Leonardo da Vinci recorded mechanical designs, anatomy, optics, and observations of nature in his notebooks. Painting was only one of his activities, and many designs remained sketches.

Mirror writing appears frequently in his notes. The drawings reveal an exploration of ideas, but they do not establish that every device was actually built.

Anatomy and physiology

Leonardo started his study of the anatomy of the human body under the apprenticeship of Verrocchio, who demanded that his students develop a deep knowledge of the subject. As an artist, he quickly became master of topographic anatomy, drawing many studies of muscles, tendons and other visible anatomical features.

As a successful artist, Leonardo was given permission to dissect human corpses at the Hospital of Santa Maria Nuova in Florence and later at hospitals in Milan and Rome. From 1510 to 1511 he collaborated in his studies with the doctor Marcantonio della Torre, professor of Anatomy at the University of Pavia. Leonardo made more than 240 detailed drawings and wrote about 13,000 words toward a treatise on anatomy. Only a small amount of the material on anatomy was published in Leonardo's Treatise on Painting. During the time that Melzi was ordering the material into chapters for publication, they were examined by anatomists and artists, including Vasari, Cellini and Albrecht Dürer, who made drawings from them.

Leonardo's anatomical drawings include many studies of the human skeleton, its parts, and the muscles and sinews. He studied the mechanical functions of the skeleton and the muscular forces that are applied to it in a manner that prefigured the modern science of biomechanics. He drew the heart and vascular system, the sex organs and other internal organs, making one of the first scientific drawings of a foetus in utero. The drawings and notation are far ahead of their time, and if published would undoubtedly have made a major contribution to medical science.

Leonardo also closely observed and recorded the effects of age and of human emotion on the physiology, studying in particular the effects of age. He drew many figures who had significant facial deformities or signs of illness. Leonardo also studied and drew the anatomy of many animals, dissecting cows, birds, monkeys, bears, and frogs, and comparing in his drawings their anatomical structure with that of humans. He also made studies of horses.

Leonardo's dissections and documentation of muscles, nerves, and vessels helped to describe the physiology and mechanics of movement. He attempted to identify the source of 'emotions' and their expression. He found it difficult to incorporate the prevailing system and theories of bodily humours, but eventually he abandoned these physiological explanations of bodily functions. He made the observations that humours were not located in cerebral spaces or ventricles. He documented that the humours were not contained in the heart or the liver, and that it was the heart that defined the circulatory system. He was the first to define atherosclerosis and liver cirrhosis. He created models of the cerebral ventricles with the use of melted wax and constructed a glass aorta to observe the circulation of blood through the aortic valve by using water and grass seed to watch flow patterns.

Engineering and inventions

During his lifetime, Leonardo was also valued as an engineer. With the same rational and analytical approach that moved him to represent the human body and to investigate anatomy, Leonardo studied and designed many machines and devices. He drew their "anatomy" with unparalleled mastery, producing the first form of the modern technical drawing, including a perfected "exploded view" technique, to represent internal components. Those studies and projects collected in his codices fill more than 5,000 pages. In a letter of 1482 to the lord of Milan Ludovico il Moro, he wrote that he could create all sorts of machines both for the protection of a city and for siege. When he fled from Milan to Venice in 1499, he found employment as an engineer and devised a system of moveable barricades to protect the city from attack.