Sa'id bin al-Musayyib (may Allah have mercy on him) says that Sayyiduna Umar and Sayyiduna Uthman (may Allah be pleased with them both) fixed, for the miltah, a head wound that reaches the thin membrane over the bone, half the blood money of a bone-exposing wound in which the bone becomes visible.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الديات / 16211