Recto: The viscera of a horse. Verso: The hemisection of a man and woman in the act of coition

Recto: an anterior view of the arteries, veins and the genito-urinary system of an animal, probably a horse; a spinal column with notes on dissection. Verso: a drawing of the coition of a hemisected man and woman; a drawing of the left side of a youth, showing the alimentary tract; two sections of the penis and a male torso squared for enlargement; details of genitalia and nerves based on Galenic theories. Leonardo’s forerunners differed on whether conception involved a material union, a spiritual union, or both. Plato, for example, believed that the ‘seed’ was a spiritual entity in the brain and spinal cord, and here Leonardo notes that ‘Avicenna [the eleventh-century Persian philosopher Ibn Sına] claims that the soul begets the soul, and the body the body’. But here Leonardo seems to depict an arrangement whereby three components are involved in conception, following the medieval division of the body: in the man he has drawn channels into the penis from the brain via the spinal cord (to transmit an ‘animal’ element, or soul), from the heart (a ‘spiritual’ element), and from the testes (a ‘material’ element). In the woman, the spine bifurcates and a branch of the spinal cord passes directly into the uterus, to transmit the woman’s soul to the child, but the ovaries and heart are not drawn, and it is thus questionable whether, at this stage of his career, Leonardo believed that there was an equal contribution to conception from the female. In the late 1480s Leonardo proposed a treatise on human anatomy. The human body was of course the principal subject matter of the Renaissance artist, and Leonardo quickly realised that it was far too complex for a mere chapter of his treatise on painting. In time anatomy would become Leonardo’s greatest scientific pursuit, and though he never completed his treatise, his later studies mark him out as one of the great scientists of the Renaissance. Text adapted from Leonardo da Vinci: A life in drawing, London, 2018