One of the greatest challenges for Leonardo was understanding how the body functioned as a coherent entity.

According to medieval tradition, the brain contains three chambers that house the intellectual and imaginative faculties, the sensus communis and the memory. In addition to their receptive and analytical functions, these also perform the vital role of transmitting the commands for human motion and emotion. The sensus communis was supposed to be the site on which all of the nerves converged, and from which all voluntary action was controlled - everything from running, walking, or lifting an arm to small details of expression like smiling, or raising an eyebrow.

This is one of a series of studies of the human skull by Leonardo. The drawings are famous for their anatomical accuracy and the virtuosity of the pen and ink technique employed.

According to Leonardo, the sensus communis was also the locus of the human soul. Here, he plots the site of the sensus communis at the intersection of upright and diagonal lines seen within the tilted plane, at a point that marks the proportional centre of the skull.

In Leonardo's words
The soul seems to reside in the judgement, and the judgement would seem to be seated in that part where all the senses meet, called the sensus communis, and is not all-pervading throughout the body, as many have thought. Rather it is entirely in one part.