14 The nature of our bodies is such, that when endued with life and feeling by conjunction with a sentient soul, they become something more than inert, insensate matter. They feel when touched, suffer when pricked, shiver with cold, feel pleasure in warmth, waste with hunger, and grow fat with food. By a certain transfusion of the soul, which supports and penetrates them, they feel pleasure or pain according to the surrounding circumstances. When the body is pricked or pierced, it is the soul which pervades it that is conscious, and suffers pain.
For instance a flesh-wound is felt even to the bone, while the fingers feel nothing when we cut the nails which protrude from the flesh. And if through some disease a limb becomes withered, it loses the feeling of living flesh: it can be cut or burnt, it feels no pain whatever, because the soul is no longer mingled with it. Also when through some grave necessity part of the body must be cut away, the soul can be lulled to sleep by drugs, which overcome the pain, and produce in the mind a death-like forgetfulness of its power of sense.
Then limbs can be cut off without pain: the flesh is dead to all feeling, and does not heed the deep thrust of the knife, because the soul within it is asleep. It is, therefore, because the body lives by admixture with a weak soul, that it is subject to the weakness of pain.
Source: On the Trinity (New Advent)