Thomas Aquinas, Compendium of Theology

In Christ three substances, the body, the soul, and the divinity of the Word, are joined together in one person. Two of these, the soul and the body, are united to form one nature. Accordingly at the death of Christ the union between body and soul was dissolved. Otherwise the body would not have been truly dead, since death of the body is nothing else than the separation of the soul from it.

But neither soul nor body was separated from the Word of God, as far as union with the person is concerned. Human nature results from the union of soul and body; hence Christ could not be said to be a man during the three days of His death, when His soul remained separated from His body by death. However, as was shown above, on account of the union of the human nature with the Word of God in one person, whatever is said of the man Christ can rightly be predicated also of the Son of God. Consequently, since the personal union of the Son of God both with the soul and with the body of Christ remained in death, whatever is said of either of them could be predicated of the Son of God. Hence the Creed asserts that the Son of God was buried, for the reason that the body united to Him lay in the tomb, and likewise that He descended into hell, because His soul descended.

We should also recall that the masculine gender designates a person, and that the neuter gender designates nature. Thus in speaking of the Trinity we say that the Son is another person (alius) than the Father, but not that He is another thing (aliud). Accordingly, during the three days of His death the whole (totus) Christ was in the sepulcher and in hell and in heaven, because of His person which remained united to His flesh reposing in the tomb and to His soul which was emptying hell, and which continued to subsist in the divine nature reigning in heaven. But we cannot say that the whole (totum) of Christ was in the sepulcher or in hell, because only a part of the human nature and not the whole of it was in the sepulcher or in hell.