Holmes then reveals the truth about what has been happening, much to the relief of Mrs. Ferguson as this is exactly what she has been hoping for: for the truth to come from someone else's lips. It turns out that the culprit is Jack, Mr. Ferguson's elder son, who is extremely jealous of his young half-brother. Holmes has confirmed this by looking at Jack's reflection in the window while his father's attention was on the baby. Jack has been attempting to murder his half-brother by shooting poisoned darts at him, and his stepmother's behaviour of sucking the baby's neck is thereby explained: she was sucking the poison out. It also explains why she struck Jack, and why she was sick when Holmes and Watson arrived. The wounds, therefore, were caused by the darts, not by her biting.