All the drawings deal with the muscles of the right shoulder, with the chest muscle, pectoralis major, split into four separate muscle bodies. At top right is one of Leonardo’s ‘thread diagrams’, in which the muscles are reduced to threads along their lines of force, conveying the complex three-dimensional structure of the shoulder in a single depiction. Most of the drawings are based on study of a live subject, but display Leonardo’s knowledge of the underlying structures obtained by dissection. Leonardo was particularly interested here in the muscles running up the neck from the clavicle (collarbone) to the head, and in the form of the chest muscle, pectoralis major, which he habitually depicted as if divided into separate segments.