I felt very strongly that I should probably think about doing something.
I wasn't sure what.
If you see someone in physical distress, I do believe your natural human inclination is to try to help them, even though by the time you've wrestled with indecision, and had a quick grapple with your inhibitions, they may well have made a full recovery and wandered off.
The first option seemed to be to tap him on the shoulder and say, 'Erm, excuse me. Do you know your head's bleeding? Because it, like, is.' Then I was worried that this might embarrass him, and he would touch his scalp gingerly and realise there was blood on his hand and then not know what to do with it, and I would offer him a tissue from my bag and he would feel awkward and uncomfortable at being mothered by someone roughly 30 years his junior.
The next potential path was simply to lunge at him with the same tissue and start dabbing away uninvited which, at best, may have caused him some considerable surprise, and at worst could have seen him standing up and shouting abuse at me, batting my ministrations away with wildly flailing arms while the bus driver called the police.
The final option was to get out of my seat, go and stand in his eyeline, cough obtrusively and attempt to spell out his plight with an elaborate sequence of head jerking, eyebrow raising and eye rolling, and the high risk of marking myself out as a bus lunatic.
I did none of these. I lifted up the four-day-old Sunday supplement I was reading so it was right in front of my face, and I could no longer see him. Before I got off at my stop, I had lowered my magazine several times to make sure I hadn't imagined it, and the blood was still there. Each time it was.