JIM McCaul remembers all their faces: the young boy who blew off his jaw playing with a gun; an elderly man sliced open with an angle-grinder while cutting a cat flap; a woman who suffered serious head injuries after being crushed beneath a car.

Faces ravaged by cancer, marred through violence and left unrecognisable after industrial accidents. Over the past two decades McCaul – a maxillofacial/head and neck consultant surgeon at the Queen Elizabeth University Hospital in Glasgow – has painstakingly put them back together again.

Unsurprisingly, his skills and expertise are much in demand. It's a baking hot July afternoon and McCaul has just wolfed down half a sandwich on the hoof as he rushes to meet me. He's been in clinic all morning and things overran. Now ensconced in his office, McCaul's brain is still whirring.

His words come out in a rapid-fire tumble as we settle down to chat about his memoir, Face to Face, which dissects his life and work. The pages conjure up powerful imagery which sears the mind's eye with stories of tragedy, loss, hope, life and death.

McCaul knew by his seventh birthday he wanted to become a surgeon. During his early childhood in Linwood, Renfrewshire, he witnessed the chronic pain that his polio-stricken mother suffered, recalling how she would wince in agony due to the constant burning ache in her leg.

In the evenings, he and his younger brother Vincent would take turns to massage her knee in the hope of alleviating some discomfort. "I remember thinking that if I learned how to be a surgeon, I would be able to make my mum's pain go away," says McCaul.

He's smiling yet there's a catch in his voice. "I did have this romantic notion as a wee boy of dark green cotton drapes and surgical gowns. I thought: 'What a good job that must be.' The idea of helping people with my hands is still appealing. It is extremely rewarding."

McCaul, now 49, studied dentistry and then medicine at Glasgow University – a maxillofacial surgeon must be qualified in both – before going on to do surgical training and hone his skills in hospitals across the west of Scotland, London and Florida.

It included a six-week stint at Jackson Memorial Hospital and Coral Gables Hospital, both in Miami, during the mid-1990s. Having grown up on the outskirts of Glasgow, McCaul was familiar with stories of gang violence. Although that largely involved knife crime. He was about to see up close the life-shattering impact of guns.

"I remember asking one of the ER doctors: 'How many gun-shot [incidents] did you have here last year?' He replied: 'Six, maybe seven?' I said: 'What? Is that all?' McCaul mimics doing an incredulous double take. "He then clarified: 'Here, in the department.'"

The rate of violent crime in Miami was almost three times the US national average. McCaul assisted on operations including a young boy who blew off his own jaw after accidentally firing a handgun and a woman shot in the back of the head as she fled a hold-up at a drive-through ATM.

"The gun crime was staggering," he says. "I dealt with cases when people were first brought in and others who were in various stages of reconstruction. The fresh cases depended on you being on-call for trauma and you couldn't do that every night of the week because it was exhausting."

McCaul doesn't mince his words when outlining the destruction that guns can cause. "It is hellish," he says. "In the movies, it is 'bang!' and you're dead. But the reality is it takes a while to die. People don't just fall down. They keep running for a while and then they slowly collapse.

"There isn't a big 'oof' when you are shot by a high velocity bullet. The momentum doesn't knock you on your back; it rips your insides. That is absolutely chilling."

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As a consultant surgeon McCaul spent eight years working at Bradford Teaching Hospitals in West Yorkshire before moving to the Royal Marsden Hospital and Northwick Park Hospital, both London, in 2014.

He returned to Glasgow last year to take up his current role at the Queen Elizabeth University Hospital. "I've had to undergo elocution lessons to get this Scottish accent back again," he quips.

McCaul's passion for his chosen vocation is palpable. In many ways, Face to Face is a love letter to both the fragility and stoic robustness of the human body, these stark opposites deftly juxtaposed.

"I still feel a frisson of excitement as I anticipate the familiar sight of the surgical blade parting the skin surface," he writes. "Exposing the yellow subcutaneous fat and, beneath it, the sheen of the silvery fascia that covers the red-brown muscles.

"The incision is as clean as a drawn line on paper for an instant, and then the bright crimson blood begins oozing from the severed edges and seeping into the surgical cut.

"At this point there is an almost overwhelming sense of entering into a sanctuary, where nothing can distract from the purpose at hand and there is total focus on the person lying in front of me. A hospital is the only building in the world where no one moves when the fire alarm goes off."

The book contains no photographs, but as we sit in his office, McCaul offers to show me some of the cases he has worked on over the years. "There is a lot of red," he warns beforehand.

We flick through the images on his laptop: a chainsaw used injudiciously; a failed suicide attempt with a shotgun; the aftermath of a heated family dispute ("That is a kitchen knife right into the eye socket and out of the inferior orbital fissure," says McCaul).

One photograph shows a farm worker whose face had been smashed with such force by an out-of-control, high-pressure slurry hose that it pushed the right eye from its socket, leaving it resting on the cheek below.

Another man had a bullet lodged near the top corner of his skull. McCaul points towards the frontal lobe area on the scan. "He looks OK, but there is scrambled brain in there," he says.

Around 80 per cent of McCaul's work involves treating patients with tumours of the face, mouth or neck. Aggressive cancers can pulverise jawbones, decimate tongues and throats, leaving those affected with scar-ridden, often unrecognisable appearances.

It is McCaul's job to reverse or repair this damage. His main method involves what is called a "free flap transfer" where a piece of human tissue – skin, flesh, muscle or bone – is cut from one site on the body and transplanted to another to remedy a trauma, excised tumour or congenital defect.

Bone from the shoulder blade, hip or fibula in the lower leg can be used to reconstruct the jaw, while skin and soft tissue from the arm or thigh can form a new tongue.

His work, like colleagues in fields such as cardiothoracic, neurosurgery and orthopaedics, is vital and often life-saving, yet McCaul professes to feeling an additional burden of responsibility.

So much of our sense of self, he says, is based on what we look like and how others see us. We are hardwired to appreciate and judge facial appearance; it is pre-programmed into the human brain.

"Walk into any cafe, restaurant or pub, you can scan the room and straightaway are able to spot the 'attractive' and 'unattractive' faces," says McCaul. "That is because evolution has programmed us to read physical differences like barcodes."

When a face has been altered by cancer, an accident, a fall, a beating, a car crash or gunshot wound, it can provoke horrified reactions from loved ones and strangers alike.

"If you had bowel, breast or other types of cancer, you can hide the scars to an extent, but with your face, that is less easy," he says. "Maintaining and restoring that again is about a whole sense of self. You can be giving people their life back."

The concept of identity is hugely interesting, asserts McCaul. "Quite often the baddie in a film has some sort of facial disfigurement," he says. "I'm currently reading Munich by Robert Harris. The Gestapo guy in that has a pockmarked face."

He talks about reconstructing a jaw using the iliac crest – a section of bone on the outside of the pelvis – and repairing damage to a throat using a flap from the stomach – the rectus abdominis muscle – which sees "a six-pack become a three-pack".

His easy manner belies the complexity of many of the surgical procedures McCaul undertakes, operations averaging between eight and 12 hours – sometimes double that.

He admits to having testing days. "I had a patient with a reconstruction for advanced cancer who bled from his carotid artery," says McCaul. "I was sitting at home at 8.11pm and by 8.23pm I had screeched my car outside on the pavement and ran into the ward.

"I arrived in my shorts because it was a sunny day and as they were wheeling him out with blood everywhere, one of the staff said: 'Sorry sir, you can't be in here, we've got an emergency' and I replied: 'Actually, he's my patient, so you do need me …'

"It took until 2am but he survived. That was the Sunday. On the Saturday I had been in theatre with two patients both with complex reconstructions."

Not all his stories have a happy ending. Among those McCaul reflects on in his memoir is Kathleen, a woman with oral cancer who faced an unenviable decision: do nothing and be slowly suffocated as the tumour in her mouth grew or go ahead with a surgery she may not recover from.

Kathleen decided to have the operation, but the odds were not in her favour. After six weeks of continuing ventilator support in intensive care following the procedure, she requested to be released from hospital to die at home and passed away later that same afternoon.

"There are patients that really stay with me," he says. "That one was tough. Knowing she was going to die. Those things are etched. I remember them all.

"You can get massive textbooks like those" – McCaul points to a stack of tomes on a nearby cabinet – "and read them from cover to cover, but when it is a person in front of you, it seems to go in a different way and stay there indelibly."

To that end, says McCaul, it is about more than simply knowing where to cut with a scalpel. "There is a saying in our world that good surgeons know how to operate, better ones when to operate, and the best when not to operate at all," he says.

There are light-hearted moments too. Gallows humour some might call it. Like when McCaul recalls Larry, a man with Parkinson's disease whose face was sliced right down the middle with an angle-grinder while cutting a cat flap.

The operation to reconnect the two halves of Larry's face, says McCaul, was one of the most successful he had performed. So, some weeks later, he was surprised to see a glum-looking Larry arrive in his consulting rooms.

Immediately concerned, McCaul asked him what was wrong, to which the bold Larry deadpanned: "Well, my kids have taken my angle-grinder off me …"

Throughout our conversation, McCaul speaks highly of the team around him: consultants, surgical fellows, scrub nurses and anaesthetists to name but a few. "Make sure you mention that," he pleads. "I don't want people thinking I see myself as The Big I Am."

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The centenary of the Somme in 2016 was the catalyst for the book and in it McCaul pays tribute to the work of his hero, Sir Harold Delf Gillies, a New Zealand-born surgeon and pioneer of facial reconstruction who transformed the lives of thousands of soldiers disfigured during the First World War.

"That was the birth of my speciality," says McCaul. "The soldiers wore helmets and because of that they started coming home with horrific facial trauma because the brain survived it. The helmets were quite flimsy – I have two at home – but they would stop a rifle bullet.

"Gillies had produced luggage labels that said: 'Please send to Queen Mary's Hospital, Sidcup' and gave them to nurses going to the front. He made 200 labels but in the end 2,000 soldiers came back from the Somme and were directed to him for help. They were shockingly young these men."

Like Gillies, McCaul is a trailblazer in his field. To date, he has authored 57 scientific papers and is currently spearheading an international clinical trial that he hopes will lead to the early detection of face, mouth and throat cancers.

"Every one of the 37.2 trillion cells in your body should store glycogen, a form of carbohydrate," he explains. "If you apply potassium iodide to that you get what are called tri-iodate molecules forming which are chocolatey brown.

"And if you don't get the chocolatey brown stain, then something is going wrong. Tumour cells use energy differently. That stain – or lack of it – will show us where there is glycogen missing."

The Queen Elizabeth University Hospital is the highest incidence site for facial, head and neck cancer in the UK. "There is so much head and neck cancer in Scotland," he says. "The big thing is finding out why people are getting it."

McCaul highlights immunotherapy and cancer vaccines as two areas he would be keen to see advance and develop in the coming years.

Examining factors such as diet or proximity to heavy industry could be key, he suggests. "Allostatic load is the term that has been coined. That is where the environment impacts on your ability to survive.

"We have pathogens to find. We know the human papillomavirus (HPV) is responsible for a lot of the increase in throat cancer but of the extra cases we are seeing – it is fastest increasing cancer in Western Europe and America – only half have the HPV virus, the other half, we don't know why."

McCaul gives short shrift to those who turn their nose up at cosmetic surgery. "Often with a facelift it is not vanity – it is dignity," he says. "I can do a facelift after someone has cancer surgery to help them look more normal and human."

Full-face transplants are still relatively in their infancy and as yet none have been carried out in the UK. There had been hopes that Glasgow could become a major centre for the procedure, but at present those aspirations appear to have been placed on hold.

McCaul is sanguine when asked if it is a surgery he would like to tackle should the opportunity – a suitable patient and donor – arise. "It is daunting. But the technical aspects of that are not as challenging as people might think compared to some of the operations we have talked about."

Away from work, family life is his anchor. McCaul is married to Lorna, a consultant and specialist in restorative dentistry and oral rehabilitation, and the couple have two children James, 23, and Katherine, 20.

Music is a life-long love. McCaul was a talented flautist from a young age and won a place in the orchestra at the Royal Scottish Academy of Music and Drama in Glasgow, now the Royal Conservatoire of Scotland, as a teenager.

He stills plays the flute – along with the piano – regularly and credits it with helping his manual dexterity. "It is almost like it sharpens up the connections between the motor cortex and your fingertips," says McCaul.

One of the best parts of the job, he says, is making the telephone call to a patient's next-of-kin to tell them that surgery has gone successfully. The worst is when he must stand by and do nothing.

"I do a lot of cancer work and one of the biggest challenges is when you have someone who might not be quite fit enough to go through the whole procedure," says McCaul. "That can be an awfully hard decision to make."

Face to Face: True Stories of Life, Death and Transformation from My Career as a Facial Surgeon by Jim McCaul is published by Bantam Press, priced £16.99