At the time of the accident, one of Gage's duties was to set explosive charges to remove unwanted sections of large rocks. Typically, a long, narrow hole was drilled into the rock which was then filled with gunpowder and ignited. Before lighting the fuse, the hole was topped off with sand, and a three-foot-long, 1.25" diameter iron tamping rod was used to pack down the gunpowder. However on 13 September 1848, Gage was distracted momentarily while in the process of preparing a blast, and he neglected to add the protective barrier of sand. When he thrust the iron tamper into the hole in the rock, it created a spark, and the gunpowder was ignited.
The resulting explosion propelled the fourteen pound iron rod straight into the air with the force of a cannon, causing it to pass through Gage's skull in the process. It entered through the bottom of his left cheekbone and exited through the top of his head, then continued to fly in an arc across the sky, landing almost 100 feet behind him.
The unscheduled explosion got the attention of his fellow railroad workers, who rushed over to see if there was a problem. What they found was Phineas Gage slumped on the ground with a hole through his skull. Amazingly, the man was still alive and breathing. Even more amazingly, within moments his eyes were open and he was speaking to his fellow workers. The injured Gage was quickly loaded into a cart, and transported back to his boarding house, some 45 minutes away.
When Dr. John Martyn Harlow arrived, Phineas was conscious and had a regular heartbeat, and both of his pupils reacted to light normally. He was reported to be "in full possession of his reason, and free from pain." He was under the care of Dr. Harlow for ten weeks, at which point he was sent home to Lebanon, New Hampshire. But while he was recovering, the doctor noted some changes in the man's demeanor and personality. People who had known him before the accident described him as hard-working, responsible, and popular with his workers, but after the traumatic injury, Phineas Gage was not the same man.
In regards to his patient, Dr. Harlow wrote:
Gage was fitful, irreverent, indulging at times in the grossest profanity (which was not previously his custom), manifesting but little deference for his fellows, impatient of restraint or advice when it conflicts with his desires, at times pertinaciously obstinate, yet capricious and vacillating, devising many plans of future operations, which are no sooner arranged than they are abandoned in turn for others appearing more feasible. A child in his intellectual capacity and manifestations, he has the animal passions of a strong man. Previous to his injury, although untrained in the schools, he possessed a well-balanced mind, and was looked upon by those who knew him as a shrewd, smart businessman, very energetic and persistent in executing all his plans of operation. In this regard his mind was radically changed, so decidedly that his friends and acquaintances said he was 'no longer Gage'.
Several months after the accident Gage felt strong enough to return to work, yet due to his personality changes, his previous employers would not entrust him with the foreman position he had previously held. In the following years, he took various jobs caring for horses, driving stagecoaches, and doing some farm work. He also briefly appeared at a museum in New York which was curated by the infamous P. T. Barnum, alongside the tamping iron which had impaled his brain.
Not much is known about his years after the injury, but eleven years after the accident, when he was aged thirty-seven years, Gage began to experience epileptic seizures. He died several months later, on 21 May 1860. His brain was not subjected to any medical examination at that time, but seven years later his body was exhumed so that his skull might be studied. It has since been subjected to much scrutiny.
There is still some controversy over the extent of damage to Phineas' brain. It is certain that it passed through the anterior frontal cortex and white matter, but it has not been determined with certainty whether the lesion involved both frontal lobes or was limited only to the left side. In any case, the damage caused by the accident was roughly equivalent to a frontal lobotomy.
Today, Gage's skull and the tamping rod which damaged it are on permanent display at Harvard’s Countway Library of Medicine. The incident did much to advance the field of neurology, as it was among the first evidence suggesting that damage to the frontal lobes could alter aspects of personality and affect social skills. Before Gage's brain injury, the frontal lobes were largely thought to have little role in behavior.