Oribasius (about 320 - 403) was a Greek medical writer and the personal physician of the Roman emperor Julian the Apostate. He studied at Alexandria under physician Zeno of Cyprus before joining Julian's retinue.
Oribasius's major works, written at the behest of Julian, are two collections of excerpts from the writings of earlier medical scholars: a collection of excerpts from Galen, and the Medical Collections (Collectiones medicae), a massive compilation of excerpts from other medical writers of the ancient world. The first of these works is entirely lost, and only 25 of the 70 (or 72) books of the Collectiones survive.
Medical Collections, Book 44, section 20, line 64, discussing treatments and surgery for fistula/abscess. Page 140 of Johannes Raeder, Oribasii Collectionum medicarum reliquiae, libri XXIV-XXV. XLIII-XLVIII, (Leipzig & Berlin: Teubner 1928–9).
When the fistula/abscess is positioned low down and the sacred bone becomes moist or decayed, one should not be afraid of doing what is needed because of its name, for it is not an essential part. But if the condition requires excision, the way to operate is described in other cases of bone decay.
Note by Jon Miles:
As with the writings of Paul of Aegina, the term 'sacred bone' cannot correspond here to the modern definition of the sacrum, since Oribasius says it is not essential and may be removed. Since the sacrum contains the end of the spinal canal and supports the hips, it is essential. The only bone in the area which can be removed without serious consequences is the coccyx, so it appears that in this case the 'sacred bone' refers only to the coccyx, excluding the sacrum.
Synopsis ad Eustathium, Section 6, line 44. Page 208 of Johannes Raeder, Oribasii Synopsis ad Eustathium, Libri ad Eunapium., (Leipzig & Berlin: Teubner, 1926).
When the part begins to turn red, we make a circle of wool of large size and place it underneath the part, and afterwards prepare a rose or myrtle salve, containing litharge or white lead, and apply. When inflammation develops, one must make a plaster of bread, with nightshade, knot-grass, plantain, or tender cabbage. But if the ulcer is spreading, it must be plastered with lentils and pomegranate peel.(6.44).
Oribasius is named by Paul of Aegina as the main source of his work. However, no account of the use of manipulation of the coccyx is found in Oribasius's surviving works.