What makes the doctor's pulse go "dot and carry one" (skip a beat)? Why?

What makes the doctor's pulse skip a beat is hearing what he calls the "cry of a man at the point of death." He thinks it is the voice of Jim Hawkins, but he finds out later he is wrong: it is Alan, the first person to be killed by the mutineers.
Dr. Livesey hears the heart-stopping cry during his trip ashore with Hunter to explore the island. Dr. Livesey is happy to find a fresh water spring on the island, for the men on the ship sorely need water. Otherwise, they are well supplied with food and wine.
Unfortunately, however, Dr. Livesey also finds that Captain Flint's men have built a cleverly devised stockade. It is constructed so that it is very difficult to attack, while affording the men inside good cover and easy pickings in shooting anyone trying to take it over.