"as before death Christ's flesh was united personally and hypostatically with the Word of God, it remained so after His death, so that the hypostasis of the Word of God was not different from that of Christ's flesh after death".

"sicut ante mortem caro Christi unita fuit secundum personam et hypostasim verbo Dei, ita et remansit unita post mortem, ut scilicet non esset alia hypostasis verbi Dei et carnis Christi post mortem".

     St. Thomas Aquinas, ST III.50.2.Resp., trans. FEDP.  Latin from Corpus Thomisticum.  I was put on to this by Aaron Riches, Ecce homo:  on the divine unity of Christ, Interventions (Grand Rapids, MI:  Eerdmans, 2016):  204-208 ("The body of Christ in the tomb"), a wonderful discussion:

"'what belongs to the body of Christ after death is predicated of the Son of God [(ST III.50.2.sed contra)].'  The entombed cadaver—maximally different from God's apatheia—is nevertheless predicable only by virtue of the hypostatic union" (205). 

"There is no autonomous particularity, no 'thingness' or 'thisness' that can be granted to any aspect of the incarnate Christ, not even to the corpse in the tomb, apart from the Son.  The whole incarnate reality of Christ exists and is real only insofar as it subsists in union with the divine hypostasis of the Son" (207) 

"'The divinity was so indissoluably united to the humanity of Christ that, although body and soul were separated from each other, nonetheless the very divinity was always perfectly present both to the soul and the body.  Therefore, the Son of God was both in the tomb with the body and descended into hell with the soul [(et ideo in sepulcro cum corpore fuit filius Dei, et ad Inferos cum anima descendit)]'" (207; St. Thomas Aquinas, Sermon conferences on the Apostles' Creed, trans. Ayo (Notre Dame, IN:  University of Notre Dame Press, 1998), 79 =Expositio in Symbolum Apostolorum 5).

And from the concluding paragraph of the book by Riches (249):

     The indissoluble union of the Incarnate Logos is 'stretched out' from the height of the Son's eternal being with the Father to the cold stone on which his cadaver is laid, from the human breast of the Mother to the region of hell in which the crucified soul of Jesus is abandoned.  This mystery at the core of all being is not a tidy fact about divinity or humanity; it is the scandal of the Incarnate Son of God.  And 'none of the rulers of this age understood this; for if they had, they would not have crucified the Lord of glory' (1 Cor 2:8).