Leonardo can be regarded as the founder of anatomy. He studied the structure of many organs in the human body at different times. He began with the skull because he regarded it as the meeting-point of all the senses and the seat of the soul; he then went on to the skeleton and bones as being the supporting elements of the human form, continued with the stomach and the intestines, and finally, the lungs, kidneys and genitals of both sexes. Almost all of his anatomical studies are kept in the Windsor Collection. Leonardo revolutionised the technique of anatomical illustration and his output is extraordinary. The innovations that he made consisted of accompanying his drawings with descriptions, drawing the same part from a number of viewpoints to make a complete three-dimensional image, and using cross-sections to analyse the interior of the organs.
His descriptive anatomy, however, was unknown for a long time and it was the work of Vesalio in 1543 that was regarded as revolutionary. The earliest trace of Leonardo's interest in anatomy is in his painting of St Jerome, which dates from his youth. The neck and shoulders already reveal some knowledge of muscle anatomy. While contemporary artists, such as Michelangelo, confined their studies to superficial anatomy, Leonardo extended his research to the innermost parts of the body. His studies of a horse's entrails and the spinal chord of a frog prove that in this phase he was making use of animal dissection. In one series of drawings, he analysed the internal dimensions of the skull. He also sought to locate the psychic faculties inside three circular intra-cerebral cavities. Leonardo espoused the traditional view of nerves as tubes through which the flow of air caused the muscles to contract by inflation. Leonardo resumed his anatomical studies in about 1510, after a break of nearly a decade. His studies in mechanics influenced his later anatomical work.
He now analysed the body's articulations as semi-articulated joints governed by the laws of the lever. Routine practice of dissection exposed him to the awesome complexity of anatomical data. He was convinced that every anatomical structure has a precise function. No detail, therefore, was to be overlooked in the visual representation. He employed innovative graphic devices already used for machines, such as "see-through" images, exploded views, drawings of the body from different vantage points, and the depiction of muscles as lines of force.
He was led by his wish to demonstrate the very close analogy between machines and the human body, both wonderful works of nature, whose rigid laws regulate not only mechanical tools but the movements of animals: "Nature," Leonardo insisted, "cannot give animals movement without mechanical tools." It is not surprising, therefore, that his anatomical research concentrated on the basic organs of the great mechanism of the body, and his drawings of these obsessively emphasise the direct analogy with mechanical devices, to the extent of suggesting that it would be possible to construct perfectly functioning artificial limbs and models.