For a long time the circulatory system presented an impenetrable puzzle. People could observe arteries and veins, and see that these blood vessels branched out into smaller and thinner vessels. Some observers speculated that blood from the arteries returned to the heart through veins. However, they had had no means to investigate this. They had reached the limits of human sight. The invention of the single lens microscope by Leeuwenhoek, who was the first to observe unicellular organisms, around 1668 allowed scientists for the first time to view the most basic element of life, the cell.
Antony van Leeuwenhoek was an unlikely scientist, born in 1632 into a family of tradesmen in Delft, Holland. He had no fortune, no university education, and knew only his native language, Dutch. His lack of any formal qualifications or wealth would seem to create great odds against him for success in the scientific community of his time.
However Leeuwenhoek's determination and curiosity allowed him to succeed in making important discoveries in the field of biology.
Leeuwenhoek is best known for his improvement of the microscope. As a hobby Leeuwenhoek ground lenses and used them to study tiny objects. Using these lenses, he observed protozoa in pond water and bacteria in the human mouth and intestine. He gave the first full descriptions of bacteria, protozoan, and spermatozoa. The lenses allowed him to discover blood corpuscles and capillaries, and also to describe the structure of muscles and nerves. His observations laid the foundations for bacteriology and protozoology.
In 1595, nearly forty years prior to Leeuwenhoek's birth, compound microscopes had been invented in England by Robert Hooke and in the Netherlands by Jan Swammerdam. These compound microscopes were similar to the ones in use today but only increased the magn/;ification power by only 20x. Leeuwenhoek's interest in...