'How many do you have?' Helena despised family conversations, but any minute now the shameless girl would be asking if he carried cameo portraits with him.

'Fifteen.' Either his wife, or more likely a succession of wives, really enjoyed being pregnant, or his pharmacopoeia didn't mention the benefits of alum wax when making love.

'I have heard that we could have Julia's tonsils removed,' Helena said, frowning at the thought. 'Madam, don't touch them!' Aedemon exclaimed at once. He sounded highly alarmed. He did not expand on the warning. Helena recoiled from his outburst and we were all silent for a while. The carriage was dawdling, stuck behind a heavy wagon that lumbered through the countryside about as fast as a snail who had spied his lunch ten yards ahead. The snail may have spotted the lettuce, but he wasn't very hungry yet and was gawping at the scenery. When the chill in conversation passed, I asked whether Aedemon had been at the Quadrumatus house when Scaeva died. He said not but I sought his opinion on the manner of death.

'I welcome expert comment, Aedemon. We don't get many severed heads in domestic murders. The only one I've seen personally was the victim of a serial killer, and she had been dismembered after death, specifically for disposal. Generally in violent death, if a quarrel flares unexpectedly women are battered by husbands and boyfriends, probably with bare fists or kitchen implements; men are attacked by fuends and workmates with fists, hammers and other tools, or personal knives. If loathing has brewed over a long period in the home, the method of choice tends to be poison. The wildly insane do run amok with specially obtained knives or swords, but they stab with them. And their victims are usually strangers in the street.' Aedemon was nodding. 'Is decapitation an easy way to kill someone?' 'No. A fit young man would hardly just stand there and let you hack off his head.' 'He would resist. Of course he would.' 'Violently – and there would be signs of his resistance on his body, Falco.' 'Were there any such signs with Scaeva, do you know?' 'No.' As Helena and I looked surprised, Aedemon explained that although he had not been in the house when Scaeva died, the family's doctors were sent for soon afterwards to give calming draughts – or whatever palliative they favoured – to the hysterical relatives. Poppy worked quickest, Aedemon said, though Drusilla Gratiana had been soothed with hemp by Cleander, who always had to be different. I said I preferred a stiff drink after a bad shock; Aedemon let his guard down and confessed that Drusilla consumed so much wine on a daily basis, it had little effect on her medicinally. 'Then all of us took a look at the corpse – curiosity, I'm afraid.' He was not really apologetic; in fact he looked gleeful. Doctors have their own arrogance. 'The death was, as you say, so unusual.'

'Quite.' I was still intrigued by how it happened. 'And puzzling. If you're the killer, you can't just walk up to Gratianus Scaeva while he's lounging on a couch and calmly saw through his neck. You'd have to find him asleep or unconscious – and even then you'd need to be damned quick.' 'Surely you would need to know what you were doing, too?' Helena added, wincing. I reinforced it. 'And bring a very sharp blade for the task?' 'Extremely sharp…' Aedemon confirmed. 'Surgically sharp, perhaps?' Helena asked. Professional caution set in fast: Aedemon pulled a face and shrugged. His mighty shoulders rose, the back of the carriage bowed outwards as he moved, then he slumped down into his rolls of fat again, to the relief of the carriage frame. The shrug was eloquent – but gurning and shrugging won't stand up in court.

'Luckily for Mastarna, he never saw his patient that day.' Watching Aedemon adopt his noncommittal face, I said, 'Or that's what he told me.' The lack of comment from Mastarna's rotund colleague continued. 'Was he summoned with the rest of you?' Aedemon looked vague. 'I believe he must have been. I certainly saw him there when we all gathered…' 'Even though his patient was dead?' I demanded scornfully. 'Somebody had a high opinion of his regenerative powers!'

'Well, none of us thought he could sew the head back on to Scaeva. I dare say, the slaves were just told to fetch all the doctors quickly. But Mastarna would have to be told what had happened.'

'And that he had lost his income?' Helena dug me in the ribs. 'So what do you think of Mastarna, Aedemon?' 'A sound physician.' 'You doctors all say that about each other. Even when you're diametrical opposites in your treatments.' 'The truth. Mastarna does good work. Different patients need different cures; different people suit different specialists.' 'And what's his practice? He's Etruscan. So is that magic and herbs?'

Apparently there is a clause in the Hippocratic Oath that says no doctor shall ever criticise another. Aedemon fired up immediately: 'Oh I think Mastarna is more modern than that! Etruscan medicine of course has a long history. It may have begun with religious healing, and that in turn may have meant herb- and root-gathering, perhaps by moonlight in order to find the plants. One should never decry folk medicine; there is a lot of sense to it.'

'It certainly helps Mastarna gather in the denarii – have you seen his house?' I jibed.

A sub-clause in the Oath says that any doctor who thinks a competitor is making more money than he does, can insult him after all: 'Patients can be very gullible!' After this flash of jealousy, Aedemon recovered smoothly: 'I would classify our friend Mastarna as fascinated by theory. His school tends to diagnose using the general history of disease -' 'He's a dogmatist?' Helena asked. Aedemon put his index fingers together and surveyed her over them as if he felt it was unhealthy for a woman to use words of more than two syllables. 'I believe so.' Since Helena was familiar with the medical schisms, he then acknowledged: 'And I am an empiricist. Our philosophical rule is, if I may say so, taking over public confidence nowadays. For very good reasons.' That was good news for laxative sellers. I wondered if the laxative market was sponsoring the empiricist school, paying salaries for empiricist teachers and handing out free samples… 'I prefer to study the patient's particular symptoms, then to base my recommendations on his history, my experience and, where appropriate, analogy with similar cases.'

To me, this did not sound too different from Mastarna's approach. But Helena saw distinctions: 'You concentrate on anatomical congestion and look to recent advances in pharmacology for treatment; he would be more likely to suggest surgery?' Aedemon looked startled. She carried on as if unaware he was impressed, 'I'm afraid I did upset him very much by suggesting that dogmatists approve of dissection of dead bodies. In fact Marcus and I had hoped, for selfish reasons, that as the young man's doctor Mastarna had examined Scaeva's corpse in detail. We hoped he could tell us about wounds or other significant factors that would assist us in investigating who killed the young man. Mastarna angrily informed me that post-mortem research is illegal, although he mentioned it had been carried out for a time in Alexandria.'

'Rarely.' Aedemon, the Alexandrian, was instantly dismissive. 'An anarchic, irreligious practice. I cure the living. I don't desecrate the dead.'

I saw Helena decide not to press him on whether surreptitious autopsy still took place nowadays. He wasn't going to tell us, even if he knew of it. She changed her approach: 'He had another patient too, I believe, at one point. V eleda? We know Mastarna discussed trepanation with Veleda. She was desperate to find somebody who would relieve the pressure in her skull. Did you have any views on that?'

'I never met the woman.' He was crisp. Too crisp? I did not think so; he was genuinely relieved to be able to deny involvement. Did that mean there were other subjects where his position might be more equivocal? Were our questions causing him anxiety?