What is "Me'or Einayim", and how does it gladden the heart?
Rashi #1: It is understanding Torah. They ask him something, and he knows what to answer.
Rashi #2: It is something that the eyes desire to see, e.g. a vegetable garden or flowing rivers. It gladdens the heart and removes sadness.
R. Yonah: Light widens man's heart. Chazal said, Halachos seek light and clarity 1 .
Magihah: He refers to Eruvin 65a - to decide Halachah, one needs clarity like a day when the north wind blows.
What is the significance that good tidings fatten bones?
Gitin 56b: It is literal. The Roman emperor died, and they decided that Aspanyainus will be the new leader. He had put on one shoe before hearing this. He was unable to put on the other, nor could he take off the first, for the good tidings fattened his bones.
R. Yonah: In addition to gladdening the heart of the hearer, it fattens his bones. This is an advantage of the ear over the eyes. Shlomo taught one of the many benefits of the ear 1 . We find that one who deafened a person, he pays his entire value (that he was worth to be sold as a slave, for now he is worthless).
Malbim: If one sees something nice with his eyes, this gladdens the heart, the abode of the powers of the Nefesh. Good tidings that he hears from afar, it only fattens his bones - physical powers of the body. Simchah of the heart is greater than fattening bones; it is spiritual, and elevated above it. The heart is the source of life for the entire body.
Sha'arei Teshuvah 2:12: Bones do not feel. A greater Simchah is needed to fatten them [than is needed to gladden the heart].
