach to anything at the distal end but are embedded in thoracic muscle. Ribs 8 to 12 are therefore called false ribs, and ribs 11 and 12 are also called floating ribs for lack of any connection to the sternum.

Pectoral GirdleThe pectoral girdle (shoulder girdle) supports the arm. It consists of two bones on each side of the body: the clavicle (collarbone) and the scapula (shoulder blade).

The medial end of the clavicle articulates with the sternum at the sternoclavicular joint, and its lateral end articulates with the scapula at the acromioclavicular Joint. The scapula also articulates with the humerus at the humeroscapular joint. These are loose attachments that result in a shoulder far more flexible than that of most other mammals, but they also make the shoulder joint easy to dislocate.

ClavicleThe clavicle is a slightly S-shaped bone, somewhat flattened dorsoventrally and easily seen and palpated on the upper thorax. The superior surface is relatively smooth, whereas the inferior surface is marked by grooves and ridges for muscle attachment.

The medial sternal end has a rounded, hammerlike head, and the lateral acromial end is markedly flattened. Near the acromial end is a rough tuberosity called the conoid tubercle—a ligament attachment that faces toward. the rear and slightly downward. The clavicle braces the shoulder and is thickened in people who do heavy manual labor. Without it, the pectoralis major muscles would pull the shoulders forward and medially, as occurs when a clavicle is fractured. Indeed, the clavicle is the mostcommonly fractured bone in the body because it is so close to the surface and because people often reach out with their arms to break a fall.

ScapulaThe scapula is a triangular plate that dorsally overlies ribs 2 to 7. The three sides of the triangle arecalled the superior, medial (vertebral), and lateral (axillary) borders, and its three angles are the superior, inferior, and lateral angles. A conspicuous suprascapular notch in the superior border provides passage for a nerve.

The broad anterior surface of the scapula, called the subscapular fossa, is slightly concave and relativelyfeatureless. The posterior surface has a transverse ridge called the spine, a deep indentation superior to the spine called the supraspinous fossa, and a broad surface inferior to it called the infraspinous fossa. The scapula is held in place by numerous muscles attached to these three fossae.

The most complex region of the scapula is its lateral angle, which has three main features:

1. The acromion is a platelike extension of the scapular spine that forms the apex of the shoulder. It articulates with the clavicle—the sole point of attachment of the arm and scapula to the rest of the skeleton.

2. The coracoid process is shaped like a finger but named for a vague resemblance to a crow’s beak; it provides attachment for the biceps brachii and other muscles of the arm.

3. The glenoid cavity is a shallow socket that articulates with the head of the humerus.

Upper Limb

The upper limb is divided into four regions containing a total of 30 bones per limb:

1. The brachium, or arm proper,extends from shoulder to elbow. It contains onlyone bone, the humerus.

2. The antebrachium, or forearm, extends fromelbow to wrist and contains two bones—the radius and ulna. In anatomical position, these bones are parallel and the radius is lateral to the ulna.

3. The carpus,or wrist, contains eight small bones arranged in two rows.

4. The manus, or hand, contains 19 bones in twogroups—5 metacarpals in the palm and 14phalanges in the fingers

Humerus

The humerus has a hemispherical head that articulates with the glenoid cavity of the scapula . The smooth surface of the head (covered with articular cartilage in life) is bordered by a groove called the anatomicalneck.

Other prominent features of the proximal end are muscle attachments called the greater and lesser tubercles and an intertubercular groove between them thataccommodates a tendon of the biceps muscle.

Radius

The proximal head of the radius is a distinctivedisc that rotates freely on the humerus when the palm is turned forward and back. It articulates with the capitulum of the humerus and radial notch of the ulna. On the shaft, immediately distal to the head, is a medial rough tuberosity, which is the insertion of the biceps muscle. The distal end ofthe radius has the following features, from lateral to medial:

1. a bony point, the styloid process, which can bepalpated proximal to the thumb;

2. two shallow depressions (articular facets) thatarticulate with the scaphoid and lunate bones of thewrist; and

3. the ulnar notch, which articulates with the end ofthe ulna.

Ulna

At the proximal end of the ulna is a deep, Cshaped trochlear notch that wraps around the trochlea of the humerus. The posterior side of this notch is formed bya prominent olecranon—the bony point where you rest your elbow on a table. The anterior side is formed by a less prominent coronoid process. Medially, the head of the ulna has a less conspicuous radial notch, which accommodates the head of the radius.

At the distal end of the ulna is a medial styloidprocess. The bony lumps you can palpate on each side of your wrist are the styloid processes of the radius and ulna. The radius and ulna are attached along their shafts by a ligament called the interosseous membrane, which is attached to an angular ridge called theinterosseous margin on the medial side of each bone.

Carpal Bones

The carpal bones, which form the wrist, are arranged in two rows of four bones each . These short bones allow movements of the wrist from side to side and up and down. The carpal bones of the proximal row, starting at the lateral (thumb) side, are the scaphoid (navicular), lunate, triquetrum, and pisiform—Latin for boat-, moon-, triangle-, and pea-shaped, respectively. Unlike the other carpal bones, the pisiform is a sesamoid bone; it develops within the tendon of the flexor carpi ulnarismuscle.

The bones of the distal row, again starting on the lateral side, are the trapezium, trapezoid, capitate,and hamate.The hamate can be recognized by a prominent hook on the palmar side.

Proximal:A=Scaphoid B=Lunate C=Triquetrum (Triangular) D=Pisiform Distal:E=Trapezium F=Trapezoid G=Capitate H=Hamate

Pelvic Girdle

The adult pelvic girdle is composed of four bones: a right and left os coxae (plural, ossa coxae), the sacrum, and the coccyx . Another term for the os coxae—arguably the most self-contradictory term in anatomy—is the innominate bone, “the bone with no name.”

The pelvic girdle supports the trunk on the legs and encloses and protects viscera of the pelvic cavity—mainly the lower colon, urinary bladder, and reproductive organs

Lower LimbThe number and arrangement of bones in the lower limb are similar to those of the upper limb. In the lower limb, however, they are adapted for weight-bearing and locomotion and are therefore shaped and articulated differently.

The lower limb is divided into four regions containing a total of 30 bones per limb:

1. The femoral region, or thigh, extends from hip to knee and contains the femur (the longest bone in the body). The patella (kneecap) is a sesamoid bone at the junction of the femoral and crural regions.

2. The crural region, or leg proper, extends from knee to ankle and contains two bones, the medial tibia and lateral fibula.

3. The tarsal region (tarsus), or ankle, is the union of the crural region with the foot. The tarsal bones are treated as part of the foot.

4. The pedal region (pes), or foot, is composed of 7 tarsal bones, 5 metatarsals, and phalanges in the toes.

FemurThe femur (FEE-mur) (fig. 8.38) has a nearly spherical head that articulates with the acetabulum of the pelvis, forming a quintessential ball-and-socket joint. A ligament extends from the acetabulum to a pit, the fovea, in the head of the femur. Distal to the head is a constricted neck and then two massive, rough processes called the greater and lesser trochanters , which are insertions for the powerful muscles of the hip.

They are connected on the posterior side by a thick oblique ridge of bone, the intertrochanteric crest, and on the anterior side by a more delicate intertrochanteric line.

The primary feature of the shaft is a posterior ridge called the linea aspera at itsmidpoint. It branches into less conspicuous lateral and medial ridges at its inferior and superior ends. The distal end of the femur flares into medial and lateral epicondyles, which serve as sites of muscle and ligament attachment.

Distal to these are two smooth round surfaces of the knee joint, the medial and lateral condyles,separated by a groove called the intercondylar fossa. On the anterior side of the femur, asmooth medial depression called the patellar surface articulates with the patella.

PatellaThe patella, or kneecap, is a roughly triangular sesamoid bone that forms within the tendon of the knee as a child begins to walk. It has a broad superior base, a pointed inferior apex, and a pair of shallow articular facets on itsposterior surface where it articulates with the femur.

The lateral facet is usually larger than the medial. The quadriceps femoris tendon extends from the anterior muscle of the thigh (the quadriceps femoris) to the patella, and it continues as the patellar ligament from the patella to the tibia.

TibiaThe leg has two bones—a thick, strong tibia and a slender, lateral fibula. The tibia, on the medial side, is the only weight-bearing bone of the crural region. Its broad superior head has two fairly flat articular surfaces, the medial and lateral condyles, separated by a ridge called the intercondylar eminence.

The condyles of the tibia articulate with those of the femur. The rough anterior surface of the tibia, the tibial tuberosity, can be palpated just below the patella. This is where the patellar ligament inserts and the thigh muscles exert their pull when they extend the leg. Distal to this, the shaft has a sharply angular anterior crest, which can be palpated in the shin. At the ankle, just above the rim of a standard dress shoe, you can palpate a prominent bony knob on each side. These are the medial and lateral.

The medial malleolus is part of the tibia, and the lateral malleolus is the part of the fibula.

FibulaThe fibula (fig. 8.39) is a slender lateral strut that helps to stabilize the ankle. It does not bear any of the body’s weight; indeed, orthopedic surgeons sometimes remove the fibula and use it to replace damaged or missing bone elsewhere in the body. The fibula is somewhat thicker and broader at its proximal end, the head, than at the distal end. The point of the head is called the apex.

The distal expansion is the lateral malleolus.Like the radius and ulna, the tibia and fibula are joined by an interosseous membrane along their shafts.

The Ankle and FootThe tarsal bones of the ankle are arranged in proximal and distal groups somewhat like the carpal bones of the wrist. Because of the load-bearing role of the ankle, however, their shapes and arrangement are conspicuously different from those of the carpal bones, and they are thoroughly integrated into the structure of the foot.

The largest tarsal bone is the calcaneus, which forms the heel. Its posterior end is the point of attachment for the calcaneal (Achilles) tendon from the calf muscles.

The second-largest tarsal bone, and the most superior, is the talus. It has three articular surfaces: an inferoposterior one that articulates with the calcaneus, a superior trochlear surface that articulates with the tibia, and an surface that articulates with a short, wide tarsal bone called the navicular. The talus, calcaneus, and navicular are considered the proximal row of tarsal bones. (Navicular is also used as a synonym for the scaphoid bone of the wrist.)

The bones of the tarsus with A=calcaneus, B=talus bone, C=cuboid bone, D=navicular bone, E=lateral cuneiform, F=intermediate cuneiform G=medial cuneiform

The distal group forms a row of four bones. Proceeding from the medial side to the lateral, these are the medial, intermediate, and lateral cuneiforms and the cuboid. The cuboid is the largest.

The remaining bones of the foot are similar in arrangement and name to those of the hand. The proximal metatarsals are similar to the metacarpals. They are metatarsals I to V from medial to lateral, metatarsal I being proximal to the great toe. (Note that Roman numeral I representsthe medial group of bones in the foot but the lateral group in the hand. In both cases, however, Roman numeral I refers to the largest digit of the limb.) Metatarsals I to III articulate with the first through third cuneiforms; metatarsals IV and V both articulate with the cuboid.

Joints and Their Classification

Arthrology is the science concerned with the anatomy, function, dysfunction, and treatment of joints. The study of musculoskeletal movement is called kinesiology.

This is a subdiscipline of biomechanics, which deals with a broad range of motions and mechanical processes, including the physics of blood circulation, respiration, and hearing.

Joints such as the shoulder, elbow, and knee are remarkable specimens of biological design—self-lubricating, almost frictionless, and able to bear heavy loads and withstand compression while executing smooth and precise movements.

Yet, it is equally important that other joints be less movable or even immovable. Such joints are better able to support the body and provide protection for delicate organs. The vertebral column, for example, must provide a combination of support and flexibility; thus its joints are only moderately movable. The immovable joints between the cranial bones afford the best possible protection for the brain and sense organs.

The name of a joint is typically derived from the names of the bones involved. For example, the atlanto-occipital joint is where the occipital condyles meet the atlas, the humeroscapular joint is where the humerus meets the scapula, and the coxal joint is where the femur meets the os coxae.

Joints can be classified according to their relative freedom of movement:

A diarthrosis is a freely movable joint such as the elbow.

An amphiarthrosis is a joint that is slightly movable, such as the intervertebral and intercarpal joints.

A synarthrosis is a joint that is capable of little or no movement, such as a suture of the skull.

Joints are also classified according to the manner in which the adjacent bones are joined. In this system, there are fibrous, cartilaginous, bony, and synovial joints.

These two systems of classification overlap. For example, synovial joints may be either diarthroses or amphiarthroses, and amphiarthroses can be any of the three structural types— synovial, fibrous, or cartilaginous

Fibrous JointsIn a fibrous joint, collagen fibers emerge from the matrix of one bone and penetrate into the matrix of another, spanning the space between them There are three types of fibrous joints: sutures, gomphoses, and syndesmoses.

In sutures and gomphoses, the collagen fibers are very short and allow for little movement. In syndesmoses, the fibers are longer and the attached bones are more movable.

Sutures are immovable fibrous joints that closely bind the bones of the skull to each other; they occur nowhere else.

Even though the teeth are not bones, the attachment of a tooth to its socket is classified as a joint called a gomphosis

Syndesmoses are joints at which two bones are bound by a ligament only. (Ligaments also bind bones together at synovial joints, but are not the exclusive means of holding those joints together.

Syndesmoses are the most movable of the fibrous joints. The radius and ulna are bound to each other side by side, as are the tibia and fibula, by a syndesmosis in which the ligament forms a broad sheet called an interosseous membrane along the shafts of the two bones

SutureBones tightly bound by minimal fiberOnly found in skull

SyndemosesBones connected by ligamentsE.g. tibiofibular ligament, interosseous membrane of radius/ulna

Gomphoses– Peg in socket joint– Only found in – teeth/alveoli

CartilaginousCartilaginous joints are joints in which the bones are attached by cartilage. These joints allow for only a little movement, such as in the spine or ribs.

Bones are united by cartilage and there is no cavity between the bones.

Cartilaginous Joints• Synchondrosis

– Hyaline cartilage unites bones– Epiphyseal growth plates– Costal cartilage-sternum

• Symphyses– Fibrocartilage unites bones– Pubic symphysis– Intervertebral disc

A synovial joint is one in which two bones are separated by a space that contains a slippery lubricant called synovial fluid. Most synovial joints, including the jaw, elbow, hip, and knee joints, are freely movable. These are not only the most common and familiar joints in the body, but they are also the most structurally complex and the most likely to develop uncomfortable and crippling dysfunctions.

The bones of a synovial joint are separated by a joint (articular) cavity containing the synovial fluid. Synovial fluid is rich in albumin and hyaluronic acid, which give it a viscous, slippery texture similar to that of raw egg white. It nourishes the articular cartilages and removes their wastes.

The adjoining surfaces of the bones are covered with a layer of hyaline articular cartilage about 2 mm thick in young, healthy joints. The cartilages and synovial fluid make movements at synovial joints almost friction-free. A fibrous joint (articular) capsule encloses the cavity and retains the fluid. It has an outer fibrous capsule continuous with the periosteum of the adjoining bones and an inner synovial membrane of areolar tissue, which secretes the fluid.

Synovial Joints• Most common joints in body• Most mobile joints• Have– Articular surfaces on bone with hyaline cartilage– Completely enclosed joint capsule formed from

ligamentous connective tissue– Synovial fluid within capsule lubricates joint– Some have meniscus or articular disc(e.g. knee,

jaw joint)

Structure of Synovial Joints

In the jaw, sternoclavicular, and knee joints, cartilage grows inward from the joint capsule and forms a pad called a meniscus between the articulating bones. The meniscus absorbs shock and pressure, guides the bones across each other, reduces the chance of dislocation, and distributes force across the entire joint instead of at just a few points of contact.

The most important accessory structures of a synovial joint are tendons, ligaments, and bursae. A tendon is a strip or sheet of tough, collagenous connective tissue that attaches a muscle to a bone. Tendons are often the most important structures in stabilizing a joint. A ligament is a similar tissue that attaches one bone to another. Several ligaments are named and illustrated in our later discussion of individual joints, and tendons are more fully considered in later lectures along with the gross anatomy of muscles.

A bursa is a fibrous sac filled with synovial fluid, located between adjacent muscles or where a tendon passes over a bone. Bursae cushion muscles, help tendons slide more easily over the joints, and sometimes enhance the mechanical effect of a muscle by modifying the direction in which its tendon pulls.

Types of Synovial JointsThere are six types of synovial joints with distinctive patterns of motion determined by the shapes of the articular surfaces of the bones (fig. 9.8; table 9.1). A bone’s movementat a joint can be described with reference to three mutually perpendicular planes in space (x, y, and z). If the bone can move in only one plane, the joint is said to be monaxial; if it can move in two planes, the joint is biaxial; and if three, it is multiaxial.

Ball and Socket Joints

These occur at the shoulder and hip, where one bone has a smooth hemispherical head that fits within a cuplike depression on the other. The head of the humerus fits into the glenoid cavity of the scapula, and the head of the femur fits into the acetabulum of the os coxae. These are the only multiaxial joints of the skeleton.

2. Hinge joints. At a hinge joint, one bone has a convex surface that fits into a concave depression of the other one. Hinge joints are monaxial—like a door hinge, they can move in only one plane.

Examples include the elbow, knee, and interphalangeal (finger and toe) joints.

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• hinge joint• trochlea of humerus• trochlear notch of ulna

• gliding joint• capitulum of humerus• head of radius

• flexion and extension• many reinforcing ligaments• stable joint

3. Saddle joint. The only saddle joint is the Trapeziometacarpal joint at the base of the thumb. Each articular surface—on metacarpal and the trapezium of the wrist—is shaped like a saddle, concave in one direction and convex in the other. This is a biaxial joint. If you compare the range of motion of your thumb with that of your fingers, you can see that a saddle joint is more movable than a condyloid or hinge joint. This is the joint responsible for that hallmark of primate anatomy, the opposable thumb.

4. Pivot joints. These are monaxial joints in which one bone has a projection that fits into a ringlike ligament of another, and the first bone rotates on its longitudinal axis relative to the other. One example is the atlantoaxial joint between the first two vertebrae. This joint pivots when you rotate your head as in gesturing “no.”

5. Gliding (plane) joints. Here, the articular surfaces are flat or only slightly concave and convex. The adjacent bones slide over each other and have rather limited monaxial movement; they are amphiarthroses in contrast to the other five types listed, which are diarthroses.

Gliding joints occur between the carpal and tarsal bones, between the articular processes of the vertebrae, and at the sternoclavicular joint. To feel a gliding joint in motion, palpate your sternoclavicular joint as you raise your arm above your head.

6. Condyloid (ellipsoid) joints. These joints exhibit an oval convex surface on one bone that fits into a similarly shaped depression on the next. The radiocarpal joint of the wrist and the metacarpophalangeal joints at the bases of the fingers are examples.

These are considered biaxial joints because they can move in two directions, forexample up and down and side to side. To demonstrate, hold your hand with your palm facing you. Flex your index finger back and forth as if gesturing to someone, “come here,” and then move the finger from side to side toward the thumb and away. This shows the biaxial motion of the condyloid joint.