Don't worry: that vertical cut was made after the subject died. In fact, it's an incredibly rare example of vertical skull dissection, most likely performed at the end of the 1800s as a teaching aid.
The skull was discovered by Jenna Dittmar from the University of Cambridge in the UK. Found in a dusty part of the University's collection, analysis of the cut reveals that it was done with reasonable precision. Unlike some of the samples that Dittmar has studied—such as samples from the 1700s, where she noticed medical practitioners had "sawed off the top of a skull horizontally, like a boiled egg," reports New Scientist. Nice. [New Scientist]
Image by Jenna Dittmar