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

Recto: a study of a torso of a man, seen from behind, with the muscles strongly marked; on the left is a similar study, but only the right half of the body and the right arm are shown; notes on the drawings.

Leonardo deliberately exaggerated the muscles of the back and arm – not just by using pen and ink alone (with no wash) to model the muscles, but also by drawing lines around some of the muscles in a darker ink. Most of the major muscle groups of the back and posterior surface of the arm are identifiable, though the shapes of many are unusual, suggesting that while Leonardo was generally familiar with the arrangement of the muscles, from a combination of surface observation and dissection, he remained uncertain about their form and function.

Trapezius is shown as at least two distinct muscles, either side of and below the vertebra prominens at the base of the neck. Latissimus dorsi, in the mid-region of the back, is rendered as a succession of bulges that do not correspond with the direction of action of that muscle; deltoid is likewise divided into so many bulges that it is impossible to tell where it ends and where infraspinatus starts. In the study to the left, biceps, brachialis and the long extensors of the wrist and hand are clearly defined, and an outcropping muscle of the thumb is indicated on the upper border of the forearm, by the wrist.

The three blocks of notes are programmatic: Leonardo states his intention to analyse every muscle of the body, starting with the spine and gradually ‘clothing’ it with the muscles before adding the nerves, vessels and viscera. He also planned to analyse differences in muscle shape for different body types:

The most prominent parts of thin people are [also] more prominent in muscular people, and likewise in fat people. But the difference which exists between the shape of the muscles in fat people in comparison with those who are muscular will be described below.

Verso: three studies of the neck and thorax, showing the ribs, the internal intercostal muscles in one, and the external intercostals in two; with notes on the drawings.