Recto: Notes on floating, and the action of water on the earth.

Verso: Studies of the thorax, trunk & diaphragm of an ox, with notes on the drawings and on the action of water; a slight sketch of architecture. Leonardo compiled these drawings and notes from two different directions, such that those on the right of the sheet appear upside-down with respect to those on the left.

The largest study at centre left shows the abdominal cavity of the ox, with the longissimus dorsi muscles – the principal component of a rib-eye steak – prominent on either side of the spine. The drawing at lower left shows the inside of the thoracic cage of the ox (as indicated by the single word bo above the drawing). A detail of this region is enlarged to upper right: the zig-zag shape coloured with red chalk is a portion of the transversus thoracis muscle, flanked by ‘a vessel directed between the muscles of the diaphragm and the middle of the diaphragm’ and ‘a vessel leading outside the muscles of the diaphragm’.

At bottom left Leonardo describes the diaphragm accurately as "a thick sinewy membrane which is surrounded by muscles, its extensors. And this diaphragm with its muscles is covered by a dense membrane composed of very fine ones. Outside it is clothed by a hard membrane [the pleura] lining the diaphragm and ribs."

Accordingly, the diagram at upper centre shows the membranes of the diaphragm peeled apart.
 
Text from M. Clayton and R. Philo, Leonardo da Vinci: Anatomist, London 2012.