A.T. Robertson Commentary Hebrews 10:7

"Then said I, Lo, I am come (In the roll of the book it is written of me) To do thy will, O God." — Hebrews 10:7 (ASV)

Then (τοτε). When it was plain that God could not be propitiated by such sacrifices.

Lo, I am come (Ιδου ηκω). The Messiah is represented as offering himself to do God's will (του ποιησα το θελημα σου, the genitive articular infinitive of purpose).

In the roll of the book it is written of me (εν κεφαλιδ βιβλιου γεγραπτα περ εμου). Stands written (γεγραπτα, perfect passive indicative). Κεφαλις is a diminutive of κεφαλη (head), a little head, then roll only here in N.T., but in the papyri. Here it refers "to the O.T. as a prediction of Christ's higher sacrifice" (Moffatt).