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The first great physician was Hippocrates, a Greek who lived almost 2,500 years ago. He developed theories about the practice of medicine and the anatomy of the human body, but Hippocrates is remembered today for a set of medical ethics that still influences medical practice. The oath that he administered to his disciples is still administered to physicians about to start practice. His 87 treatises on medicine, known as the "Hippocratic Collection," are believed to be the first authoritative record of early medical theory and practice. Hippocratic physicians believed in the theory that health was maintained by a proper balance of four "humors" in the body: blood, phlegm, black bile, and yellow bile.

Another Greek physician, Galen, influenced medical thought for more than a thousand years. During the Middle Ages, his works were translated into Arabic and Syriac.

The great civilizations of Egypt, India, and China all developed medical theories of diagnosis and treatment that influenced later cultures of their own countries and those of other countries. The school of medicine at Alexandria, Egypt, for example, incorporated the theories of the ancient Greeks as well as those of the Egyptians. This great medical school flourished and was influential for several hundred years. Research specialists there learned more about human anatomy than had ever been learned before.

The theories and practices of medicine were kept alive almost entirely during the Middle Ages by monks in monasteries. Few new theories were developed during this period, but the medical records of most of the great early civilizations were carefully preserved and copied.

The Renaissance saw a renewal of interest in medical research. Swiss physician Paracelsus publicly burned the writings of Galen and Avicenna (a Persian physicist and philosopher), signifying a break with the past. Concepts of psychology and psychiatry were introduced by Juan Luis Vives, a Spanish humanist and physician.

In the 17th century, English physician William Harvey discovered that blood, propelled by the pumping action of the heart, circulates through the body. Many inventions in other fields helped the progress of medicine. Anton van Leeuwenhoek, a Dutch lens grinder, made instruments that magnified up to 270 times. He also studied blood circulation and composition, and was the first to see bacteria and protozoans.

During the 18th century the Dutch physician Hermann Boerhaave introduced clinical instruction (teaching at the bedside of patients). Edward Jenner discovered a vaccination against smallpox. Specialization grew rapidly, as did the growth of medical schools, hospitals, and dispensaries.

The 19th century saw advances in more precise instruments, such as the stethoscope, the ophthalmoscope, and X-rays. Doctors began to use anesthetics like ether and nitrous oxide and antiseptics. Knowledge of the cell, digestion, metabolism, and the vasomotor system increased.

Among the 20th century discoveries and developments have been the identification of four blood types, the discovery of insulin, development of antibiotics, and immunizations such as the polio vaccine. Technological advances have included the electron microscope, pacemakers, ultrasound, heart-lung machines, dialysis machines, and prostheses, to name only a few. Medical research and practice made giant strides toward the relief of human distress and the prolonging of human life. Every day brings new discoveries and the possibility of major breakthroughs in the areas that have long plagued humans.

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