In 1776, Philadelphia-based surgeon John Jones published Plain concise practical remarks, on the treatment of wounds and fractures, specifically intended to aid "young military and naval surgeons, in North-America." You can find the book here. In his eight chapter (starting on page 69 of the text), Jones addressed wounds to the head, a "difficult and dangerous" branch of surgery. For skull fractures - a type of injury it could take days to detect due to the slow development of symptoms - Jones outlined the process of trepanning in chapter 10 (page 80). This procedure entailed drilling a hole in the skull to remove broken bone chips and cut a drainage hole in the dura matter, the "bag" the brain sits in. This drain would allow the brain to swell without a fatal increase in pressure from the cerebral-spinal fluid trapped between the skull and the expanding brain. There are numerous surviving examples of trephines from the period, and, I gather, skulls with both healed and unhealed treppaning holes.

While this was a difficult and risky surgery, it was still considered worth doing. Jones addressed some cases where surgeons could be too eager to start drilling. As such, a skull fracture was not viewed as a guaranteed death sentence. John can save his affections for the plague rat for another day.