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Skeletal System Overview (Morphology)

Skeletal System Overview (Morphology)

When death has occurred weeks or months before a body is discovered, decomposition removes much of the body fluids , muscle, and tissue from a corpse. What remains is the supporting skeleton. The arrangement of the bones in a skeleton, their condition, and markings that can be present (such as the scrape or gouging left by a knife blade) can tell a forensic investigator much about the deceased.

The bones of the skeletal system can be classified according to their shape and location. The types of bones categorized by shapelong, short, flat, and irregularalso provide evidence of their function. Long bones consist of an elongated shaft called the diaphysis. Each end of the diaphysis is an expanded portion of the shaft and is called an epiphysis. Examples of long bones include the femur in the thigh and the humerus in the arm. These bones function as levers when muscles contract, thus providing support to enable movement. Short bones often have equal dimensions, like those of a cube. Compared to long bones, short bones have a limited range of motion but are able to withstand force. Examples of short bones include the carpals in the wrist and the tarsals in the ankle. Flat bones are thin bones that protect internal organs and provide sites for muscle attachment. The ribs, cranial bones, and scapula are all examples of flat bones. Irregular bones are not shaped like any of the three aforementioned bones and therefore form their own category. The vertebrae and facial bones are categorized as irregular bones.

Location rather than shape classifies other types of bones, such as sesamoid and sutural bones. Sesamoid bones bear pressure as the result of being buried in tendons. The kneecap, or patella, is the best-known example of a sesamoid bone. Sutural bones are tiny bones located between the joints, or sutures, of the cranial bones.

The adult skeleton consists of 206 bones. A baby is born with 270 bones, many of which fuse together during adolescence and adulthood. The bones of males and females differ in that male bones tend to be larger and heavier than female bones.

The skeletal system can be divided into the axial skeleton and the appendicular skeleton. The axial skeleton is composed of the bones that surround the midline or axis of the body, forming the head and trunk. These bones include the skull bones, auditory ossicles, hyoid bone, vertebral column, sternum, and ribs.

The skull can be subdivided into eight cranial bones and fourteen facial bones. The cranial bones include the frontal bone, two parietal bones, two temporal bones, occipital bone, sphenoid bone, and ethmoid bone. The facial bones include two lacrimal bones, two nasal bones, two inferior nasal conchae, vomer, two zygomatic bones, two maxillae, two palatine bones, and mandible. Within the middle ear are three auditory ossicles: the maleus, incus, and stapes. These tiny bones transmit vibrations from the eardrum to the inner ear. The hyoid bone is located in the superior part of the neck and attaches the muscles of the tongue.

The vertebral column typically consists of twenty-six vertebrae that protect the spinal cord and provide attachment sites for ribs and back muscles. The seven most superior vertebrae are the cervical vertebrae. The first vertebra is called the atlas and enables the head to move forward and backward. The second vertebra, the axis, is unique in that it is the only vertebra that has a process called the dens or odontoid process. The axis enables the head to rotate from side to side. The vertebrae immediately inferior to the cervical vertebrae are the twelve thoracic vertebrae. These vertebrae are larger than the cervical vertebrae and, except for the eleventh and twelfth thoracic vertebrae, have facets that articulate with the ribs. (The point where two bones meet forms a joint and the bones are said to articulate with one another.) Just below the thoracic vertebrae are the five lumbar vertebrae. The lumbar vertebrae are the largest of the vertebrae because they support a tremendous amount of the body's weight. The five sacral vertebrae are actually fused together in adults to form the sacrum. Inferior to the lumbar vertebrae, the sacrum articulates with the pelvic girdle to form the pelvis. The four remaining bones of the vertebral column constitute the coccyx. These individual bones also become fused together in adults.

The sternum, also known as the breastbone, consists of three parts. The manubrium and the body are the superior and middle parts of the sternum that articulate with the ribs. Additionally, the manubrium articulates with the clavicles. The xiphoid process is the inferior part of the sternum that provides attachment for abdominal muscles.

There are twelve pairs of ribs that make up the rib cage. The first seven pairs are true ribs because they are attached directly to the sternum by cartilage. The next three pairs of ribs are false ribs because they are indirectly attached to the sternum by the cartilage of the seventh pair. The two remaining ribs are known as floating ribs because they do not connect to the sternum at all.

The appendicular skeleton is comprised of two pectoral girdles, two pelvic girdles, and the bones of the upper and lower extremities. Each pectoral girdle, or shoulder girdle, includes the clavicle and scapula responsible for attaching the upper extremities to the axial skeleton. The clavicle, or collarbone, is the anterior component of the shoulder that articulates with the scapula and manubrium of the sternum. The scapula, or shoulder blade, is positioned posterior to the clavicle and articulates with the humerus. The humerus constitutes the upper arm and articulates with the two bones of the forearm. The radius is the lateral bone and the ulna is the medial bone of the forearm. The distal end of the radius articulates with the carpals, the first row of bones in the hand. The proximal row of carpals located from lateral to medial includes the scaphoid, lunate, triquetrum, and pisiform. The distal row of carpals that articulates with the metacarpals are the trapezium, trapezoid, capitate, and hamate. The metacarpals are numbered one through five, beginning on the lateral palm of the hand extending medially. The fourteen bones of the fingers, named phalanges, articulate with the metacarpals. Each finger has a proximal, middle, and distal phalanx except for the thumb, which only has two phalanges.

Each pelvic girdle, or hipbone, in an adult is made of three fused bones. Also known as the coxal bones, the hipbones consist of the ilium, ischium, and pubis. The ilium articulates posteriorly with the sacrum. The ischium connects the ilium and pubis. The two pubis bones meet anteriorly to form the pubis symphysis. Together, the hipbones, sacrum, and coccyx constitute the pelvis. One major difference between the male and female skeleton is the bones of the pelvis. In the female, the pelvic bones form a wide, round opening called the pelvic inlet to accommodate for childbirth. The pelvic inlet of males is heart shaped and much narrower than in women. Additionally, the sacrum is wider and shorter in women than in men, allowing the forensic identification of the sex of the deceased

The bones of the lower extremities include the femur, patella, tibia, fibula, tarsals, metatarsals, and phalanges. The femur is the leg bone that articulates with the pelvic girdle. The distal end of the femur articulates with the foreleg to form the knee. Anterior to the knee lies the patella, or kneecap. Each foreleg consists of two bones: tibia and fibula. The tibia is the larger of the two bones and forms the shin. The fibula is the lateral bone in the foreleg. At the distal end of the forelegs are the proximal bones of the foot called the tarsals. The tarsals include the calcaneus, talus, navicular, cuboid, and three cuneiforms. The metatarsals form the sole of the foot and are labeled one through five beginning on the medial side of the foot. Each toe consists of three phalanges, the proximal, middle, and distal phalanx. The exception is the big toe, which contains only two phalanges.

see also Asphyxiation (signs of); Bite analysis; Exhumation; Skull.

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Skeletal System

Skeletal system

Inside every person is a skeleton, a sturdy framework of 206 bones. The skeleton protects the body's organs, supports the body, and provides attachment points for muscles to enable body movement. Bones also produce blood cells and act as a storage site for minerals such as calcium and phosphorus.

All humans are born with over 300 bones. But some bones, such as those in the skull and lower spine, fuse (join together) during growth, thereby reducing the number. The skeletal system is made up of living material, with networks of blood vessels running throughout. Living mature bone is about 60 percent calcium compounds and about 40 percent collagen (a fibrous protein). Hence, bone is strong, hard, and slightly elastic. Although mature bones consist largely of calcium, most bones in the human skeleton began as cartilage. Cartilage is a type of connective tissue that contains collagen and elastin fibers.

Individual bones meet at areas called joints and are held in place by connective tissue. Cartilage lines the surface of many joints and helps reduce friction between bones. The connective tissues linking the skeleton together at the joints are ligaments and tendons. Both are made up of collagen, but serve different functions. Ligaments link bones together and help prevent dislocated joints. Tendons link bone to muscle.

Because the bones making up the human skeleton are inside the body, the skeleton is called an endoskeleton. Some animals, such as the crab, have an external skeleton called an exoskeleton.


The human skeletal system is divided into two main groups: the axial skeleton and the appendicular skeleton. The axial skeleton includes bones associated with the body's main axis, the spine. This includes the spine, the skull, and the rib cage. The appendicular skeleton is attached to the axial skeleton and consists of the bones associated with the body's appendagesthe arms and legs. This includes the bones of the pectoral girdle (shoulder area), the pelvic girdle (hip area), and the arms and legs.

Axial skeleton. There are 28 bones in the skull. In adults, the bones of the cranium (part of the skull that encloses the brain) are flat and interlocking at their joints. In infants, cartilage fills the spaces between the cranial bones. Known as soft spots or fontanelles, these spaces allow the skull bones to move slightly during birth. This makes birth easier and helps prevent skull fractures. Eventually, the fontanelles are replaced by bone. In addition to protecting the brain, skull bones also support and protect the organs responsible for sight, hearing, smell, and taste.

The skull rests atop the spine, which encases and protects the spinal cord. The spine, also called the spinal column or backbone, consists of 33 stacked vertebrae, the lower ones fused. The spinal column helps to support the weight of the body and protects the spinal cord. Disks of cartilage lie between the bony vertebrae of the back and provide cushioning, like shock absorbers. The vertebrae of the spine are capable of only limited movement, such as bending and some twisting.

Twelve pair of ribs (a total of 24) extend forward from the vertebrae of the upper back. Most of the ribs (the first seven pair) attach in the front of the body via cartilage to the long, flat breastbone, or sternum. These ribs are called true ribs. The next three pair of ribs, called false ribs, do not attach to the sternum. They are connected by cartilage to the ribs above them. The lower two pair of ribs that do not attach in the front are called floating ribs. Ribs give shape to the chest and support and protect the body's major organs, such as the heart and lungs. The rib cage also provides attachment points for connective tissue, to help hold organs in place.

Appendicular skeleton. The appendicular skeleton joins with the axial skeleton at the shoulders and hips. Forming a loose attachment with the sternum is the pectoral girdle, or shoulder. Two bones, the clavicle (collar bone) and scapula (shoulder blade) form one shoulder. The major advantage to the loose attachment of the pectoral girdle is that it allows for a wide range of shoulder motions and greater overall freedom of movement.

Unlike the pectoral girdle, the pelvic girdle, or hips, is strong and dense. Each hip, left and right, consists of three fused bonesthe ilium, ischium, and pubic. The pelvic girdle is bowl-shaped, with an opening at the bottom. In a pregnant woman, this bony opening is a passageway through which her baby must pass during birth. The pelvic girdle of women is generally wider than that of men, which helps to ease birth. The pelvic girdle protects the lower abdominal organs, such as the intestines, and helps supports the weight of the body above it.

The arms and legs, appendages of the body, are very similar in form. The upper arm bone, the humerus, is the long bone between the elbow and the shoulder. It connects the arm to the pectoral girdle. In the leg, the thigh bone, or femur, is the long bone between the knee and hip that connects the leg to the pelvic girdle. The humerus and femur are sturdy bones, especially the femur, which is the longest bone in the body.

At the elbow the humerus attaches to a set of parallel bonesthe ulna and radiusthe bones of the forearm. These bones attach to the eight small carpal bones of the wrist. The hand is made up of 19 bones.

Similarly, in the leg, the femur attaches to a set of bones of the lower leg, the fibula and tibia. The tibia, or shin bone, is larger than the fibula and forms the joint behind the patella (kneecap) with the femur. At the ankle joint, the fibula and tibia connect to the seven tarsal bones forming the ankle and heel. These, in turn, are connected to the 19 bones that make up the foot.

Bone structure

Bones may be classified according to their various traits, such as shape and texture. Four types are recognized based on shape. These are long bones, short bones, flat bones, and irregular bones. The smooth, hard outer layer of bones is called compact bone. Inside the compact bone is cancellous bone, sometimes called the bone marrow. Cancellous bone appears open and spongy, but is actually very strong, like compact bone. Together, these two types of bone produce a light, but strong, skeleton.

Bones and medicine

Even though bones are very strong, they may be broken. Fortunately, most fractures will fully heal with proper care. In children, bones often heal without a trace.

Bones are affected by poor diet and are also subject to a number of diseases and disorders. Some examples include scurvy, rickets, osteoporosis, and arthritis. Scurvy results from the lack of vitamin C. In infants, scurvy causes poor bone development. It also causes membranes surrounding the bone to bleed. Rickets is a children's disease resulting from a deficiency of vitamin D. This vitamin enables the body to absorb calcium and phosphorus. Without it, bones become soft and weak and actually bend, or bow out, under the body's weight.

The elderly, especially women who had several children in a row, sometimes suffer from osteoporosis. This condition develops when a

body's calcium level is low and calcium from bones is dissolved into the blood to maintain a proper balance. Weak, brittle bones dotted with pits and pores are the result.

Another condition commonly afflicting the elderly is arthritis, an often painful inflammation of the joints. Arthritis is not, however, restricted to the elderly, as even young people may suffer from this condition. Arthritis basically involves the inflammation and deterioration of cartilage and bone at the joint surface.

[See also Orthopedics ]

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Musculoskeletal System

Musculoskeletal System

The musculoskeletal system includes bones, joints, skeletal muscles, tendons, and ligaments. Muscles generate force; tendons transfer it to bones; and the bones move if enough force is transmitted. The force must be enough to overcome the weight of the moving body part, gravity, and other external resistance. Motion occurs at joints associated with one or both ends of the bone.

The force is produced in the muscle belly, which consists of muscle tissue. Tendons are basically connected bundles of collagen. They are classified as dense regular connective tissue and arise partially from the connective tissue coverings of muscle fibers and fiber groups. Tendons attach to the external membrane of a bone, the periosteum, which covers the bone except at joint surfaces. A few muscles bypass tendons and attach directly to the periosteum. Other muscles attach to skin (muscles of facial expression), to other muscles, or to fascia, which are connective tissue sheets between muscles.

The surfaces of the bone making up the joint have a layer of hyaline cartilage, the articular cartilage, which forms a smooth surface for easy movement. Bone ends may be surrounded by a joint capsule, which secretes fluid for lubrication and nutrition. Joint motion is usually pain free, but age, injury, and some diseases damage the articular cartilage, resulting in arthritis.

Biomechanics applies the principles of physics to human movement. Some joints work like levers, others like pulleys, and still others like a wheel-axle mechanism. Most motion uses the principle of levers. A lever consists of a rigid "bar" that pivots around a stationary fulcrum . In the human body, the fulcrum is the joint axis, bones are the levers, skeletal muscles usually create the motion, and resistance can be the weight of a body part, the weight of an object one is acting upon, the tension of an antagonistic muscle, and so forth.

Levers are classified by first, second, and third class, depending upon the relations among the fulcrum, the effort, and the resistance. First-class levers have the fulcrum in the middle, like a seesaw. Nodding the head employs a first-class lever, with the top of the spinal column as the fulcrum. Second-class levers have a resistance in the middle, like a load in a wheel-barrow. The body acts as second-class lever when one engages in a full-body push-up. The foot is the fulcrum, the body weight is the resistance, and the effort is applied by the hands against the ground.

Third-class levers have the effort (the muscle) in the middle. Most of the human body's musculoskeletal levers are third class. These levers are built for speed and range of motion. Muscle attachments are usually close to the joint. As the length of the lever increases, the possible speed increases, but so does the force required to produce it. For instance, the forearm is a third-class lever, controlled by the biceps muscle. A longer forearm can produce faster motion of the hand, but requires more effort to move than a shorter forearm.

A few muscle-bone connections work on the principle of a pulley, which changes the direction of an applied force. A classic example is the patella (kneecap), which alters the direction in which the quadriceps (patellar) tendon pulls on the tibia.

Muscles play four roles in producing joint movements: agonist (prime mover), antagonist, synergist, and fixator. A given muscle can play any of these roles, often moving from one to the next in a series during an action. Agonists and antagonists are opposing muscles. This means that when an agonist creates tension, the antagonist produces an opposing tension, thereby contributing to control at the joint. When one lifts a glass of water from the table to one's mouth, for example, the biceps brachii muscle acts as an agonist to flex the elbow, while the triceps brachii acts as an antagonist to keep the elbow from flexing too fast or too far. Synergists aid the motion of an agonist.

Although every musculotendinous unit (muscle belly and tendons attaching it to the bone) has a specific name, it is common to group muscles according to the motion they create. Flexors create motion that would bring the distal segment closer to the torso, while abductors cause a limb to move laterally , away from the body.

see also Bone; Muscle; Skeletons

Karen Jensen


Arthritis Answers. <www.arthritis.org/Answers/disease_center.asp>.

Kreighbaum, Ellen, and Katharine Barthels. Biomechanics: A Qualitative Approach for Studying Human Movement. New York: Allyn and Bacon, 1996.

Marieb, Elaine. Essentials of Human Anatomy and Physiology. San Francisco, CA: Benjamin/Cummings, 2000.

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