Enzymes have been called the "agents of life" because all life processes are dependent on them. Enzymes are protein molecules that act as catalysts (they speed up chemical reactions without undergoing any change themselves).


Epinephrine, also known as adrenaline, is a hormone secreted by the medulla (inner part) of the adrenal glands, located on the kidneys. The adrenal glands are one of the body's endocrine glands (glands producing substances that are distributed by way of the bloodstream).


Ether is a colorless, transparent, and very volatile (readily vaporizable) liquid. It has a characteristic odor and is highly flammable.


Eyeglasses are corrective lenses mounted in frames. They are used to help people with vision problems see clearly.

False Teeth

Replacements for decayed or lost teeth have been produced for a few thousand years. The Etruscans (people from the ancient country of Etruria in western Italy) made skillfully designed false teeth out of ivory and bone.

Finsen Light

The finsen light is named for its inventor, Niels Tyberg Finsen (1860-1903). It was a powerful light used to cure people of the skin disorder lupus.

Fluoride Treatment, Dental

Fluoride is a chemical found in many substances. In the human body, fluoride acts to prevent tooth decay by strengthening tooth enamel and inhibiting the growth of plaque-forming bacteria.

Folic Acid

Folic acid is a member of the vitamin B family. It plays an important role in the synthesis of the amino acids and of components of the nucleic acids.

Fractures, Treatments and Devices

Fractures, or broken bones, have always occurred. Prompt treatment is important if a patient is to regain full use of an injured arm or leg.

Gamete Intrafallopian Transfer (GIFT)

A gamete is a mature male or female reproductive cell. At the time of sex cell formation, the two factors that exist for each heredity characteristic separate equally into two gametes.


The gene is the physical unit of heredity. For each physical trait—such as eye color, height, hair color—a person inherits two genes or two groups of genes, one from each parent.

Gene Therapy

Gene therapy is a new field in which normal genes are given to patients to cure genetic disorders. Some successes have occurred, as well as some failures.

Genetic Code

The genetic code, which carries the instructions on what a human (or any other living creature) will be like, from color of eyes to tendencies toward disease, is located in specific molecules called nucleotides inside the nucleus (center) of body cells. The genetic material, which is made up of acids in combination with sugar and phosphate molecules is called DNA (deoxyribonucleic acid).

Genetic Engineering

Genetic engineering is the human altering of the genetic material of living cells to make them capable of producing new substances or performing new functions. The technique became possible during the 1950s when Francis Crick (1916-) and James Watson (1928-) discovered the structure of DNA molecules.

Genetic Fingerprinting

Fingerprints are unique to each individual. Methods of recording and matching fingerprints have allowed police to correctly identify many criminals.

Genetically Engineered Blood-Clotting Factor

Excessive, uncontrolled bleeding can be fatal. Hemophilia is the most common chronic condition that involves uncontrolled bleeding.


Hallucinogens are natural and synthetic (synthesized) substances that, when ingested (taken into the body), significantly alter one's state of consciousness. Hallucinogenic compounds often cause people to see (or think they see) random colors, patterns, events, and objects that do not exist.

Hearing Aids and Implants

Hearing aids are electronic devices that amplify sound for people with impaired (damaged) hearing. Millions of hearing aids are sold annually.

Heart-Lung Machine

One of the major milestones in medicine was the development of artificial circulation, also known as heart-lung bypass. Before the heart-lung machine was invented, heart surgeons operated blindly, with the heart still pumping, or by slowly chilling the patient's body until circulation nearly stopped, or by connecting the patient's circulatory system to a second person's system during the operation.


Hormones are chemical messengers that regulate bodily processes such as growth, reproduction, metabolism, digestion, mineral and fluid balance, and the functioning of various organs. In animals, hormones are secreted by organs, tissues, and glands of the endocrine system directly into the blood by and carried in the bloodstream to target organs.