A folio from Leonardo's 'Anatomical Manuscript B'.

Recto: two studies of a right leg with the knee flexed; with notes, those above being a list of anatomical propositions, the others on movement as exemplified in the drawings.

Leonardo has removed most of the muscles of the right thigh, in particular the quadriceps femoris complex, to reveal the femur and display the position and action of sartorius (running diagonally across the front of the thigh; cf. RCIN 919035), semitendinosus (behind and to the right of sartorius as we look at the drawings) and tensor fasciae latae (on the left edge of each drawing). The muscles of the calf, gastrocnemius and soleus, are depicted in the study to the left; these are removed in the second drawing to show the tibia and fibula, and the thigh muscles are reduced in thickness to demonstrate their origins and insertions even more clearly.

The sartorius muscle is partly responsible for rotating the leg medially (moving the knee outwards and the foot inwards) when the knee is flexed – it cannot do that when the knee is extended and locked into full stance – but none of the notes explore this action explicitly. Instead, rather puzzlingly, Leonardo seems to suggest that he has here depicted all the muscles responsible for upward movement of the thigh and flexion of the knee:

"In this demonstration made from different aspects, account is taken of all the muscles which move the leg; which muscles are attached to the lip of the pelvis [iliac crest] from which also arise the muscles which move the thigh from the knee upwards; and also of those which bend the knee when one kneels."

The five lines at the head of the page were in fact written during the earlier period of compilation of the notebook, c.1489, and list possible subjects to be investigated in Leonardo’s proposed treatise(cf. cat. 919059):

The ramification of the veins of the shoulders upwards, and from the spleen to the lung.
The ramification of the nerves and of the reversive nerves to the heart.
Of the shape and position of the intestines.
Where the umbilicus is attached.
Of the muscles of the body and of the loins.

Verso: further notes on the scheme of the book, to treat of prenatal life, growth in childhood, man, woman, their proportions, expressions, attitudes and senses.

Text adapted from M. Clayton and R. Philo, Leonardo da Vinci: Anatomist, London 2012.