Skeletal Tree

The human skeleton is divided into two parts: the axial and the appendicular (Fig 1.4). The axial skeleton shapes the longitudinal axis of the human body. It is composed of 22 bones of the skull, 7 bones associated with the skull, 26 bones of the vertebral column, and 24 ribs and 1 sternum comprising the thoracic cage. It is acted on by approximately 420 different skeletal muscles. The axial skeleton transmits the weight of the head and the trunk and the upper limbs to the lower limbs at the hip joint. The muscles of the axial skeleton position the head and the spinal column, and move the rib cage so as to make breathing possible. They are also responsible for the minute and complex movements of facial features.

The vertebral column begins at the support of the skull with a vertebra called the atlas and ends with an insert into the hip bone (Fig. 1.5a). The average length of the vertebral column among adults is 71 cm. The vertebral column protects the spinal cord. In addition, it provides a firm support for the trunk, head, and upper limbs. From a mechanical viewpoint, it is a flexible rod charged with maintaining the upright position of the body (Fig. 1.5b). The vertebral column fulfills this role with the help of a large number of ligaments and muscles attached to it.

A typical vertebra is made of the vertebral body (found anteriorly) and the vertebral arch (positioned posteriorly). The vertebral body is in the form of a flat cylinder. It is the weight-bearing part of the vertebra. Between the vertebral bodies are 23 intervertebral disks that are made of relatively deformable fibrous cartilage. These disks make up approximately one-quarter of the total length of the vertebral column. They allow motion between the vertebrae. The shock absorbance characteristics of the vertebral disks are essential for physical activity. The compressive force acting on the spine of a weight lifter or a male figure skater during landing of triple jumps peak at many times the body weight. Without shock absorbants, the spine would suffer irreparable damage.

axial skeleton (80)

skull (29)

thoracic cage (25)

vertebral column (26)

cranium (B)

auditory ossicles (S)

appendicular skeleton (126)

cranium (B)

auditory ossicles (S)

clavicle (2) scapula (2)

humerus (2) ulna (2) radius (2) carpals (16) metacarpals (10) phalanges (28)

hip bone (2) pelvic girdle (2)

femur (2) patella (2) tibia (2) fibula (2) tarsals (14) metatarsals (10) phalanges (28)

Figure 1.4. Frontal view of the human skeleton. The skeleton is composed of 206 bones. it is divided into two parts: the axial skeleton and appendicular skeleton. The numbers in parentheses indicate the number of bones of a certain type (or in a certain subgroup). The names of the major bones of the skeleton are identified in the figure.

clavicle (2) scapula (2)

humerus (2) ulna (2) radius (2) carpals (16) metacarpals (10) phalanges (28)

pectoral girdle (4)

upper extremities (60)

hip bone (2) pelvic girdle (2)

femur (2) patella (2) tibia (2) fibula (2) tarsals (14) metatarsals (10) phalanges (28)

lower extremities (60)

Figure 1.4. Frontal view of the human skeleton. The skeleton is composed of 206 bones. it is divided into two parts: the axial skeleton and appendicular skeleton. The numbers in parentheses indicate the number of bones of a certain type (or in a certain subgroup). The names of the major bones of the skeleton are identified in the figure.

The vertebral disks are also instrumental in determining the curvature of the spinal column. Most of the body weight lies in front of the vertebral column during standing, walking, and running. Individual disks are not of uniform thickness, but are slightly wedged. The curvatures in the cervical (neck) and lumbar (pelvic) regions are primarily caused by the greater anterior thickness of the disks in that region. The reverse S shape of the vertebral column in the standing position brings the weight in line with the body axis.

The bodies of the vertebrae are held together by longitudinal ligaments that extend the entire length of the vertebral column. There are also a

sacral thoraci lum ce sacral thoraci lum ce

Figure 1.5a,b. Side view of the spinal column (a). The spinal column is like a string of beads of irregular shape. It would collapse under its own weight in the absence of the large number of ligaments and muscles that are attached to it. Most of the body weight lies anterior to the spinal column, and to balance it, ligaments and the erector spinae muscles pull the spine to its curved shape (b).

Figure 1.5a,b. Side view of the spinal column (a). The spinal column is like a string of beads of irregular shape. It would collapse under its own weight in the absence of the large number of ligaments and muscles that are attached to it. Most of the body weight lies anterior to the spinal column, and to balance it, ligaments and the erector spinae muscles pull the spine to its curved shape (b).

multitude of ligaments that connect arches of the adjacent vertebrae. The supraspinous ligament runs posteriorly along the axis of the vertebral column and plays an important role in restoring the upper body from flexion to a extension. Contractile muscles that are attached to the vertebral column provide mobility as well as stability.

The thoracic cage is directly connected to the vertebral column. The ribs arise on or between thoracic vertebrae and are connected to the sternum by cartilaginous extensions. There are 12 pairs of ribs in the thoracic cage. The joints of the axial skeleton are heavily reinforced by an array of ligaments, and as a result they permit only limited movement.

The appendicular skeleton consists of the bones of the upper and lower limbs and the supporting elements (girdles) that connect them to the trunk (see Fig. 1.4). Each arm articulates with the trunk at the shoulder (the pectoral girdle), and the lower extremities are attached to the trunk at the pelvic girdle. There are 126 bones in the appendicular skeleton, and approximately 300 muscles act on them to cause movement or to sustain a certain pause.

The upper limbs are connected to the trunk at the shoulder (pectoral) girdle. The shoulder girdle consists of the S-shaped clavicle (collarbone) and a broad, flat scapula (the shoulder blade). The clavicle joins at one end to the sternum and at the other end meets the scapulae. The only direct connection between the shoulder girdle and the axial skeleton is the joint between the clavicle and sternum. Skeletal muscles support and position the scapula, which has no direct bony or ligamentous connections to the rib cage. Once the shoulder joint is in position, muscles that originate on the pectoral girdle help to move the upper extremity.

The bone of the upper arm, the humerus, articulates with the scapula on the proximal end. At its distal end, it articulates with the bones of the forearm, the radius and ulna. These are parallel bones that support the forearm. Their distal ends form joints with the bones of the wrist. The radius and the ulna are connected through their entire length by a flexible interosseus membrane.

The wrist is composed of eight carpal bones that are arranged in two rows, proximal and distal carpals. In the hand, five metacarpals articulate with the distal carpals of the wrist and support the palm. Distally, the metacarpals articulate with the finger bones or phalanges. There are 14 phalanges bones in each hand.

The pelvic girdle attaches the lower limbs to the axial skeleton. The pelvis is a composite structure that is composed of the hip bone (coxae) of the appendicular skeleton and the sacrum and coccyx, the last two elements of the vertebral column. An extensive network of fibers connect the elements of the pelvis, increasing the stability of this structure under various types of loading conditions. Because the bones of the pelvic girdle bear the weight of the human body, they are more massive than those of the pectoral girdle. Similarly, the bones of the thigh and the lower leg are more massive than those of the arm and the forearm.

The long bone of the thigh, the femur, is the longest and heaviest bone in the body. More than 7% of all stress fractures in the human occur in the femur. The head of the femur joins the pelvis and the other end articulates with the tibia of the leg at the knee joint. The other bone of the lower leg, the fibula, is slender in comparison with the tibia. The fibrous membrane between these two bones stabilizes their position and provides additional surface area for muscle attachment. The fibula is excluded from the knee joint and generally does not transfer weight to the ankle and the foot. However, it is an important site for muscle attachment. In addition, the distal tip of the fibula extends laterally to the ankle joint, providing lateral stability to the ankle. About half of all stress fractures in the human occur in the tibia. These fractures are usually the result of repetitive, cyclic loading of the bone such as occurs during running, ballet, and jumping sports. As we shall see later in the text, high-impact activities drastically increase the loads carried by the bones of the lower leg. The reaction forces at the feet may be 5 to 10 times higher than the body weight during sprinting or jumping. Usually the strong muscles and mobile joints act as shock absorbers, damping the intensity of the peak load transmitted to the bone. Muscle fatigue, and stiff or immobile joints have been implicated in increased load on bone.

The patella (kneecap) is a large sesamoid bone that forms within the tendon of the quadriceps femoris, a group of muscles that extend the leg.

The kneecap prevents the knee from extensive damage caused by an impact force. It also increases the lever arm of the quadriceps muscle group, making the muscle more efficient in extending the knee.

The ankle (also called the tarsus) consists of seven tarsals. The bones of the foot include the five long bones that form the sole of the foot. Tarsals bear 25% of the stress fractures in the human. The phalanges (the toes) have the same anatomical organization as fingers. Together, they contain 14 phalanges in each foot.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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