Three types of levers: (a) A first-class lever is used in a pair of scissors, (b) a second-class lever is used in a wheelbarrow, and (c) a third-class lever is used in a pair of forceps.


When seven-year-old Jacob fell from the tree limb he had been hanging from and held out his arm at an odd angle, it was obvious that he had broken a bone. An X ray at the hospital emergency room confirmed this, and Jacob spent the next six weeks with his broken arm immobilized in a cast.

Many of us have experienced fractured, or broken, bones. A fracture is classified by its cause and the nature of the break. For example, a break due to injury is a traumatic fracture, whereas one resulting from disease is a spontaneous, or pathologic, fracture.

A broken bone exposed to the outside by an opening in the skin is termed a compound {open) fracture. It has the added danger of infection, because microorganisms enter through the broken skin. A break protected by uninjured skin is a closed fracture. Figure 7A shows several types of traumatic fractures.

Repair of a Fracture

Whenever a bone breaks, blood vessels within it and its periosteum rupture, and the periosteum is likely to tear. Blood escaping from the broken vessels spreads through the damaged area and soon forms a blood clot, or hematoma. Vessels in surrounding tissues dilate, swelling and inflaming tissues.

Within days or weeks, developing blood vessels and large numbers of osteoblasts originating from the periosteum invade the hematoma. The osteoblasts rapidly multiply in the regions close to the new blood vessels, building spongy bone nearby. Granulation tissue develops, and in regions farther from a blood supply, fibroblasts produce masses of fibrocartilage.

Meanwhile, phagocytic cells begin to remove the blood clot as well as any dead or damaged cells in the affected area. Osteoclasts also appear and resorb bone fragments, aiding in "cleaning up" debris.

In time, fibrocartilage fills the gap between the ends of the broken bone. This mass, termed a cartilaginous callus, is later replaced by bone tissue in much the same way that the hyaline cartilage of a developing endochondral bone is replaced. That is, the cartilaginous callus breaks down, blood vessels and osteoblasts invade the area, and a bony callus fills the space.

Typically, more bone is produced at the site of a healing fracture than is necessary to replace the

A greenstick fracture is incomplete, and the break occurs on the convex surface of the bend in the bone.

A fissured fracture involves an incomplete longitudinal break.

A comminuted fracture is complete and fragments the bone.

A transverse fracture is complete, and the break occurs at a right angle to the axis of the bone.

Figure 7A

Various types of fractures.

An oblique fracture occurs at an angle other than a right angle to the axis of the bone.

A spiral fracture is caused by twisting a bone excessively.

damaged tissues. Osteoclasts remove the excess, and the final result is a bone shaped very much like the original. Figure 7B shows the steps in the healing of a fracture.

The rate of fracture repair depends upon several factors. For instance, if the ends of the broken bone are close together, healing is more rapid than if they are far apart. Setting fractured bones and using casts or metal pins to keep the broken ends together help speed healing, as well as aligning the fractured parts. Also, some bones naturally heal more rapidly than others. The long bones of the upper limbs, for example, may heal in half the time required by the long bones of the lower limbs, as Jacob was happy to discover. He also healed quickly because of his young age. ■

Compact bone

Medullary cavity


Compact bone

Medullary cavity


Fibrocartilage Spongy bone

(a) Blood escapes from ruptured blood vessels and forms a hematoma.

(b) Spongy bone forms in regions close to developing blood vessels, and fibrocartilage forms in more distant regions.

Bony callus

Bony callus

Medullary cavity


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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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