EXUDATES.
Barbet was one of the first to confront the idea that fresh liquid blood, which he guessed would spread out and soak in around its source by capillary action was not an appropriate medium for making the well defined edges of the trickles on the arms and across the back. Perhaps he tried dribbling blood on a sheet, perhaps he tried blotting it with cloth; I don't know, but whatever it was, he failed. He was also aware that blood dries surprisingly quickly, and that it doesn't flow well from dead bodies, which again could not account for the trickles. However, he was unable to let go of the idea that the stains are really blood, so had to speculate, and the exudate was born. Trouble was, exudate, as well he knew, is yellow to colourless, and the marks he was trying to explain are red. The solution to that was that the red blood cells had largely hemolysed, and their content spread out into the plasma. He hoped that by a process similar to chromatography, the serum would spread out further than the red colour, and imagined he could identify serum borders to the wounds. As far as I know, nobody checked whether this was a credible hypothesis by lifting a dressing off a clotting wound to find out what had happened, and hypothesis became gospel first by authority and then by antiquity. Some rather gruesome images found by Googling "wound dressing removal" show what happens, and the marks look nothing like those on the shroud...