to the high mortality rate. It was used only for some interventions, such as the treatment of hemorrhages, depressed fractures and penetrating the head. Also the name for the surgery changed from trepanning to craniotomy. In the late 1860s, the archaeologist E.G. Squier discovered a skull in an ancient Inca cemetery. This specific skull was anticipated to be of pre-Columbian era. The skull exhibited a large rectangle-shaped hole on the top. The skull was brought it back to the United States, and his findings presented to the New York Academy of Medicine. Squier argued that the brain was injected with a tool called a burin which was used on woods and metals before. Traces showed human hand prints. He concluded that the skull and brain evidenced recovery from prehistoric brain surgery, potentially prolonging the patients life. Metallurgy was a technique that allowed the use of saws and scalpels. Other cultures came about experimenting through the usage of glass. Application A cranial drill is currently used for neurosurgery operations. The procedure of trepanning is applied to patients who suffer for example a traumatic brain injury or a stroke. In these cases, it might be necessary to drill a hole in the skull to be able to access the dura mater or the brain itself, and to relieve brain pressure or blood clots. With the use of modern types of cranial drills, surgeons are able to create holes in the bone structure without traumatizing underlying brain tissue. The drill's working tool tip consists of a spiral blade that is framed by a guard device with an angled cranium guide that rests against the inner layer of the skull bone. The dura guard pushes the dura mater downward while the craniotome is moved forward thus preventing dural tearing. Types and design A cranial drill is an essential instrument used by surgeons to drill in to the skull bone. Various types of drills are used by surgeons from the craniotomy, or oral surgeries. The cranial drill can be differentiated by the examinations of what kind of surgery have to be performed. They can be manually operated, operated by electricity, or by pneumatic motors. Manual cranial drill The rotating crank is typically connected to several cogs that sets pressure on the skull. This specific drill is not connected to any external power, and is used very little in today's operations. The manual cranial drill is