From Nagarjuna's Treatise on the Great Perfection of Wisdom
What is meant by "internal" giving? [It refers] to not stinting in the giving to beings of [even] one's own physical life. It is as discussed in the past-life causes and conditions when Shakyamuni Buddha as a bodhisattva was serving as the king of a great country. The world was without a buddha, without the Dharma and without the bhikshu Sangha. This king searched in the four directions for the Dharma of the Buddha but was finally unable to find it. At that time there was a brahman who said, "I know a verse uttered by the Buddha. If an offering is made to me I will give it to you."
The King then asked, "What sort of offering are you seeking."
He replied, "If you are able to break open your flesh and turn it into a torch as an offering to me, then I shall give it to you."
The King then thought to himself, "This body of mine is fragile and impure. The amount of suffering which I have undergone on its behalf in life after life is incalculable. It has never been for the sake of Dharma. Only now does it begin to be truly useful. This body is truly not to be cherished now.
After having these thoughts, he called a ca.n.daalaa, and ordered him to scrape the surface of his entire body so that it might serve as a torch. Then he wrapped the King's flesh in white cloth, drenched it in ghee, and set fire to his entire body. The fire blazed up and only then did the brahman give that single verse.
Additionally, in a previous life, Shakyamuni Buddha was a pigeon in the snowy mountains. One time there was a great blizzard. There was a man who had lost his way. He was poor and in miserable straits, undergoing bitterness and suffering. Hunger and cold were both upon him and at that moment his life hung in the balance. The pigeon saw this man and immediately flew in search of fire, piling up twigs and then lighting them. He then additionally cast his body upon the fire as a gift to this starving man. In a manner such as this he gave his head, eyes, marrow and brains to beings. It would be appropriate at this point to draw upon all sorts of cases from the Sutra on the Causes and Conditions of Previous Lives. All sorts of instances such as this illustrate what is meant by "internal" giving.