It is specifically a brain dissection being carried out in this artwork and seems ideally suited to the artist's method of darkening almost all of a scene, other than the key focal point. In this case your eyes are drawn to the victim's skull as he lies motionless.
As Deijman completes the work, his assistant Gijsbert Calkonen stands to the left, holding the section of skull that had already been removed. The subject's feet are placed close to the viewer, making us feel closer to the activity.
Rembrandt's skills in depicting human flesh and faces more generally helps to hover the feet tantalisingly at us, helping us to imagine the sight and sounds of this morbid scene.
A further element of darkness to the atmosphere is that the subject lying down, a tailor by the name of Joris 'Black Jack' Fonteijn, was actually executed just several days prior. His permission for this anatomical examination was likely never requested nor given and the body of a convicted criminal was considered fair game for all manner of activities.
Very few famous artists would make use of such morbid scenes as these for their work but Rembrandt van Rijn himself also brought us The Anatomy Lesson of Dr Nicolaes Tulp which is considered one of his most famous paintings and is also featured in our Rembrandt paintings section.