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Leonardo's anatomical drawings: 1489-1515

In about 1489 Leonardo da Vinci begins a series of anatomical drawings. For accuracy of observation they are far in advance of anything previously attempted. Over the next twenty-five years he dissects about thirty human corpses, many of them at a mortuary in Rome - until in 1515 the pope, Leo X, orders him to stop.

His drawings, amounting to some 750, include studies of bone structures, muscles, internal organs, the brain and even the position of the foetus in the womb. His studies of the heart suggest that he was on the verge of discovering the concept of the circulation of the blood.

Vesalius and the science of anatomy: 1533-1543

A young medical student, born in Brussels and known to history as Vesalius, attends anatomy lectures in the university of Paris. The lecturer explains human anatomy, as revealed by Galen more than 1000 years earlier, while an assistant points to the equivalent details in a dissected corpse. Often the assistant cannot find the organ as described, but invariably the corpse rather than Galen is held to be in error.

Vesalius decides that he will dissect corpses himself and trust to the evidence of what he finds. His approach is highly controversial. But his evident skill leads to his appointment in 1537 as professor of surgery and anatomy at the university of Padua.

In 1540 Vesalius gives a public demonstration of the inaccuracies of Galen's anatomical theories, which are still the orthodoxy of the medical profession.

Galen did many of his experiments on apes. Vesalius now has on display - for comparison - the skeletons of a human being and of an ape.

Vesalius is able to show that in many cases Galen's observations are indeed correct for the ape, but bear little relation to the man. Clearly what is needed is a new account of human anatomy.

Vesalius sets himself the task of providing it, illustrated in a series of dissections and drawings. He has at his disposal a method, relatively new in Europe, of ensuring accurate distribution of an image in printed form - the art of the woodcut. His studies inaugurate the modern science of anatomy.

At Basel, in Switzerland, Vesalius publishes in 1543 his great work - De humani corporis fabrica (The Structure of the Human Body). There are seven volumes including numerous magnificent woodcut illustrations. The book is an immediate success, though naturally it enrages the traditionalists who follow Galen. Galen's theories have, after all, the clear merit of seniority. They are by now some 1400 years old.

But for those willing to look with clear eyes, the plates in Vesalius's volumes are a revelation. For the first time human beings can peer beneath their own skins, in these strikingly clear images of what lies hidden.

Harvey and the circulation of the blood: 1628

A book is published in 1628 which provides one of the greatest breakthroughs in the understanding of the human body - indeed perhaps the greatest until the discovery of the structure of DNA in the 20th century.

The book consists of just fifty-two tightly argued pages. Its text is in Latin. Its title is Exercitatio anatomica de motu cordis et sanguinis in animalibus ('The Anatomical Function of the Movement of the Heart and the Blood in Animals'). Its author is William Harvey. In this book he demonstrates beyond any reasonable doubt an entirely new concept. Blood, he shows, does not drift in the body in any sort of random ebb and flow. Instead it is pumped endlessly round a very precise circuit.

Until now it has been assumed that the blood in arteries and the blood in veins are different in kind. It is well known that they are of a different colour, and there have been many theories as to what each supply of blood does.

The most commonly held belief is that arterial blood carries some sort of energy connected with air to the body (not far from the truth), and that blood in the veins distributes food from the liver (less accurate).

By a long series of dissections (from dogs and pigs down to slugs and oysters), and by a process of logical argument, Harvey is able to prove that the body contains only a single supply of blood; and that the heart is a muscle pumping it round a circuit.

This circuit, as he can demonstrate, brings the blood up from the veins into the right ventricle of the heart; sends it from there through the lungs to the left ventricle of the heart; and then distributes it through the arteries back to the various regions of the body.

After much initial opposition, Harvey's argument eventually convinces most of his contemporaries. But there are two missing ingredients. His theory implies that there must be a network of tiny blood vessels bringing the blood from the arterial system to the venous system and completing the circuit. But his dissections are not adequate to demonstrate this. It is not till four years after his death that Marcello Malpighi observes the capillaries.

And Harvey is unable to explain why the heart should circulate the blood. That explanation will have to await the discovery of oxygen.

Malpighi and the microscope: 1661

Marcello Malpighi, a lecturer in theoretical medicine at the university of Bologna, has been pioneering the use of the microscope in biology.

One evening in 1661, on a hill near Bologna, he uses the setting sun as his light source, shining it into his lens through a thin prepared section of a frog's lung. In the enlarged image it is clear that the blood is all contained within little tubes.

Malpighi thus becomes the first scientist to observe the capillaries, the tiny blood vessels in which blood circulates through flesh . They are so fine, and so numerous, that each of our bodies contains more than 100,000 kilometres of these microscopic ducts.

With their discovery, the missing link in Harvey's circulation of the blood has been found. For the capillaries are literally the link through which oxygen-rich blood from the arteries first delivers its energy to the cells of the body and then finds its way back to the veins to be returned to the heart.

Leeuwenhoek and the microscope: 1674-1683

Malpighi's pioneering work with the microscope is taken further by the Dutch researcher Anton van Leeuwenhoek. Teaching himself to grind lenses to a very high degree of accuracy and clarity (some of them providing a magnification of 300x), he uses a simple microscope with a single lens - in effect a tiny and extremely powerful magnifying glass.

With instruments of this kind he is able to observe phenomena previously too small to be seen. In 1674 he is the first scientist to give an accurate description of red blood corpuscles. In 1677 he observes and depicts spermatozoa in the semen of a dog. In 1683 he provides a drawing of animalculae (or bacteria) seen in saliva and dental plaque.

His discoveries, published for the most part in the Philosophical Transactions of the Royal Society in London (though he himself lives in Delft), vividly suggest the excitement of being the first to wander with such enlarged vision among the minutiae of the animal kingdom.

His account of the common flea follows its development from egg to the practical perfection of its adult anatomy. His researches demonstrate for the first time that the tiniest living things have a life cycle and generative systems like any larger creature.

Microscopic anatomy: 17th - 20th century

With Malphighi's discovery of the capillaries, the major anatomy of the human body is known. With Leeuwenhoek's meticulous study of previously invisible aspects of living material, the subject moves into a more arcane phase - that of microscopic anatomy.

The first great milestones on this new route occur in the 1830s.

Félix Dujardin in 1835 identifies a viscous translucent substance as being common to all forms of life; it is later given the name protoplasm. Meanwhile others observe that living material is organized in a repeated structural form. Robert Brown discovers in plants, in 1831, the nucleus at the centre of every cell. In 1839 Matthias Schleiden and Theodor Swann give the first coherent account of cell formation as the building process of all life (a theme long guessed at by others, but not resolved or demonstrated).

Yet further along this journey, deep into the centre of living matter, is the discovery in 1953 of the structure of DNA.

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