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

Recto: eight drawings of the skeleton of the arm, shown in various states of flexion, and supine and prone positions of the hands; two small studies of the muscles of a right shoulder; a study of the skeleton of the pelvis and legs, seen from behind; notes on the drawings.

Most of the drawings are concerned with pronation and supination (turning the hand) when the arm is flexed or extended at the elbow, and demonstrate how this occurs without rotation of the humerus. There is no verbal explanation of the action (the role of biceps brachii was treated fully on RCIN 919000v) and the exceptionally beautiful drawings are left to speak for themselves.

An ancillary theme is the apparent change in the length of the arm during flexion and extension. In particular, Leonardo observed – as shown in the drawings at centre left – that during flexion, the olecranon process of the ulna (at the lowest point of the drawing to the left of centre) emerges from the olecranon fossa of the humerus to form the point of the elbow, so increasing the apparent length of the upper arm.

In the notes at lower right, Leonardo queries the purpose of the prominence f, about half-way down the humerus. He makes a note to ‘see what purpose the gibbosity of the arm at f serves’, followed by ‘I have looked at it, and find that the gibbosity f serves as the attachment of the muscle that raises the humerus’. This is the deltoid tuberosity, where the deltoid muscle is attached to the humerus.

In several of the drawings there is a jagged appearance to a portion of the inner margins of the ulna and radius. This may be the result of a healed bad break of both bones, or of ossification or calcification of the interosseous membrane that joined the two bones, due to ageing, diet or hard labour. Leonardo did not realise that these gobbi (‘bumps’) were not design features, and with faint pen lines at centre left he posited that they might be the sites of insertion of muscles of the upper arm.The drawing at the centre of the sheet examines the lower spine, sacrum, pelvic girdle and lower limb. The number of foramina (holes) in the sacrum is correct, and the pelvic girdle is shown properly articulated and tilted – even Vesalius failed to capture this. The small note between the femurs states that during childbirth the coccyx moves, and indeed it is pressed backwards by between 2.5 and 3.5 cm (1–1½”) to allow a significant increase in the size of the birth canal.

On the verso: two studies of an old man in profile, both showing the surface muscles of the neck and upper arm; two studies of the right shoulder and arm.

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